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Angular - Observables compared to other techniques

Observables compared to other techniques

Observables compared to promises
Creation and subscription
Chaining
Cancellation
Error handling
Cheat sheet
Observables compared to events API
Observables compared to arrays

You can often use observables instead of promises to deliver values asynchronously. Similarly, observables can take the place of event handlers. Finally, because observables deliver multiple values, you can use them where you might otherwise build and operate on arrays.

Observables behave somewhat differently from the alternative techniques in each of these situations, but offer some significant advantages. Here are detailed comparisons of the differences.

Observables compared to promises

Observables are often compared to promises. Here are some key differences:

Observables are declarative; computation does not start until subscription. Promises execute immediately on creation. This makes observables useful for defining recipes that can be run whenever you need the result.
Observables provide many values. Promises provide one. This makes observables useful for getting multiple values over time.
Observables differentiate between chaining and subscription. Promises only have .then() clauses. This makes observables useful for creating complex transformation recipes to be used by other part of the system, without causing the work to be executed.
Observables subscribe() is responsible for handling errors. Promises push errors to the child promises. This makes observables useful for centralized and predictable error handling.

Creation and subscription

Observables are not executed until a consumer subscribes. The subscribe() executes the defined behavior once, and it can be called again. Each subscription has its own computation. Resubscription causes recomputation of values.

src/observables.ts (observable)

      
      // declare a publishing operation
const observable = new Observable<number>(observer => {
  // Subscriber fn...
});

// initiate execution
observable.subscribe(value => {
  // observer handles notifications
});

Promises execute immediately, and just once. The computation of the result is initiated when the promise is created. There is no way to restart work. All then clauses (subscriptions) share the same computation.

src/promises.ts (promise)

      
      // initiate execution
let promise = new Promise<number>(resolve => {
  // Executer fn...
});
promise.then(value => {
  // handle result here
});

Chaining

Observables differentiate between transformation function such as a map and subscription. Only subscription activates the subscriber function to start computing the values.
src/observables.ts (chain)
```
      
      observable.pipe(map(v => 2 * v));
    
```
Promises do not differentiate between the last .then clauses (equivalent to subscription) and intermediate .then clauses (equivalent to map).
src/promises.ts (chain)
```
      
      promise.then(v => 2 * v);
    
```

Cancellation

Observable subscriptions are cancellable. Unsubscribing removes the listener from receiving further values, and notifies the subscriber function to cancel work.

src/observables.ts (unsubscribe)

      
      const subscription = observable.subscribe(() => {
  // observer handles notifications
});

subscription.unsubscribe();

Promises are not cancellable.

Error handling

Observable execution errors are delivered to the subscriber's error handler, and the subscriber automatically unsubscribes from the observable.
src/observables.ts (error)
```
      
      observable.subscribe(() => {
  throw new Error('my error');
});
    
```

Promises push errors to the child promises.

src/promises.ts (error)

      
      promise.then(() => {
  throw new Error('my error');
});

Cheat sheet

The following code snippets illustrate how the same kind of operation is defined using observables and promises.

Operation	Observable	Promise
Creation	`new Observable((observer) => { observer.next(123); });`	`new Promise((resolve, reject) => { resolve(123); });`
Transform	`obs.pipe(map((value) => value * 2));`	`promise.then((value) => value * 2);`
Subscribe	`sub = obs.subscribe((value) => { console.log(value) });`	`promise.then((value) => { console.log(value); });`
Unsubscribe	`sub.unsubscribe();`	Implied by promise resolution.

Observables compared to events API

Observables are very similar to event handlers that use the events API. Both techniques define notification handlers, and use them to process multiple values delivered over time. Subscribing to an observable is equivalent to adding an event listener. One significant difference is that you can configure an observable to transform an event before passing the event to the handler.

Using observables to handle events and asynchronous operations can have the advantage of greater consistency in contexts such as HTTP requests.

Here are some code samples that illustrate how the same kind of operation is defined using observables and the events API.

Observable Events API

Creation & cancellation

	Observable	Events API
Creation & cancellation	`// Setup const clicks$ = fromEvent(buttonEl, 'click'); // Begin listening const subscription = clicks$ .subscribe(e => console.log('Clicked', e)) // Stop listening subscription.unsubscribe();`	`function handler(e) { console.log('Clicked', e); } // Setup & begin listening button.addEventListener('click', handler); // Stop listening button.removeEventListener('click', handler);`
Subscription	`observable.subscribe(() => { // notification handlers here });`	`element.addEventListener(eventName, (event) => { // notification handler here });`
Configuration	Listen for keystrokes, but provide a stream representing the value in the input. `fromEvent(inputEl, 'keydown').pipe( map(e => e.target.value) );`	Does not support configuration. `element.addEventListener(eventName, (event) => { // Cannot change the passed Event into another // value before it gets to the handler });`

      
      // Setup 
const clicks$ = fromEvent(buttonEl, 'click'); 
// Begin listening 
const subscription = clicks$ 
  .subscribe(e => console.log('Clicked', e)) 
// Stop listening 
subscription.unsubscribe();

      
      function handler(e) { 
  console.log('Clicked', e); 
} 
// Setup & begin listening 
button.addEventListener('click', handler); 
// Stop listening 
button.removeEventListener('click', handler);

Subscription

      
      observable.subscribe(() => { 
  // notification handlers here 
});

      
      element.addEventListener(eventName, (event) => { 
  // notification handler here 
});

Configuration

Listen for keystrokes, but provide a stream representing the value in the input.

      
      fromEvent(inputEl, 'keydown').pipe( 
  map(e => e.target.value) 
);

Does not support configuration.

      
      element.addEventListener(eventName, (event) => { 
  // Cannot change the passed Event into another 
  // value before it gets to the handler 
});

Observables compared to arrays

An observable produces values over time. An array is created as a static set of values. In a sense, observables are asynchronous where arrays are synchronous. In the following examples, → implies asynchronous value delivery.

Values	Observable	Array
Given	`obs: →1→2→3→5→7` `obsB: →'a'→'b'→'c'`	`arr: [1, 2, 3, 5, 7]` `arrB: ['a', 'b', 'c']`
`concat()`	`concat(obs, obsB)` `→1→2→3→5→7→'a'→'b'→'c'`	`arr.concat(arrB)` `[1,2,3,5,7,'a','b','c']`
`filter()`	`obs.pipe(filter((v) => v>3))` `→5→7`	`arr.filter((v) => v>3)` `[5, 7]`
`find()`	`obs.pipe(find((v) => v>3))` `→5`	`arr.find((v) => v>3)` `5`
`findIndex()`	`obs.pipe(findIndex((v) => v>3))` `→3`	`arr.findIndex((v) => v>3)` `3`
`forEach()`	`obs.pipe(tap((v) => { console.log(v); })) 1 2 3 5 7`	`arr.forEach((v) => { console.log(v); }) 1 2 3 5 7`
`map()`	`obs.pipe(map((v) => -v))` `→-1→-2→-3→-5→-7`	`arr.map((v) => -v)` `[-1, -2, -3, -5, -7]`
`reduce()`	`obs.pipe(reduce((s,v)=> s+v, 0))` `→18`	`arr.reduce((s,v) => s+v, 0)` `18`

Last reviewed on Mon Feb 28 2022

Angular - Complex animation sequences

Complex animation sequences

Prerequisites
The query() function
Animate multiple elements using query() and stagger() functions
Parallel animation using group() function
Sequential vs. parallel animations
Filter animation example
Animating the items of a reordering list
Animations and Component View Encapsulation
Animation sequence summary
More on Angular animations

Prerequisites

A basic understanding of the following concepts:

Introduction to Angular animations
Transition and triggers

So far, we've learned simple animations of single HTML elements. Angular also lets you animate coordinated sequences, such as an entire grid or list of elements as they enter and leave a page. You can choose to run multiple animations in parallel, or run discrete animations sequentially, one following another.

The functions that control complex animation sequences are:

Functions	Details
`query()`	Finds one or more inner HTML elements.
`stagger()`	Applies a cascading delay to animations for multiple elements.
`group()`	Runs multiple animation steps in parallel.
`sequence()`	Runs animation steps one after another.

The query() function

Most complex animations rely on the query() function to find child elements and apply animations to them, basic examples of such are:

Examples	Details
`query()` followed by `animate()`	Used to query simple HTML elements and directly apply animations to them.
`query()` followed by `animateChild()`	Used to query child elements, which themselves have animations metadata applied to them and trigger such animation (which would be otherwise be blocked by the current/parent element's animation).

The first argument of query() is a css selector string which can also contain the following Angular-specific tokens:

Tokens	Details
`:enter` `:leave`	For entering/leaving elements.
`:animating`	For elements currently animating.
`@*` `@triggerName`	For elements with any—or a specific—trigger.
`:self`	The animating element itself.

Entering and Leaving Elements

Not all child elements are actually considered as entering/leaving; this can, at times, be counterintuitive and confusing. Please see the query api docs for more information.

You can also see an illustration of this in the animations live example (introduced in the animations introduction section) under the Querying tab.

Animate multiple elements using query() and stagger() functions

After having queried child elements via query() , the stagger() function lets you define a timing gap between each queried item that is animated and thus animates elements with a delay between them.

The following example demonstrates how to use the query() and stagger() functions to animate a list (of heroes) adding each in sequence, with a slight delay, from top to bottom.

Use query() to look for an element entering the page that meets certain criteria
For each of these elements, use style() to set the same initial style for the element. Make it transparent and use transform to move it out of position so that it can slide into place.
Use stagger() to delay each animation by 30 milliseconds
Animate each element on screen for 0.5 seconds using a custom-defined easing curve, simultaneously fading it in and un-transforming it

src/app/hero-list-page.component.ts

      
      animations: [
  trigger('pageAnimations', [
    transition(':enter', [
      query('.hero', [
        style({opacity: 0, transform: 'translateY(-100px)'}),
        stagger(30, [
          animate('500ms cubic-bezier(0.35, 0, 0.25, 1)',
          style({ opacity: 1, transform: 'none' }))
        ])
      ])
    ])
  ]),

Parallel animation using group() function

You've seen how to add a delay between each successive animation. But you might also want to configure animations that happen in parallel. For example, you might want to animate two CSS properties of the same element but use a different easing function for each one. For this, you can use the animation group() function.

NOTE :
The group() function is used to group animation steps , rather than animated elements.

The following example uses group() s on both :enter and :leave for two different timing configurations, thus applying two independent animations to the same element in parallel.

src/app/hero-list-groups.component.ts (excerpt)

      
      animations: [
  trigger('flyInOut', [
    state('in', style({
      width: '*',
      transform: 'translateX(0)', opacity: 1
    })),
    transition(':enter', [
      style({ width: 10, transform: 'translateX(50px)', opacity: 0 }),
      group([
        animate('0.3s 0.1s ease', style({
          transform: 'translateX(0)',
          width: '*'
        })),
        animate('0.3s ease', style({
          opacity: 1
        }))
      ])
    ]),
    transition(':leave', [
      group([
        animate('0.3s ease', style({
          transform: 'translateX(50px)',
          width: 10
        })),
        animate('0.3s 0.2s ease', style({
          opacity: 0
        }))
      ])
    ])
  ])
]

Sequential vs. parallel animations

Complex animations can have many things happening at once. But what if you want to create an animation involving several animations happening one after the other? Earlier you used group() to run multiple animations all at the same time, in parallel.

A second function called sequence() lets you run those same animations one after the other. Within sequence() , the animation steps consist of either style() or animate() function calls.

Use style() to apply the provided styling data immediately.
Use animate() to apply styling data over a given time interval.

Filter animation example

Take a look at another animation on the live example page. Under the Filter/Stagger tab, enter some text into the Search Heroes text box, such as Magnet or tornado .

The filter works in real time as you type. Elements leave the page as you type each new letter and the filter gets progressively stricter. The heroes list gradually re-enters the page as you delete each letter in the filter box.

The HTML template contains a trigger called filterAnimation .

src/app/hero-list-page.component.html

      
      <label for="search">Search heroes: </label>
<input type="text" id="search" #criteria
       (input)="updateCriteria(criteria.value)"
       placeholder="Search heroes">

<ul class="heroes" [@filterAnimation]="heroesTotal">
  <li *ngFor="let hero of heroes" class="hero">
    <div class="inner">
      <span class="badge">{{ hero.id }}</span>
      <span class="name">{{ hero.name }}</span>
    </div>
  </li>
</ul>

The filterAnimation in the component's decorator contains three transitions.

src/app/hero-list-page.component.ts

      
      @Component({
  animations: [
    trigger('filterAnimation', [
      transition(':enter, * => 0, * => -1', []),
      transition(':increment', [
        query(':enter', [
          style({ opacity: 0, width: 0 }),
          stagger(50, [
            animate('300ms ease-out', style({ opacity: 1, width: '*' })),
          ]),
        ], { optional: true })
      ]),
      transition(':decrement', [
        query(':leave', [
          stagger(50, [
            animate('300ms ease-out', style({ opacity: 0, width: 0 })),
          ]),
        ])
      ]),
    ]),
  ]
})
export class HeroListPageComponent implements OnInit {
  heroesTotal = -1;

  get heroes() { return this._heroes; }
  private _heroes: Hero[] = [];

  ngOnInit() {
    this._heroes = HEROES;
  }

  updateCriteria(criteria: string) {
    criteria = criteria ? criteria.trim() : '';

    this._heroes = HEROES.filter(hero => hero.name.toLowerCase().includes(criteria.toLowerCase()));
    const newTotal = this.heroes.length;

    if (this.heroesTotal !== newTotal) {
      this.heroesTotal = newTotal;
    } else if (!criteria) {
      this.heroesTotal = -1;
    }
  }
}

The code in this example performs the following tasks:

Skips animations when the user first opens or navigates to this page (the filter animation narrows what is already there, so it only works on elements that already exist in the DOM)
Filters heroes based on the search input's value

For each change:

Hides an element leaving the DOM by setting its opacity and width to 0
Animates an element entering the DOM over 300 milliseconds. During the animation, the element assumes its default width and opacity.
If there are multiple elements entering or leaving the DOM, staggers each animation starting at the top of the page, with a 50-millisecond delay between each element

Animating the items of a reordering list

Although Angular animates correctly *ngFor list items out of the box, it will not be able to do so if their ordering changes. This is because it will lose track of which element is which, resulting in broken animations. The only way to help Angular keep track of such elements is by assigning a TrackByFunction to the NgForOf directive. This makes sure that Angular always knows which element is which, thus allowing it to apply the correct animations to the correct elements all the time.

IMPORTANT :
If you need to animate the items of an *ngFor list and there is a possibility that the order of such items will change during runtime, always use a TrackByFunction .

Animations and Component View Encapsulation

Angular animations are based on the components DOM structure and do not directly take View Encapsulation into account, this means that components using ViewEncapsulation.Emulated behave exactly as if they were using ViewEncapsulation.None ( ViewEncapsulation.ShadowDom behaves differently as we'll discuss shortly).

For example if the query() function (which you'll see more of in the rest of the Animations guide) were to be applied at the top of a tree of components using the emulated view encapsulation, such query would be able to identify (and thus animate) DOM elements on any depth of the tree.

On the other hand the ViewEncapsulation.ShadowDom changes the component's DOM structure by "hiding" DOM elements inside ShadowRoot elements. Such DOM manipulations do prevent some of the animations implementation to work properly since it relies on simple DOM structures and doesn't take ShadowRoot elements into account. Therefore it is advised to avoid applying animations to views incorporating components using the ShadowDom view encapsulation.

Animation sequence summary

Angular functions for animating multiple elements start with query() to find inner elements; for example, gathering all images within a <div> . The remaining functions, stagger() , group() , and sequence() , apply cascades or let you control how multiple animation steps are applied.

Component interaction

Pass data from parent to child with input binding
Test it for Pass data from parent to child with input binding
Intercept input property changes with a setter
Test it for Intercept input property changes with a setter
Intercept input property changes with ngOnChanges()
Test it for Intercept input property changes with ngOnChanges()
Parent listens for child event
Test it for Parent listens for child event
Parent interacts with child using local variable
Test it for Parent interacts with child using local variable
Parent calls an @ViewChild()
Test it for Parent calls an @ViewChild()
Parent and children communicate using a service
Test it for Parent and children communicate using a service

This cookbook contains recipes for common component communication scenarios in which two or more components share information.

See the live example / download example .

Pass data from parent to child with input binding

HeroChildComponent has two input properties , typically adorned with @Input() decorator.

component-interaction/src/app/hero-child.component.ts

      
      import { Component, Input } from '@angular/core';

import { Hero } from './hero';

@Component({
  selector: 'app-hero-child',
  template: `
    <h3>{{hero.name}} says:</h3>
    <p>I, {{hero.name}}, am at your service, {{masterName}}.</p>
  `
})
export class HeroChildComponent {
  @Input() hero!: Hero;
  @Input('master') masterName = '';
}

The second @Input aliases the child component property name masterName as 'master' .

The HeroParentComponent nests the child HeroChildComponent inside an *ngFor repeater, binding its master string property to the child's master alias, and each iteration's hero instance to the child's hero property.

component-interaction/src/app/hero-parent.component.ts

      
      import { Component } from '@angular/core';

import { HEROES } from './hero';

@Component({
  selector: 'app-hero-parent',
  template: `
    <h2>{{master}} controls {{heroes.length}} heroes</h2>

    <app-hero-child
      *ngFor="let hero of heroes"
      [hero]="hero"
      [master]="master">
    </app-hero-child>
  `
})
export class HeroParentComponent {
  heroes = HEROES;
  master = 'Master';
}

The running application displays three heroes:

Test it for Pass data from parent to child with input binding

E2E test that all children were instantiated and displayed as expected:

component-interaction/e2e/src/app.e2e-spec.ts

      
      // ...
const heroNames = ['Dr. IQ', 'Magneta', 'Bombasto'];
const masterName = 'Master';

it('should pass properties to children properly', async () => {
  const parent = element(by.tagName('app-hero-parent'));
  const heroes = parent.all(by.tagName('app-hero-child'));

  for (let i = 0; i < heroNames.length; i++) {
    const childTitle = await heroes.get(i).element(by.tagName('h3')).getText();
    const childDetail = await heroes.get(i).element(by.tagName('p')).getText();
    expect(childTitle).toEqual(heroNames[i] + ' says:');
    expect(childDetail).toContain(masterName);
  }
});
// ...

Intercept input property changes with a setter

Use an input property setter to intercept and act upon a value from the parent.

The setter of the name input property in the child NameChildComponent trims the whitespace from a name and replaces an empty value with default text.

component-interaction/src/app/name-child.component.ts

      
      import { Component, Input } from '@angular/core';

@Component({
  selector: 'app-name-child',
  template: '<h3>"{{name}}"</h3>'
})
export class NameChildComponent {
  @Input()
  get name(): string { return this._name; }
  set name(name: string) {
    this._name = (name && name.trim()) || '<no name set>';
  }
  private _name = '';
}

Here's the NameParentComponent demonstrating name variations including a name with all spaces:

component-interaction/src/app/name-parent.component.ts

      
      import { Component } from '@angular/core';

@Component({
  selector: 'app-name-parent',
  template: `
    <h2>Master controls {{names.length}} names</h2>

    <app-name-child *ngFor="let name of names" [name]="name"></app-name-child>
  `
})
export class NameParentComponent {
  // Displays 'Dr. IQ', '<no name set>', 'Bombasto'
  names = ['Dr. IQ', '   ', '  Bombasto  '];
}

Test it for Intercept input property changes with a setter

E2E tests of input property setter with empty and non-empty names:

component-interaction/e2e/src/app.e2e-spec.ts

      
      // ...
it('should display trimmed, non-empty names', async () => {
  const nonEmptyNameIndex = 0;
  const nonEmptyName = '"Dr. IQ"';
  const parent = element(by.tagName('app-name-parent'));
  const hero = parent.all(by.tagName('app-name-child')).get(nonEmptyNameIndex);

  const displayName = await hero.element(by.tagName('h3')).getText();
  expect(displayName).toEqual(nonEmptyName);
});

it('should replace empty name with default name', async () => {
  const emptyNameIndex = 1;
  const defaultName = '"<no name set>"';
  const parent = element(by.tagName('app-name-parent'));
  const hero = parent.all(by.tagName('app-name-child')).get(emptyNameIndex);

  const displayName = await hero.element(by.tagName('h3')).getText();
  expect(displayName).toEqual(defaultName);
});
// ...

Intercept input property changes with `ngOnChanges()`

Detect and act upon changes to input property values with the ngOnChanges() method of the OnChanges lifecycle hook interface.

You might prefer this approach to the property setter when watching multiple, interacting input properties.

Learn about ngOnChanges() in the Lifecycle Hooks chapter.

This VersionChildComponent detects changes to the major and minor input properties and composes a log message reporting these changes:

component-interaction/src/app/version-child.component.ts

      
      import { Component, Input, OnChanges, SimpleChanges } from '@angular/core';

@Component({
  selector: 'app-version-child',
  template: `
    <h3>Version {{major}}.{{minor}}</h3>
    <h4>Change log:</h4>
    <ul>
      <li *ngFor="let change of changeLog">{{change}}</li>
    </ul>
  `
})
export class VersionChildComponent implements OnChanges {
  @Input() major = 0;
  @Input() minor = 0;
  changeLog: string[] = [];

  ngOnChanges(changes: SimpleChanges) {
    const log: string[] = [];
    for (const propName in changes) {
      const changedProp = changes[propName];
      const to = JSON.stringify(changedProp.currentValue);
      if (changedProp.isFirstChange()) {
        log.push(`Initial value of ${propName} set to ${to}`);
      } else {
        const from = JSON.stringify(changedProp.previousValue);
        log.push(`${propName} changed from ${from} to ${to}`);
      }
    }
    this.changeLog.push(log.join(', '));
  }
}

The VersionParentComponent supplies the minor and major values and binds buttons to methods that change them.

component-interaction/src/app/version-parent.component.ts

      
      import { Component } from '@angular/core';

@Component({
  selector: 'app-version-parent',
  template: `
    <h2>Source code version</h2>
    <button type="button" (click)="newMinor()">New minor version</button>
    <button type="button" (click)="newMajor()">New major version</button>
    <app-version-child [major]="major" [minor]="minor"></app-version-child>
  `
})
export class VersionParentComponent {
  major = 1;
  minor = 23;

  newMinor() {
    this.minor++;
  }

  newMajor() {
    this.major++;
    this.minor = 0;
  }
}

Here's the output of a button-pushing sequence:

Test it for Intercept input property changes with `ngOnChanges()`

Test that both input properties are set initially and that button clicks trigger the expected ngOnChanges calls and values:

component-interaction/e2e/src/app.e2e-spec.ts

      
      // ...
// Test must all execute in this exact order
it('should set expected initial values', async () => {
  const actual = await getActual();

  const initialLabel = 'Version 1.23';
  const initialLog = 'Initial value of major set to 1, Initial value of minor set to 23';

  expect(actual.label).toBe(initialLabel);
  expect(actual.count).toBe(1);
  expect(await actual.logs.get(0).getText()).toBe(initialLog);
});

it("should set expected values after clicking 'Minor' twice", async () => {
  const repoTag = element(by.tagName('app-version-parent'));
  const newMinorButton = repoTag.all(by.tagName('button')).get(0);

  await newMinorButton.click();
  await newMinorButton.click();

  const actual = await getActual();

  const labelAfter2Minor = 'Version 1.25';
  const logAfter2Minor = 'minor changed from 24 to 25';

  expect(actual.label).toBe(labelAfter2Minor);
  expect(actual.count).toBe(3);
  expect(await actual.logs.get(2).getText()).toBe(logAfter2Minor);
});

it("should set expected values after clicking 'Major' once", async () => {
  const repoTag = element(by.tagName('app-version-parent'));
  const newMajorButton = repoTag.all(by.tagName('button')).get(1);

  await newMajorButton.click();
  const actual = await getActual();

  const labelAfterMajor = 'Version 2.0';
  const logAfterMajor = 'major changed from 1 to 2, minor changed from 23 to 0';

  expect(actual.label).toBe(labelAfterMajor);
  expect(actual.count).toBe(2);
  expect(await actual.logs.get(1).getText()).toBe(logAfterMajor);
});

async function getActual() {
  const versionTag = element(by.tagName('app-version-child'));
  const label = await versionTag.element(by.tagName('h3')).getText();
  const ul = versionTag.element((by.tagName('ul')));
  const logs = ul.all(by.tagName('li'));

  return {
    label,
    logs,
    count: await logs.count(),
  };
}
// ...

Parent listens for child event

The child component exposes an EventEmitter property with which it emits events when something happens. The parent binds to that event property and reacts to those events.

The child's EventEmitter property is an output property , typically adorned with an @Output() decorator as seen in this VoterComponent :

component-interaction/src/app/voter.component.ts

      
      import { Component, EventEmitter, Input, Output } from '@angular/core';

@Component({
  selector: 'app-voter',
  template: `
    <h4>{{name}}</h4>
    <button type="button" (click)="vote(true)"  [disabled]="didVote">Agree</button>
    <button type="button" (click)="vote(false)" [disabled]="didVote">Disagree</button>
  `
})
export class VoterComponent {
  @Input()  name = '';
  @Output() voted = new EventEmitter<boolean>();
  didVote = false;

  vote(agreed: boolean) {
    this.voted.emit(agreed);
    this.didVote = true;
  }
}

Clicking a button triggers emission of a true or false , the boolean payload .

The parent VoteTakerComponent binds an event handler called onVoted() that responds to the child event payload $event and updates a counter.

component-interaction/src/app/votetaker.component.ts

      
      import { Component } from '@angular/core';

@Component({
  selector: 'app-vote-taker',
  template: `
    <h2>Should mankind colonize the Universe?</h2>
    <h3>Agree: {{agreed}}, Disagree: {{disagreed}}</h3>

    <app-voter
      *ngFor="let voter of voters"
      [name]="voter"
      (voted)="onVoted($event)">
    </app-voter>
  `
})
export class VoteTakerComponent {
  agreed = 0;
  disagreed = 0;
  voters = ['Dr. IQ', 'Celeritas', 'Bombasto'];

  onVoted(agreed: boolean) {
    if (agreed) {
      this.agreed++;
    } else {
      this.disagreed++;
    }
  }
}

The framework passes the event argument —represented by $event — to the handler method, and the method processes it:

Test it for Parent listens for child event

Test that clicking the Agree and Disagree buttons update the appropriate counters:

component-interaction/e2e/src/app.e2e-spec.ts

      
      // ...
it('should not emit the event initially', async () => {
  const voteLabel = element(by.tagName('app-vote-taker')).element(by.tagName('h3'));
  expect(await voteLabel.getText()).toBe('Agree: 0, Disagree: 0');
});

it('should process Agree vote', async () => {
  const voteLabel = element(by.tagName('app-vote-taker')).element(by.tagName('h3'));
  const agreeButton1 = element.all(by.tagName('app-voter')).get(0)
    .all(by.tagName('button')).get(0);

  await agreeButton1.click();

  expect(await voteLabel.getText()).toBe('Agree: 1, Disagree: 0');
});

it('should process Disagree vote', async () => {
  const voteLabel = element(by.tagName('app-vote-taker')).element(by.tagName('h3'));
  const agreeButton1 = element.all(by.tagName('app-voter')).get(1)
    .all(by.tagName('button')).get(1);

  await agreeButton1.click();

  expect(await voteLabel.getText()).toBe('Agree: 0, Disagree: 1');
});
// ...

Parent interacts with child using local variable

A parent component cannot use data binding to read child properties or invoke child methods. Do both by creating a template reference variable for the child element and then reference that variable within the parent template as seen in the following example.

The following is a child CountdownTimerComponent that repeatedly counts down to zero and launches a rocket. The start and stop methods control the clock and a countdown status message displays in its own template.

component-interaction/src/app/countdown-timer.component.ts

      
      import { Component, OnDestroy } from '@angular/core';

@Component({
  selector: 'app-countdown-timer',
  template: '<p>{{message}}</p>'
})
export class CountdownTimerComponent implements OnDestroy {

  intervalId = 0;
  message = '';
  seconds = 11;

  ngOnDestroy() { this.clearTimer(); }

  start() { this.countDown(); }
  stop()  {
    this.clearTimer();
    this.message = `Holding at T-${this.seconds} seconds`;
  }

  private clearTimer() { clearInterval(this.intervalId); }

  private countDown() {
    this.clearTimer();
    this.intervalId = window.setInterval(() => {
      this.seconds -= 1;
      if (this.seconds === 0) {
        this.message = 'Blast off!';
      } else {
        if (this.seconds < 0) { this.seconds = 10; } // reset
        this.message = `T-${this.seconds} seconds and counting`;
      }
    }, 1000);
  }
}

The CountdownLocalVarParentComponent that hosts the timer component is as follows:

component-interaction/src/app/countdown-parent.component.ts

      
      import { Component } from '@angular/core';
import { CountdownTimerComponent } from './countdown-timer.component';

@Component({
  selector: 'app-countdown-parent-lv',
  template: `
    <h3>Countdown to Liftoff (via local variable)</h3>
    <button type="button" (click)="timer.start()">Start</button>
    <button type="button" (click)="timer.stop()">Stop</button>
    <div class="seconds">{{timer.seconds}}</div>
    <app-countdown-timer #timer></app-countdown-timer>
  `,
  styleUrls: ['../assets/demo.css']
})
export class CountdownLocalVarParentComponent { }

The parent component cannot data bind to the child's start and stop methods nor to its seconds property.

Place a local variable, #timer , on the tag <app-countdown-timer> representing the child component. That gives you a reference to the child component and the ability to access any of its properties or methods from within the parent template.

This example wires parent buttons to the child's start and stop and uses interpolation to display the child's seconds property.

Here, the parent and child are working together.

Test it for Parent interacts with child using local variable

Test that the seconds displayed in the parent template match the seconds displayed in the child's status message. Test also that clicking the Stop button pauses the countdown timer:

component-interaction/e2e/src/app.e2e-spec.ts

      
      // ...
// The tests trigger periodic asynchronous operations (via `setInterval()`), which will prevent
// the app from stabilizing. See https://angular.io/api/core/ApplicationRef#is-stable-examples
// for more details.
// To allow the tests to complete, we will disable automatically waiting for the Angular app to
// stabilize.
beforeEach(() => browser.waitForAngularEnabled(false));
afterEach(() => browser.waitForAngularEnabled(true));

it('timer and parent seconds should match', async () => {
  const parent = element(by.tagName(parentTag));
  const startButton = parent.element(by.buttonText('Start'));
  const seconds = parent.element(by.className('seconds'));
  const timer = parent.element(by.tagName('app-countdown-timer'));

  await startButton.click();

  // Wait for `<app-countdown-timer>` to be populated with any text.
  await browser.wait(() => timer.getText(), 2000);

  expect(await timer.getText()).toContain(await seconds.getText());
});

it('should stop the countdown', async () => {
  const parent = element(by.tagName(parentTag));
  const startButton = parent.element(by.buttonText('Start'));
  const stopButton = parent.element(by.buttonText('Stop'));
  const timer = parent.element(by.tagName('app-countdown-timer'));

  await startButton.click();
  expect(await timer.getText()).not.toContain('Holding');

  await stopButton.click();
  expect(await timer.getText()).toContain('Holding');
});
// ...

Parent calls an `@ViewChild()`

The local variable approach is straightforward. But it is limited because the parent-child wiring must be done entirely within the parent template. The parent component itself has no access to the child.

You can't use the local variable technique if the parent component's class relies on the child component's class . The parent-child relationship of the components is not established within each component's respective class with the local variable technique. Because the class instances are not connected to one another, the parent class cannot access the child class properties and methods.

When the parent component class requires that kind of access, inject the child component into the parent as a ViewChild .

The following example illustrates this technique with the same Countdown Timer example. Neither its appearance nor its behavior changes. The child CountdownTimerComponent is the same as well.

The switch from the local variable to the ViewChild technique is solely for the purpose of demonstration.

Here is the parent, CountdownViewChildParentComponent :

component-interaction/src/app/countdown-parent.component.ts

      
      import { AfterViewInit, ViewChild } from '@angular/core';
import { Component } from '@angular/core';
import { CountdownTimerComponent } from './countdown-timer.component';

@Component({
  selector: 'app-countdown-parent-vc',
  template: `
    <h3>Countdown to Liftoff (via ViewChild)</h3>
    <button type="button" (click)="start()">Start</button>
    <button type="button" (click)="stop()">Stop</button>
    <div class="seconds">{{ seconds() }}</div>
    <app-countdown-timer></app-countdown-timer>
  `,
  styleUrls: ['../assets/demo.css']
})
export class CountdownViewChildParentComponent implements AfterViewInit {

  @ViewChild(CountdownTimerComponent)
  private timerComponent!: CountdownTimerComponent;

  seconds() { return 0; }

  ngAfterViewInit() {
    // Redefine `seconds()` to get from the `CountdownTimerComponent.seconds` ...
    // but wait a tick first to avoid one-time devMode
    // unidirectional-data-flow-violation error
    setTimeout(() => this.seconds = () => this.timerComponent.seconds, 0);
  }

  start() { this.timerComponent.start(); }
  stop() { this.timerComponent.stop(); }
}

It takes a bit more work to get the child view into the parent component class .

First, you have to import references to the ViewChild decorator and the AfterViewInit lifecycle hook.

Next, inject the child CountdownTimerComponent into the private timerComponent property using the @ViewChild property decoration.

The #timer local variable is gone from the component metadata. Instead, bind the buttons to the parent component's own start and stop methods and present the ticking seconds in an interpolation around the parent component's seconds method.

These methods access the injected timer component directly.

The ngAfterViewInit() lifecycle hook is an important wrinkle. The timer component isn't available until after Angular displays the parent view. So it displays 0 seconds initially.

Then Angular calls the ngAfterViewInit lifecycle hook at which time it is too late to update the parent view's display of the countdown seconds. Angular's unidirectional data flow rule prevents updating the parent view's in the same cycle. The application must wait one turn before it can display the seconds.

Use setTimeout() to wait one tick and then revise the seconds() method so that it takes future values from the timer component.

Test it for Parent calls an `@ViewChild()`

Use the same countdown timer tests as before.

Parent and children communicate using a service

A parent component and its children share a service whose interface enables bidirectional communication within the family .

The scope of the service instance is the parent component and its children. Components outside this component subtree have no access to the service or their communications.

This MissionService connects the MissionControlComponent to multiple AstronautComponent children.

component-interaction/src/app/mission.service.ts

      
      import { Injectable } from '@angular/core';
import { Subject } from 'rxjs';

@Injectable()
export class MissionService {

  // Observable string sources
  private missionAnnouncedSource = new Subject<string>();
  private missionConfirmedSource = new Subject<string>();

  // Observable string streams
  missionAnnounced$ = this.missionAnnouncedSource.asObservable();
  missionConfirmed$ = this.missionConfirmedSource.asObservable();

  // Service message commands
  announceMission(mission: string) {
    this.missionAnnouncedSource.next(mission);
  }

  confirmMission(astronaut: string) {
    this.missionConfirmedSource.next(astronaut);
  }
}

The MissionControlComponent both provides the instance of the service that it shares with its children (through the providers metadata array) and injects that instance into itself through its constructor:

component-interaction/src/app/missioncontrol.component.ts

      
      import { Component } from '@angular/core';

import { MissionService } from './mission.service';

@Component({
  selector: 'app-mission-control',
  template: `
    <h2>Mission Control</h2>
    <button type="button" (click)="announce()">Announce mission</button>

    <app-astronaut
      *ngFor="let astronaut of astronauts"
      [astronaut]="astronaut">
    </app-astronaut>

    <h3>History</h3>
    <ul>
      <li *ngFor="let event of history">{{event}}</li>
    </ul>
  `,
  providers: [MissionService]
})
export class MissionControlComponent {
  astronauts = ['Lovell', 'Swigert', 'Haise'];
  history: string[] = [];
  missions = ['Fly to the moon!',
              'Fly to mars!',
              'Fly to Vegas!'];
  nextMission = 0;

  constructor(private missionService: MissionService) {
    missionService.missionConfirmed$.subscribe(
      astronaut => {
        this.history.push(`${astronaut} confirmed the mission`);
      });
  }

  announce() {
    const mission = this.missions[this.nextMission++];
    this.missionService.announceMission(mission);
    this.history.push(`Mission "${mission}" announced`);
    if (this.nextMission >= this.missions.length) { this.nextMission = 0; }
  }
}

The AstronautComponent also injects the service in its constructor. Each AstronautComponent is a child of the MissionControlComponent and therefore receives its parent's service instance:

component-interaction/src/app/astronaut.component.ts

      
      import { Component, Input, OnDestroy } from '@angular/core';

import { MissionService } from './mission.service';
import { Subscription } from 'rxjs';

@Component({
  selector: 'app-astronaut',
  template: `
    <p>
      {{astronaut}}: <strong>{{mission}}</strong>
      <button
        type="button"
        (click)="confirm()"
        [disabled]="!announced || confirmed">
        Confirm
      </button>
    </p>
  `
})
export class AstronautComponent implements OnDestroy {
  @Input() astronaut = '';
  mission = '<no mission announced>';
  confirmed = false;
  announced = false;
  subscription: Subscription;

  constructor(private missionService: MissionService) {
    this.subscription = missionService.missionAnnounced$.subscribe(
      mission => {
        this.mission = mission;
        this.announced = true;
        this.confirmed = false;
    });
  }

  confirm() {
    this.confirmed = true;
    this.missionService.confirmMission(this.astronaut);
  }

  ngOnDestroy() {
    // prevent memory leak when component destroyed
    this.subscription.unsubscribe();
  }
}

Notice that this example captures the subscription and unsubscribe() when the AstronautComponent is destroyed. This is a memory-leak guard step. There is no actual risk in this application because the lifetime of a AstronautComponent is the same as the lifetime of the application itself. That would not always be true in a more complex application.

You don't add this guard to the MissionControlComponent because, as the parent, it controls the lifetime of the MissionService .

The History log demonstrates that messages travel in both directions between the parent MissionControlComponent and the AstronautComponent children, facilitated by the service:

Test it for Parent and children communicate using a service

Tests click buttons of both the parent MissionControlComponent and the AstronautComponent children and verify that the history meets expectations:

component-interaction/e2e/src/app.e2e-spec.ts

      
      // ...
it('should announce a mission', async () => {
  const missionControl = element(by.tagName('app-mission-control'));
  const announceButton = missionControl.all(by.tagName('button')).get(0);
  const history = missionControl.all(by.tagName('li'));

  await announceButton.click();

  expect(await history.count()).toBe(1);
  expect(await history.get(0).getText()).toMatch(/Mission.* announced/);
});

it('should confirm the mission by Lovell', async () => {
  await testConfirmMission(1, 'Lovell');
});

it('should confirm the mission by Haise', async () => {
  await testConfirmMission(3, 'Haise');
});

it('should confirm the mission by Swigert', async () => {
  await testConfirmMission(2, 'Swigert');
});

async function testConfirmMission(buttonIndex: number, astronaut: string) {
  const missionControl = element(by.tagName('app-mission-control'));
  const announceButton = missionControl.all(by.tagName('button')).get(0);
  const confirmButton = missionControl.all(by.tagName('button')).get(buttonIndex);
  const history = missionControl.all(by.tagName('li'));

  await announceButton.click();
  await confirmButton.click();

  expect(await history.count()).toBe(2);
  expect(await history.get(1).getText()).toBe(`${astronaut} confirmed the mission`);
}
// ...

Last reviewed on Mon Feb 28 2022

Read article

Angular - Angular components overview

Angular components overview

Prerequisites
Creating a component
Creating a component using the Angular CLI
Creating a component manually
Specifying a component's CSS selector
Defining a component's template
Declaring a component's styles
Next steps

Components are the main building block for Angular applications. Each component consists of:

An HTML template that declares what renders on the page
A TypeScript class that defines behavior
A CSS selector that defines how the component is used in a template
Optionally, CSS styles applied to the template

This topic describes how to create and configure an Angular component.

To view or download the example code used in this topic, see the live example / download example .

Prerequisites

To create a component, verify that you have met the following prerequisites:

Install the Angular CLI.
Create an Angular workspace with initial application. If you don't have a project, create one using ng new <project-name> , where <project-name> is the name of your Angular application.

Creating a component

The best way to create a component is with the Angular CLI. You can also create a component manually.

Creating a component using the Angular CLI

To create a component using the Angular CLI:

From a terminal window, navigate to the directory containing your application.
Run the ng generate component <component-name> command, where <component-name> is the name of your new component.

By default, this command creates the following:

A directory named after the component
A component file, <component-name>.component.ts
A template file, <component-name>.component.html
A CSS file, <component-name>.component.css
A testing specification file, <component-name>.component.spec.ts

Where <component-name> is the name of your component.

You can change how ng generate component creates new components. For more information, see ng generate component in the Angular CLI documentation.

Creating a component manually

Although the Angular CLI is the best way to create an Angular component, you can also create a component manually. This section describes how to create the core component file within an existing Angular project.

To create a new component manually:

Navigate to your Angular project directory.
Create a new file, <component-name>.component.ts .

At the top of the file, add the following import statement.

      
      import { Component } from '@angular/core';

After the import statement, add a @Component decorator.

      
      @Component({
})

Choose a CSS selector for the component.

      
      @Component({
  selector: 'app-component-overview',
})

For more information on choosing a selector, see Specifying a component's selector.

Define the HTML template that the component uses to display information. In most cases, this template is a separate HTML file.
```
      
      @Component({
  selector: 'app-component-overview',
  templateUrl: './component-overview.component.html',
})
    
```
For more information on defining a component's template, see Defining a component's template.

Select the styles for the component's template. In most cases, you define the styles for your component's template in a separate file.

      
      @Component({
  selector: 'app-component-overview',
  templateUrl: './component-overview.component.html',
  styleUrls: ['./component-overview.component.css']
})

Add a class statement that includes the code for the component.

      
      export class ComponentOverviewComponent {

}

Specifying a component's CSS selector

Every component requires a CSS selector . A selector instructs Angular to instantiate this component wherever it finds the corresponding tag in template HTML. For example, consider a component hello-world.component.ts that defines its selector as app-hello-world . This selector instructs Angular to instantiate this component any time the tag <app-hello-world> appears in a template.

Specify a component's selector by adding a selector statement to the @Component decorator.

      
      @Component({
  selector: 'app-component-overview',
})

Defining a component's template

A template is a block of HTML that tells Angular how to render the component in your application. Define a template for your component in one of two ways: by referencing an external file, or directly within the component.

To define a template as an external file, add a templateUrl property to the @Component decorator.

      
      @Component({
  selector: 'app-component-overview',
  templateUrl: './component-overview.component.html',
})

To define a template within the component, add a template property to the @Component decorator that contains the HTML you want to use.

      
      @Component({
  selector: 'app-component-overview',
  template: '<h1>Hello World!</h1>',
})

If you want your template to span multiple lines, use backticks ( ` ). For example:

      
      @Component({
  selector: 'app-component-overview',
  template: `
    <h1>Hello World!</h1>
    <p>This template definition spans multiple lines.</p>
  `
})

An Angular component requires a template defined using template or templateUrl . You cannot have both statements in a component.

Declaring a component's styles

Declare component styles used for its template in one of two ways: By referencing an external file, or directly within the component.

To declare the styles for a component in a separate file, add a styleUrls property to the @Component decorator.

      
      @Component({
  selector: 'app-component-overview',
  templateUrl: './component-overview.component.html',
  styleUrls: ['./component-overview.component.css']
})

To declare the styles within the component, add a styles property to the @Component decorator that contains the styles you want to use.

      
      @Component({
  selector: 'app-component-overview',
  template: '<h1>Hello World!</h1>',
  styles: ['h1 { font-weight: normal; }']
})

The styles property takes an array of strings that contain the CSS rule declarations.

Next steps

For an architectural overview of components, see Introduction to components and templates
For additional options to use when creating a component, see Component in the API Reference
For more information on styling components, see Component styles
For more information on templates, see Template syntax

Last reviewed on Mon Feb 28 2022

Read article

Angular - Component styles

Component styles

Using component styles
Component styling best practices
Authoring a component to support customization
Special selectors
:host
:host-context
(deprecated) /deep/ , >>> , and ::ng-deep
Loading component styles
Styles in component metadata
Style files in component metadata
Template inline styles
Template link tags
CSS @imports
External and global style files
Non-CSS style files

Angular applications are styled with standard CSS. That means you can apply everything you know about CSS stylesheets, selectors, rules, and media queries directly to Angular applications.

Additionally, Angular can bundle component styles with components, enabling a more modular design than regular stylesheets.

This page describes how to load and apply these component styles.

Run the live example / download example in Stackblitz and download the code from there.

Using component styles

For every Angular component you write, you can define not only an HTML template, but also the CSS styles that go with that template, specifying any selectors, rules, and media queries that you need.

One way to do this is to set the styles property in the component metadata. The styles property takes an array of strings that contain CSS code. Usually you give it one string, as in the following example:

src/app/hero-app.component.ts

      
      @Component({
  selector: 'app-root',
  template: `
    <h1>Tour of Heroes</h1>
    <app-hero-main [hero]="hero"></app-hero-main>
  `,
  styles: ['h1 { font-weight: normal; }']
})
export class HeroAppComponent {
/* . . . */
}

Component styling best practices

See View Encapsulation for information on how Angular scopes styles to specific components.

You should consider the styles of a component to be private implementation details for that component. When consuming a common component, you should not override the component's styles any more than you should access the private members of a TypeScript class. While Angular's default style encapsulation prevents component styles from affecting other components, global styles affect all components on the page. This includes ::ng-deep , which promotes a component style to a global style.

Authoring a component to support customization

As component author, you can explicitly design a component to accept customization in one of four different ways.

1. Use CSS Custom Properties (recommended)

You can define a supported customization API for your component by defining its styles with CSS Custom Properties, alternatively known as CSS Variables. Anyone using your component can consume this API by defining values for these properties, customizing the final appearance of the component on the rendered page.

While this requires defining a custom property for each customization point, it creates a clear API contract that works in all style encapsulation modes.

2. Declare global CSS with `@mixin`

While Angular's emulated style encapsulation prevents styles from escaping a component, it does not prevent global CSS from affecting the entire page. While component consumers should avoid directly overwriting the CSS internals of a component, you can offer a supported customization API via a CSS preprocessor like Sass.

For example, a component may offer one or more supported mixins to customize various aspects of the component's appearance. While this approach uses global styles in its implementation, it allows the component author to keep the mixins up to date with changes to the component's private DOM structure and CSS classes.

3. Customize with CSS `::part`

If your component uses Shadow DOM, you can apply the part attribute to specify elements in your component's template. This allows consumers of the component to author arbitrary styles targeting those specific elements with the ::part pseudo-element.

While this lets you limit the elements within your template that consumers can customize, it does not limit which CSS properties are customizable.

4. Provide a TypeScript API

You can define a TypeScript API for customizing styles, using template bindings to update CSS classes and styles. This is not recommended because the additional JavaScript cost of this style API incurs far more performance cost than CSS.

Special selectors

Component styles have a few special selectors from the world of shadow DOM style scoping (described in the CSS Scoping Module Level 1 page on the W3C site). The following sections describe these selectors.

:host

Every component is associated within an element that matches the component's selector. This element, into which the template is rendered, is called the host element . The :host pseudo-class selector may be used to create styles that target the host element itself, as opposed to targeting elements inside the host.

src/app/host-selector-example.component.ts

      
      @Component({
  selector: 'app-main',
  template: `
      <h1>It Works!</h1>
      <div>
        Start editing to see some magic happen :)
      </div>
  `
})
export class HostSelectorExampleComponent {

}

Creating the following style will target the component's host element. Any rule applied to this selector will affect the host element and all its descendants (in this case, italicizing all contained text).

src/app/hero-details.component.css

      
      :host {
  font-style: italic;
}

The :host selector only targets the host element of a component. Any styles within the :host block of a child component will not affect parent components.

Use the function form to apply host styles conditionally by including another selector inside parentheses after :host .

In this example the host's content also becomes bold when the active CSS class is applied to the host element.

src/app/hero-details.component.css

      
      :host {
  font-style: italic;
}

:host(.active) {
  font-weight: bold;
}

The :host selector can also be combined with other selectors. Add selectors behind the :host to select child elements, for example using :host h2 to target all <h2> elements inside a component's view.

You should not add selectors (other than :host-context ) in front of the :host selector to style a component based on the outer context of the component's view. Such selectors are not scoped to a component's view and will select the outer context, but it's not built-in behavior. Use :host-context selector for that purpose instead.

:host-context

Sometimes it's useful to apply styles to elements within a component's template based on some condition in an element that is an ancestor of the host element. For example, a CSS theme class could be applied to the document <body> element, and you want to change how your component looks based on that.

Use the :host-context() pseudo-class selector, which works just like the function form of :host() . The :host-context() selector looks for a CSS class in any ancestor of the component host element, up to the document root. The :host-context() selector is only useful when combined with another selector.

The following example italicizes all text inside a component, but only if some ancestor element of the host element has the CSS class active .

src/app/hero-details.component.css

      
      :host-context(.active) {
  font-style: italic;
}

NOTE :
Only the host element and its descendants will be affected, not the ancestor with the assigned active class.

(deprecated) `/deep/` , `>>>` , and `::ng-deep`

Component styles normally apply only to the HTML in the component's own template.

Applying the ::ng-deep pseudo-class to any CSS rule completely disables view-encapsulation for that rule. Any style with ::ng-deep applied becomes a global style. In order to scope the specified style to the current component and all its descendants, be sure to include the :host selector before ::ng-deep . If the ::ng-deep combinator is used without the :host pseudo-class selector, the style can bleed into other components.

The following example targets all <h3> elements, from the host element down through this component to all of its child elements in the DOM.

src/app/hero-details.component.css

      
      :host ::ng-deep h3 {
  font-style: italic;
}

The /deep/ combinator also has the aliases >>> , and ::ng-deep .

Use /deep/ , >>> , and ::ng-deep only with emulated view encapsulation. Emulated is the default and most commonly used view encapsulation. For more information, see the View Encapsulation section.

The shadow-piercing descendant combinator is deprecated and support is being removed from major browsers and tools. As such we plan to drop support in Angular (for all 3 of /deep/ , >>> , and ::ng-deep ). Until then ::ng-deep should be preferred for a broader compatibility with the tools.

Loading component styles

There are several ways to add styles to a component:

By setting styles or styleUrls metadata
Inline in the template HTML
With CSS imports

The scoping rules outlined earlier apply to each of these loading patterns.

Styles in component metadata

Add a styles array property to the @Component decorator.

Each string in the array defines some CSS for this component.

src/app/hero-app.component.ts (CSS inline)

      
      @Component({
  selector: 'app-root',
  template: `
    <h1>Tour of Heroes</h1>
    <app-hero-main [hero]="hero"></app-hero-main>
  `,
  styles: ['h1 { font-weight: normal; }']
})
export class HeroAppComponent {
/* . . . */
}

Reminder: These styles apply only to this component . They are not inherited by any components nested within the template nor by any content projected into the component.

The Angular CLI command ng generate component defines an empty styles array when you create the component with the --inline-style flag.

      
      ng generate component hero-app --inline-style

Style files in component metadata

Load styles from external CSS files by adding a styleUrls property to a component's @Component decorator:

src/app/hero-app.component.ts (CSS in file)

src/app/hero-app.component.css

      
      @Component({
  selector: 'app-root',
  template: `
    <h1>Tour of Heroes</h1>
    <app-hero-main [hero]="hero"></app-hero-main>
  `,
  styleUrls: ['./hero-app.component.css']
})
export class HeroAppComponent {
/* . . . */
}

Reminder: the styles in the style file apply only to this component . They are not inherited by any components nested within the template nor by any content projected into the component.

You can specify more than one styles file or even a combination of styles and styleUrls .

When you use the Angular CLI command ng generate component without the --inline-style flag, it creates an empty styles file for you and references that file in the component's generated styleUrls .

      
      ng generate component hero-app

Template inline styles

Embed CSS styles directly into the HTML template by putting them inside <style> tags.

src/app/hero-controls.component.ts

      
      @Component({
  selector: 'app-hero-controls',
  template: `
    <style>
      button {
        background-color: white;
        border: 1px solid #777;
      }
    </style>
    <h3>Controls</h3>
    <button type="button" (click)="activate()">Activate</button>
  `
})

Template link tags

You can also write <link> tags into the component's HTML template.

src/app/hero-team.component.ts

      
      @Component({
  selector: 'app-hero-team',
  template: `
    <!-- We must use a relative URL so that the AOT compiler can find the stylesheet -->
    <link rel="stylesheet" href="../assets/hero-team.component.css">
    <h3>Team</h3>
    <ul>
      <li *ngFor="let member of hero.team">
        {{member}}
      </li>
    </ul>`
})

When building with the CLI, be sure to include the linked style file among the assets to be copied to the server as described in the Assets configuration guide.

Once included, the CLI includes the stylesheet, whether the link tag's href URL is relative to the application root or the component file.

CSS @imports

Import CSS files into the CSS files using the standard CSS @import rule. For details, see @import on the MDN site.

In this case, the URL is relative to the CSS file into which you're importing.

src/app/hero-details.component.css (excerpt)

      
      /* The AOT compiler needs the `./` to show that this is local */
@import './hero-details-box.css';

External and global style files

When building with the CLI, you must configure the angular.json to include all external assets , including external style files.

Register global style files in the styles section which, by default, is pre-configured with the global styles.css file.

See the Styles configuration guide to learn more.

Non-CSS style files

If you're building with the CLI, you can write style files in sass, or less, and specify those files in the @Component.styleUrls metadata with the appropriate extensions ( .scss , .less ) as in the following example:

      
      @Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.scss']
})
…

The CLI build process runs the pertinent CSS preprocessor.

When generating a component file with ng generate component , the CLI emits an empty CSS styles file ( .css ) by default. Configure the CLI to default to your preferred CSS preprocessor as explained in the Workspace configuration guide.

Last reviewed on Mon Feb 28 2022

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Last reviewed on Mon Aug 22 2022

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Angular

link

Observables compared to other techniques

Observables compared to promises

Creation and subscription

Chaining

Cancellation

Error handling

Cheat sheet

Observables compared to events API

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Complex animation sequences

Prerequisites

The query() function

Animate multiple elements using query() and stagger() functions

Parallel animation using group() function

Sequential vs. parallel animations

Filter animation example

Animating the items of a reordering list

Animations and Component View Encapsulation

Animation sequence summary

More on Angular animations

Component interaction

Pass data from parent to child with input binding

Test it for Pass data from parent to child with input binding

Intercept input property changes with a setter

Test it for Intercept input property changes with a setter

Intercept input property changes with `ngOnChanges()`

Test it for Intercept input property changes with `ngOnChanges()`

Parent listens for child event

Test it for Parent listens for child event

Parent interacts with child using local variable

Test it for Parent interacts with child using local variable

Parent calls an `@ViewChild()`

Test it for Parent calls an `@ViewChild()`

Parent and children communicate using a service

Test it for Parent and children communicate using a service

Angular components overview

Prerequisites

Creating a component

Creating a component using the Angular CLI

Creating a component manually

Specifying a component's CSS selector

Defining a component's template

Declaring a component's styles

Next steps

Component styles

Using component styles

Component styling best practices

Authoring a component to support customization

1. Use CSS Custom Properties (recommended)

2. Declare global CSS with `@mixin`

3. Customize with CSS `::part`

4. Provide a TypeScript API

Special selectors

:host

:host-context

(deprecated) `/deep/` , `>>>` , and `::ng-deep`

Loading component styles

Styles in component metadata

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External and global style files

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