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NgZone

A zone is an execution context that persists across async tasks. You can think of it as thread-local storage for JavaScript VMs. This guide describes how to use Angular's NgZone to automatically detect changes in the component to update HTML.

Fundamentals of change detection

To understand the benefits of NgZone, it is important to have a clear grasp of what change detection is and how it works.

Displaying and updating data in Angular

In Angular, you can display data by binding controls in an HTML template to the properties of an Angular component.

In addition, you can bind DOM events to a method of an Angular component. In such methods, you can also update a property of the Angular component, which updates the corresponding data displayed in the template.

In both of the above examples, the component's code updates only the property of the component. However, the HTML is also updated automatically. This guide describes how and when Angular renders the HTML based on the data from the Angular component.

Detecting changes with plain JavaScript

To clarify how changes are detected and values updated, consider the following code written in plain JavaScript.

<html>
  <div id="dataDiv"></div>
  <button id="btn">updateData</button>
  <canvas id="canvas"></canvas>
  <script>
    let value = 'initialValue';
    // initial rendering
    detectChange();

    function renderHTML() {
      document.getElementById('dataDiv').innerText = value;
    }

    function detectChange() {
      const currentValue = document.getElementById('dataDiv').innerText;
      if (currentValue !== value) {
        renderHTML();
      }
    }

    // Example 1: update data inside button click event handler
    document.getElementById('btn').addEventListener('click', () => {
      // update value
      value = 'button update value';
      // call detectChange manually
      detectChange();
    });

    // Example 2: HTTP Request
    const xhr = new XMLHttpRequest();
    xhr.addEventListener('load', function() {
      // get response from server
      value = this.responseText;
      // call detectChange manually
      detectChange();
    });
    xhr.open('GET', serverUrl);
    xhr.send();

    // Example 3: setTimeout
    setTimeout(() => {
      // update value inside setTimeout callback
      value = 'timeout update value';
      // call detectChange manually
      detectChange();
    }, 100);

    // Example 4: Promise.then
    Promise.resolve('promise resolved a value').then(v => {
      // update value inside Promise thenCallback
      value = v;
      // call detectChange manually
      detectChange();
    }, 100);

    // Example 5: some other asynchronous APIs
    document.getElementById('canvas').toBlob(blob => {
      // update value when blob data is created from the canvas
      value = `value updated by canvas, size is ${blob.size}`;
      // call detectChange manually
      detectChange();
    });
  </script>
</html>

After you update the data, you need to call detectChange() manually to check whether the data changed. If the data changed, you render the HTML to reflect the updated data.

In Angular, this step is unnecessary. Whenever you update the data, your HTML is updated automatically.

When apps update HTML

To understand how change detection works, first consider when the application needs to update the HTML. Typically, updates occur for one of the following reasons:

  1. Component initialization. For example, when bootstrapping an Angular application, Angular loads the bootstrap component and triggers the ApplicationRef.tick() to call change detection and View Rendering.

  2. Event listener. The DOM event listener can update the data in an Angular component and also trigger change detection, as in the following example.

  1. HTTP Data Request. You can also get data from a server through an HTTP request. For example:
@Component({
  selector: 'app-root',
  template: '<div>{{data}}</div>';
})
export class AppComponent implements OnInit {
  data = 'initial value';
  serverUrl = 'SERVER_URL';
  constructor(private httpClient: HttpClient) {}

  ngOnInit() {
    this.httpClient.get(this.serverUrl).subscribe(response => {
      // user does not need to trigger change detection manually
      this.data = response.data;
    });
  }
}
  1. MacroTasks, such as setTimeout() or setInterval(). You can also update the data in the callback function of a macroTask such as setTimeout(). For example:
@Component({
  selector: 'app-root',
  template: '<div>{{data}}</div>';
})
export class AppComponent implements OnInit {
  data = 'initial value';

  ngOnInit() {
    setTimeout(() => {
      // user does not need to trigger change detection manually
      this.data = 'value updated';
    });
  }
}
  1. MicroTasks, such as Promise.then(). Other asynchronous APIs return a Promise object (such as fetch), so the then() callback function can also update the data. For example:
@Component({
  selector: 'app-root',
  template: '<div>{{data}}</div>';
})
export class AppComponent implements OnInit {
  data = 'initial value';

  ngOnInit() {
    Promise.resolve(1).then(v => {
      // user does not need to trigger change detection manually
      this.data = v;
    });
  }
}
  1. Other async operations. In addition to addEventListener(), setTimeout() and Promise.then(), there are other operations that can update the data asynchronously. Some examples include WebSocket.onmessage() and Canvas.toBlob().

The preceding list contains most common scenarios in which the application might change the data. Angular runs change detection whenever it detects that data could have changed. The result of change detection is that the DOM is updated with new data. Angular detects the changes in different ways. For component initialization, Angular calls change detection explicitly. For asynchronous operations, Angular uses a zone to detect changes in places where the data could have possibly mutated and it runs change detection automatically.

Zones and execution contexts

A zone provides an execution context that persists across async tasks. Execution Context is an abstract concept that holds information about the environment within the current code being executed. Consider the following example:

const callback = function() {
  console.log('setTimeout callback context is', this);
}

const ctx1 = { name: 'ctx1' };
const ctx2 = { name: 'ctx2' };

const func = function() {
  console.log('caller context is', this);
  setTimeout(callback);
}

func.apply(ctx1);
func.apply(ctx2);

The value of this in the callback of setTimeout() might differ depending on when setTimeout() is called. Thus, you can lose the context in asynchronous operations.

A zone provides a new zone context other than this, the zone context that persists across asynchronous operations. In the following example, the new zone context is called zoneThis.

zone.run(() => {
  // now you are in a zone
  expect(zoneThis).toBe(zone);
  setTimeout(function() {
    // the zoneThis context will be the same zone
    // when the setTimeout is scheduled
    expect(zoneThis).toBe(zone);
  });
});

This new context, zoneThis, can be retrieved from the setTimeout() callback function, and this context is the same when the setTimeout() is scheduled. To get the context, you can call Zone.current.

Zones and async lifecycle hooks

Zone.js can create contexts that persist across asynchronous operations as well as provide lifecycle hooks for asynchronous operations.

const zone = Zone.current.fork({
  name: 'zone',
  onScheduleTask: function(delegate, curr, target, task) {
    console.log('new task is scheduled:', task.type, task.source);
    return delegate.scheduleTask(target, task);
  },
  onInvokeTask: function(delegate, curr, target, task, applyThis, applyArgs) {
    console.log('task will be invoked:', task.type, task.source);
    return delegate.invokeTask(target, task, applyThis, applyArgs);
  },
  onHasTask: function(delegate, curr, target, hasTaskState) {
    console.log('task state changed in the zone:', hasTaskState);
    return delegate.hasTask(target, hasTaskState);
  },
  onInvoke: function(delegate, curr, target, callback, applyThis, applyArgs) {
    console.log('the callback will be invoked:', callback);
    return delegate.invoke(target, callback, applyThis, applyArgs);
  }
});
zone.run(() => {
  setTimeout(() => {
    console.log('timeout callback is invoked.');
  });
});

The above example creates a zone with several hooks.

The onXXXTask hooks trigger when the status of the task changes. The concept of a Zone Task is very similar to the JavaScript VM Task concept:

  • macroTask: such as setTimeout()
  • microTask: such as Promise.then()
  • eventTask: such as element.addEventListener()

These hooks trigger under the following circumstances:

  • onScheduleTask: triggers when a new asynchronous task is scheduled, such as when you call setTimeout().
  • onInvokeTask: triggers when an asynchronous task is about to execute, such as when the callback of setTimeout() is about to execute.
  • onHasTask: triggers when the status of one kind of task inside a zone changes from stable to unstable or from unstable to stable. A status of "stable" means there are no tasks inside the zone, while "unstable" means a new task is scheduled in the zone.
  • onInvoke: triggers when a synchronous function is going to execute in the zone.

With these hooks, Zone can monitor the status of all synchronous and asynchronous operations inside a zone.

The above example returns the following output:

the callback will be invoked: () => {
  setTimeout(() => {
    console.log('timeout callback is invoked.');
  });
}
new task is scheduled: macroTask setTimeout
task state changed in the zone: { microTask: false,
  macroTask: true,
  eventTask: false,
  change: 'macroTask' }
task will be invoked macroTask: setTimeout
timeout callback is invoked.
task state changed in the zone: { microTask: false,
  macroTask: false,
  eventTask: false,
  change: 'macroTask' }

All of the functions of Zone are provided by a library called Zone.js. This library implements those features by intercepting asynchronous APIs through monkey patching. Monkey patching is a technique to add or modify the default behavior of a function at runtime without changing the source code.

NgZone

While Zone.js can monitor all the states of synchronous and asynchronous operations, Angular additionally provides a service called NgZone. This service creates a zone named angular to automatically trigger change detection when the following conditions are satisfied:

  1. When a sync or async function is executed.
  2. When there is no microTask scheduled.

NgZone run() and runOutsideOfAngular()

Zone handles most asynchronous APIs such as setTimeout(), Promise.then(), and addEventListener(). For the full list, see the Zone Module document. Therefore in those asynchronous APIs, you don't need to trigger change detection manually.

There are still some third party APIs that Zone does not handle. In those cases, the NgZone service provides a run() method that allows you to execute a function inside the Angular zone. This function, and all asynchronous operations in that function, trigger change detection automatically at the correct time.

export class AppComponent implements OnInit {
  constructor(private ngZone: NgZone) {}
  ngOnInit() {
    // New async API is not handled by Zone, so you need to
    // use ngZone.run() to make the asynchronous operation in the Angular zone
    // and trigger change detection automatically.
    this.ngZone.run(() => {
      someNewAsyncAPI(() => {
        // update the data of the component
      });
    });
  }
}

By default, all asynchronous operations are inside the Angular zone, which triggers change detection automatically. Another common case is when you don't want to trigger change detection. In that situation, you can use another NgZone method: runOutsideAngular().

export class AppComponent implements OnInit {
  constructor(private ngZone: NgZone) {}
  ngOnInit() {
    // You know no data will be updated,
    // so you don't want to trigger change detection in this
    // specified operation. Instead, call ngZone.runOutsideAngular()
    this.ngZone.runOutsideAngular(() => {
      setTimeout(() => {
        // update component data
        // but don't trigger change detection.
      });
    });
  }
}

Setting up Zone.js

To make Zone.js available in Angular, you need to import the zone.js package. If you are using the Angular CLI, this step is done automatically, and you will see the following line in the src/polyfills.ts:

/***************************************************************************************************
 * Zone JS is required by default for Angular itself.
 */
import 'zone.js';  // Included with Angular CLI.

Before importing the zone.js package, you can set the following configurations:

  • You can disable some asynchronous API monkey patching for better performance. For example, you can disable the requestAnimationFrame() monkey patch, so the callback of requestAnimationFrame() will not trigger change detection. This is useful if, in your application, the callback of the requestAnimationFrame() will not update any data.
  • You can specify that certain DOM events do not run inside the Angular zone; for example, to prevent a mousemove or scroll event to trigger change detection.

There are several other settings you can change. To make these changes, you need to create a zone-flags.ts file, such as the following.

// disable patching requestAnimationFrame
(window as any).__Zone_disable_requestAnimationFrame = true;

// disable patching specified eventNames
(window as any).__zone_symbol__UNPATCHED_EVENTS = ['scroll', 'mousemove'];

Next, import zone-flags before you import zone.js in the polyfills.ts:

/***************************************************************************************************
 * Zone JS is required by default for Angular.
 */
import `./zone-flags`;
import 'zone.js';  // Included with Angular CLI.

For more information about what you can configure, see the Zone.js documentation.

NoopZone

Zone helps Angular know when to trigger change detection and let the developers focus on the application development. By default, Zone is loaded and works without additional configuration. However, you don't necessarily have to use Zone to make Angular work. Instead, you can opt to trigger change detection on your own.

Disabling Zone

If you disable Zone, you will need to trigger all change detection at the correct timing yourself, which requires comprehensive knowledge of change detection.

To remove Zone.js, make the following changes.

  1. Remove the zone.js import from polyfills.ts:
/***************************************************************************************************
 * Zone JS is required by default for Angular itself.
 */
// import 'zone.js';  // Included with Angular CLI.
  1. Bootstrap Angular with the noop zone in src/main.ts:
platformBrowserDynamic().bootstrapModule(AppModule, { ngZone: 'noop' })
  .catch(err => console.error(err));