Resolving zone pollution link

Zone.js is a signaling mechanism that Angular uses to detect when an application state might have changed. It captures asynchronous operations like setTimeout , network requests, and event listeners. Angular schedules change detection based on signals from Zone.js

In some cases scheduled tasks or microtasks don’t make any changes in the data model, which makes running change detection unnecessary. Common examples are:

• requestAnimationFrame , setTimeout or setInterval
• Task or microtask scheduling by third-party libraries

This section covers how to identify such conditions, and how to run code outside the Angular zone to avoid unnecessary change detection calls.

Identifying unnecessary change detection calls link

You can detect unnecessary change detection calls using Angular DevTools. Often they appear as consecutive bars in the profiler’s timeline with source setTimeout , setInterval , requestAnimationFrame , or an event handler. When you have limited calls within your application of these APIs, the change detection invocation is usually caused by a third-party library.

In the image above, there is a series of change detection calls triggered by event handlers associated with an element. That’s a common challenge when using third-party, non-native Angular components, which do not alter the default behavior of NgZone .

Run tasks outside NgZone link

In such cases, you can instruct Angular to avoid calling change detection for tasks scheduled by a given piece of code using NgZone.

      
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      import { Component, NgZone, OnInit } from '@angular/core';
@Component(...)
class AppComponent implements OnInit {
  constructor(private ngZone: NgZone) {}
  ngOnInit() {
    this.ngZone.runOutsideAngular(() => setInterval(pollForUpdates), 500);
  }
}
    

The preceding snippet instructs Angular to call setInterval outside the Angular Zone and skip running change detection after pollForUpdates runs.

Third-party libraries commonly trigger unnecessary change detection cycles because they weren't authored with Zone.js in mind. Avoid these extra cycles by calling library APIs outside the Angular zone:

      
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      import { Component, NgZone, OnInit } from '@angular/core';
import * as Plotly from 'plotly.js-dist-min';

@Component(...)
class AppComponent implements OnInit {
  constructor(private ngZone: NgZone) {}
  ngOnInit() {
    this.ngZone.runOutsideAngular(() => {
      Plotly.newPlot('chart', data);
    });
  }
}
    

Running Plotly.newPlot('chart', data); within runOutsideAngular instructs the framework that it shouldn’t run change detection after the execution of tasks scheduled by the initialization logic.

For example, if Plotly.newPlot('chart', data) adds event listeners to a DOM element, Angular does not run change detection after the execution of their handlers.

Angular change detection and runtime optimization link

Change detection is the process through which Angular checks to see whether your application state has changed, and if any DOM needs to be updated. At a high level, Angular walks your components from top to bottom, looking for changes. Angular runs its change detection mechanism periodically so that changes to the data model are reflected in an application’s view. Change detection can be triggered either manually or through an asynchronous event (for example, a user interaction or an XMLHttpRequest completion).

Change detection is a highly optimized performant, but it can still cause slowdowns if the application runs it too frequently.

In this guide, you’ll learn how to control and optimize the change detection mechanism by skipping parts of your application and running change detection only when necessary.

Watch this video if you prefer to learn more about performance optimizations in a media format:

Class and style binding link

Use class and style bindings to add and remove CSS class names from an element's class attribute and to set styles dynamically.

Prerequisites link

• Property binding

Binding to a single CSS class link

To create a single class binding, type the following:

[class.sale]="onSale"

Angular adds the class when the bound expression, onSale is truthy, and it removes the class when the expression is falsy—with the exception of undefined . See styling delegation for more information.

Binding to multiple CSS classes link

To bind to multiple classes, type the following:

[class]="classExpression"

The expression can be one of:

• A space-delimited string of class names.
• An object with class names as the keys and truthy or falsy expressions as the values.
• An array of class names.

With the object format, Angular adds a class only if its associated value is truthy.

If there are multiple bindings to the same class name, Angular uses styling precedence to determine which binding to use.

The following table summarizes class binding syntax.

Binding to a single style link

To create a single style binding, use the prefix style followed by a dot and the name of the CSS style.

For example, to set the width style, type the following: [style.width]="width"

Angular sets the property to the value of the bound expression, which is usually a string. Optionally, you can add a unit extension like em or % , which requires a number type.

1. To write a style in dash-case, type the following:

         
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         <nav [style.background-color]="expression"></nav>
       

2. To write a style in camelCase, type the following:

         
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         <nav [style.backgroundColor]="expression"></nav>
       

Binding to multiple styles link

To toggle multiple styles, bind to the [style] attribute—for example, [style]="styleExpression" . The styleExpression can be one of:

• A string list of styles such as "width: 100px; height: 100px; background-color: cornflowerblue;" .
• An object with style names as the keys and style values as the values, such as {width: '100px', height: '100px', backgroundColor: 'cornflowerblue'} .

Note that binding an array to [style] is not supported.

Single and multiple-style binding example link

      
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      @Component({
  selector: 'app-nav-bar',
  template: `
<nav [style]='navStyle'>
  <a [style.text-decoration]="activeLinkStyle">Home Page</a>
  <a [style.text-decoration]="linkStyle">Login</a>
</nav>`
})
export class NavBarComponent {
  navStyle = 'font-size: 1.2rem; color: cornflowerblue;';
  linkStyle = 'underline';
  activeLinkStyle = 'overline';
  /* . . . */
}
    

If there are multiple bindings to the same style attribute, Angular uses styling precedence to determine which binding to use.

The following table summarizes style binding syntax.

Styling precedence link

A single HTML element can have its CSS class list and style values bound to multiple sources (for example, host bindings from multiple directives).

What’s next link

• Component styles
• Introduction to Angular animations

Angular CLI builders link

A number of Angular CLI commands run a complex process on your code, such as linting, building, or testing. The commands use an internal tool called Architect to run CLI builders , which apply another tool to accomplish the wanted task.

With Angular version 8, the CLI Builder API is stable and available to developers who want to customize the Angular CLI by adding or modifying commands. For example, you could supply a builder to perform an entirely new task, or to change which third-party tool is used by an existing command.

This document explains how CLI builders integrate with the workspace configuration file, and shows how you can create your own builder.

CLI builders link

The internal Architect tool delegates work to handler functions called builders . A builder handler function receives two arguments; a set of input options (a JSON object), and a context (a BuilderContext object).

The separation of concerns here is the same as with schematics, which are used for other CLI commands that touch your code (such as ng generate ).

• The options object is provided by the CLI user, while the context object is provided by the CLI Builder API
• In addition to the contextual information, the context object, which is an instance of the BuilderContext , also provides access to a scheduling method, context.scheduleTarget() . The scheduler executes the builder handler function with a given target configuration.

The builder handler function can be synchronous (return a value) or asynchronous (return a Promise), or it can watch and return multiple values (return an Observable). The return value or values must always be of type BuilderOutput . This object contains a Boolean success field and an optional error field that can contain an error message.

Angular provides some builders that are used by the CLI for commands such as ng build and ng test . Default target configurations for these and other built-in CLI builders can be found (and customized) in the "architect" section of the workspace configuration file, angular.json . Also, extend and customize Angular by creating your own builders, which you can run using the ng run CLI command.

Builder project structure link

A builder resides in a "project" folder that is similar in structure to an Angular workspace, with global configuration files at the top level, and more specific configuration in a source folder with the code files that define the behavior. For example, your myBuilder folder could contain the following files.

Publish the builder to npm (see Publishing your Library). If you publish it as @example/my-builder , install it using the following command.

      
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      npm install @example/my-builder
    

Creating a builder link

As an example, create a builder that copies a file. To create a builder, use the createBuilder() CLI Builder function, and return a Promise<BuilderOutput> object.

      
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      import { BuilderContext, BuilderOutput, createBuilder } from '@angular-devkit/architect';
import { JsonObject } from '@angular-devkit/core';

interface Options extends JsonObject {
  source: string;
  destination: string;
}

export default createBuilder(copyFileBuilder);

async function copyFileBuilder(
  options: Options,
  context: BuilderContext,
): Promise<BuilderOutput> {
}
    

Now let's add some logic to it. The following code retrieves the source and destination file paths from user options and copies the file from the source to the destination (using the Promise version of the built-in NodeJS copyFile() function). If the copy operation fails, it returns an error with a message about the underlying problem.

      
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      import { BuilderContext, BuilderOutput, createBuilder } from '@angular-devkit/architect';
import { JsonObject } from '@angular-devkit/core';
import { promises as fs } from 'fs';

interface Options extends JsonObject {
  source: string;
  destination: string;
}

export default createBuilder(copyFileBuilder);

async function copyFileBuilder(
  options: Options,
  context: BuilderContext,
): Promise<BuilderOutput> {
  try {
    await fs.copyFile(options.source, options.destination);
  } catch (err) {
    return {
      success: false,
      error: err.message,
    };
  }

  return { success: true };
}
    

Handling output link

By default, copyFile() does not print anything to the process standard output or error. If an error occurs, it might be difficult to understand exactly what the builder was trying to do when the problem occurred. Add some additional context by logging additional information using the Logger API. This also lets the builder itself be executed in a separate process, even if the standard output and error are deactivated (as in an Electron app).

You can retrieve a Logger instance from the context.

      
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      import { BuilderContext, BuilderOutput, createBuilder } from '@angular-devkit/architect';
import { JsonObject } from '@angular-devkit/core';
import { promises as fs } from 'fs';

interface Options extends JsonObject {
  source: string;
  destination: string;
}

export default createBuilder(copyFileBuilder);

async function copyFileBuilder(
  options: Options,
  context: BuilderContext,
): Promise<BuilderOutput> {
  try {
    await fs.copyFile(options.source, options.destination);
  } catch (err) {
    context.logger.error('Failed to copy file.');
    return {
      success: false,
      error: err.message,
    };
  }

  return { success: true };
}
    

Progress and status reporting link

The CLI Builder API includes progress and status reporting tools, which can provide hints for certain functions and interfaces.

To report progress, use the context.reportProgress() method, which takes a current value, (optional) total, and status string as arguments. The total can be any number; for example, if you know how many files you have to process, the total could be the number of files, and current should be the number processed so far. The status string is unmodified unless you pass in a new string value.

You can see an example of how the tslint builder reports progress.

In our example, the copy operation either finishes or is still executing, so there's no need for a progress report, but you can report status so that a parent builder that called our builder would know what's going on. Use the context.reportStatus() method to generate a status string of any length.

Pass an empty string to remove the status.

      
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      import { BuilderContext, BuilderOutput, createBuilder } from '@angular-devkit/architect';
import { JsonObject } from '@angular-devkit/core';
import { promises as fs } from 'fs';

interface Options extends JsonObject {
  source: string;
  destination: string;
}

export default createBuilder(copyFileBuilder);

async function copyFileBuilder(
  options: Options,
  context: BuilderContext,
): Promise<BuilderOutput> {
  context.reportStatus(`Copying ${options.source} to ${options.destination}.`);
  try {
    await fs.copyFile(options.source, options.destination);
  } catch (err) {
    context.logger.error('Failed to copy file.');
    return {
      success: false,
      error: err.message,
    };
  }

  context.reportStatus('Done.');
  return { success: true };
}
    

Builder input link

You can invoke a builder indirectly through a CLI command, or directly with the Angular CLI ng run command. In either case, you must provide required inputs, but can let other inputs default to values that are pre-configured for a specific target , provide a pre-defined, named override configuration, and provide further override option values on the command line.

Input validation link

You define builder inputs in a JSON schema associated with that builder. The Architect tool collects the resolved input values into an options object, and validates their types against the schema before passing them to the builder function. (The Schematics library does the same kind of validation of user input.)

For our example builder, you expect the options value to be a JsonObject with two keys: A source and a destination , each of which are a string.

You can provide the following schema for type validation of these values.

      
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      {
  "$schema": "http://json-schema.org/schema",
  "type": "object",
  "properties": {
    "source": {
      "type": "string"
    },
    "destination": {
      "type": "string"
    }
  }
}
    

To link our builder implementation with its schema and name, you need to create a builder definition file, which you can point to in package.json .

Create a file named builders.json that looks like this:

      
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      {
  "builders": {
    "copy": {
      "implementation": "./dist/my-builder.js",
      "schema": "./src/schema.json",
      "description": "Copies a file."
    }
  }
}
    

In the package.json file, add a builders key that tells the Architect tool where to find our builder definition file.

      
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      {
  "name": "@example/copy-file",
  "version": "1.0.0",
  "description": "Builder for copying files",
  "builders": "builders.json",
  "dependencies": {
    "@angular-devkit/architect": "~0.1200.0",
    "@angular-devkit/core": "^12.0.0"
  }
}
    

The official name of our builder is now @example/copy-file:copy . The first part of this is the package name (resolved using node resolution), and the second part is the builder name (resolved using the builders.json file).

Using one of our options is very straightforward. You did this in the previous section when you accessed options.source and options.destination .

      
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      context.reportStatus(`Copying ${options.source} to ${options.destination}.`);
try {
  await fs.copyFile(options.source, options.destination);
} catch (err) {
  context.logger.error('Failed to copy file.');
  return {
    success: false,
    error: err.message,
  };
}

context.reportStatus('Done.');
return { success: true };
    

Target configuration link

A builder must have a defined target that associates it with a specific input configuration and project.

Targets are defined in the angular.json CLI configuration file. A target specifies the builder to use, its default options configuration, and named alternative configurations. The Architect tool uses the target definition to resolve input options for a given run.

The angular.json file has a section for each project, and the "architect" section of each project configures targets for builders used by CLI commands such as 'build', 'test', and 'lint'. By default, for example, the build command runs the builder @angular-devkit/build-angular:browser to perform the build task, and passes in default option values as specified for the build target in angular.json .

      
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      {
  "myApp": {
    …
    "architect": {
      "build": {
        "builder": "@angular-devkit/build-angular:browser",
        "options": {
          "outputPath": "dist/myApp",
          "index": "src/index.html",
          …
        },
        "configurations": {
          "production": {
            "fileReplacements": [
              {
                "replace": "src/environments/environment.ts",
                "with": "src/environments/environment.prod.ts"
              }
            ],
            "optimization": true,
            "outputHashing": "all",
            …
          }
        }
      },
      …
    

The command passes the builder the set of default options specified in the "options" section. If you pass the --configuration=production flag, it uses the override values specified in the production alternative configuration. Specify further option overrides individually on the command line. You might also add more alternative configurations to the build target, to define other environments such as stage or qa .

Target strings link

The generic ng run CLI command takes as its first argument a target string of the following form.

      
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      project:target[:configuration]
    

If your builder calls another builder, it might need to read a passed target string. Parse this string into an object by using the targetFromTargetString() utility function from @angular-devkit/architect .

Schedule and run link

Architect runs builders asynchronously. To invoke a builder, you schedule a task to be run when all configuration resolution is complete.

The builder function is not executed until the scheduler returns a BuilderRun control object. The CLI typically schedules tasks by calling the context.scheduleTarget() function, and then resolves input options using the target definition in the angular.json file.

Architect resolves input options for a given target by taking the default options object, then overwriting values from the configuration used (if any), then further overwriting values from the overrides object passed to context.scheduleTarget() . For the Angular CLI, the overrides object is built from command line arguments.

Architect validates the resulting options values against the schema of the builder. If inputs are valid, Architect creates the context and executes the builder.

For more information see Workspace Configuration.

Default architect configuration link

Let's create a simple angular.json file that puts target configurations into context.

You can publish the builder to npm (see Publishing your Library), and install it using the following command:

      
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      npm install @example/copy-file
    

If you create a new project with ng new builder-test , the generated angular.json file looks something like this, with only default builder configurations.

      
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      {
  // …
  "projects": {
    // …
    "builder-test": {
      // …
      "architect": {
        // …
        "build": {
          "builder": "@angular-devkit/build-angular:browser",
          "options": {
            // … more options…
            "outputPath": "dist/builder-test",
            "index": "src/index.html",
            "main": "src/main.ts",
            "polyfills": "src/polyfills.ts",
            "tsConfig": "src/tsconfig.app.json"
          },
          "configurations": {
            "production": {
              // … more options…
              "optimization": true,
              "aot": true,
              "buildOptimizer": true
            }
          }
        }
      }
    }
  }
  // …
}
    

Adding a target link

Add a new target that will run our builder to copy a file. This target tells the builder to copy the package.json file.

You need to update the angular.json file to add a target for this builder to the "architect" section of our new project.

• We'll add a new target section to the "architect" object for our project

• The target named "copy-package" uses our builder, which you published to @example/copy-file . (See Publishing your Library.)

• The options object provides default values for the two inputs that you defined; source , which is the existing file you are copying, and destination , the path you want to copy to

• The configurations key is optional, we'll leave it out for now

      
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      {
  "projects": {
    "builder-test": {
      "architect": {
        "copy-package": {
          "builder": "@example/copy-file:copy",
          "options": {
            "source": "package.json",
            "destination": "package-copy.json"
          }
        },
        "build": {
          "builder": "@angular-devkit/build-angular:browser",
          "options": {
            "outputPath": "dist/builder-test",
            "index": "src/index.html",
            "main": "src/main.ts",
            "polyfills": "src/polyfills.ts",
            "tsConfig": "src/tsconfig.app.json"
          },
          "configurations": {
            "production": {
              "fileReplacements": [
                {
                  "replace": "src/environments/environment.ts",
                  "with": "src/environments/environment.prod.ts"
                }
              ],
              "optimization": true,
              "aot": true,
              "buildOptimizer": true
            }
          }
        }
      }
    }
  }
}
    

Running the builder link

To run our builder with the new target's default configuration, use the following CLI command.

      
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      ng run builder-test:copy-package
    

This copies the package.json file to package-copy.json .

Use command-line arguments to override the configured defaults. For example, to run with a different destination value, use the following CLI command.

      
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      ng run builder-test:copy-package --destination=package-other.json
    

This copies the file to package-other.json instead of package-copy.json . Because you did not override the source option, it will copy from the package.json file (the default value provided for the target).

Testing a builder link

Use integration testing for your builder, so that you can use the Architect scheduler to create a context, as in this example.

• In the builder source directory, you have created a new test file my-builder.spec.ts . The code creates new instances of JsonSchemaRegistry (for schema validation), TestingArchitectHost (an in-memory implementation of ArchitectHost ), and Architect .

• We've added a builders.json file next to the builder's package.json file, and modified the package file to point to it.

Here's an example of a test that runs the copy file builder. The test uses the builder to copy the package.json file and validates that the copied file's contents are the same as the source.

      
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      import { Architect } from '@angular-devkit/architect';
import { TestingArchitectHost } from '@angular-devkit/architect/testing';
import { schema } from '@angular-devkit/core';
import { promises as fs } from 'fs';

describe('Copy File Builder', () => {
  let architect: Architect;
  let architectHost: TestingArchitectHost;

  beforeEach(async () => {
    const registry = new schema.CoreSchemaRegistry();
    registry.addPostTransform(schema.transforms.addUndefinedDefaults);

    // TestingArchitectHost() takes workspace and current directories.
    // Since we don't use those, both are the same in this case.
    architectHost = new TestingArchitectHost(__dirname, __dirname);
    architect = new Architect(architectHost, registry);

    // This will either take a Node package name, or a path to the directory
    // for the package.json file.
    await architectHost.addBuilderFromPackage('..');
  });

  it('can copy files', async () => {
    // A "run" can have multiple outputs, and contains progress information.
    const run = await architect.scheduleBuilder('@example/copy-file:copy', {
      source: 'package.json',
      destination: 'package-copy.json',
    });

    // The "result" member (of type BuilderOutput) is the next output.
    const output = await run.result;

    // Stop the builder from running. This stops Architect from keeping
    // the builder-associated states in memory, since builders keep waiting
    // to be scheduled.
    await run.stop();

    // Expect that the copied file is the same as its source.
    const sourceContent = await fs.readFile('package.json', 'utf8');
    const destinationContent = await fs.readFile('package-copy.json', 'utf8');
    expect(destinationContent).toBe(sourceContent);
  });
});
    

Watch mode link

Architect expects builders to run once (by default) and return. This behavior is not entirely compatible with a builder that watches for changes (like Webpack, for example). Architect can support watch mode, but there are some things to look out for.

• To be used with watch mode, a builder handler function should return an Observable. Architect subscribes to the Observable until it completes and might reuse it if the builder is scheduled again with the same arguments.

• The builder should always emit a BuilderOutput object after each execution. Once it's been executed, it can enter a watch mode, to be triggered by an external event. If an event triggers it to restart, the builder should execute the context.reportRunning() function to tell Architect that it is running again. This prevents Architect from stopping the builder if another run is scheduled.

When your builder calls BuilderRun.stop() to exit watch mode, Architect unsubscribes from the builder's Observable and calls the builder's teardown logic to clean up. (This behavior also allows for long-running builds to be stopped and cleaned up.)

In general, if your builder is watching an external event, you should separate your run into three phases.

Summary link

The CLI Builder API provides a new way of changing the behavior of the Angular CLI by using builders to execute custom logic.

• Builders can be synchronous or asynchronous, execute once or watch for external events, and can schedule other builders or targets

• Builders have option defaults specified in the angular.json configuration file, which can be overwritten by an alternate configuration for the target, and further overwritten by command line flags

• We recommend that you use integration tests to test Architect builders. Use unit tests to validate the logic that the builder executes.

• If your builder returns an Observable, it should clean up in the teardown logic of that Observable