Hooking into the component lifecycle

A component instance has a lifecycle that starts when Angular instantiates the component class and renders the component view along with its child views. The lifecycle continues with change detection, as Angular checks to see when data-bound properties change, and updates both the view and the component instance as needed. The lifecycle ends when Angular destroys the component instance and removes its rendered template from the DOM. Directives have a similar lifecycle, as Angular creates, updates, and destroys instances in the course of execution.

Your application can use lifecycle hook methods to tap into key events in the lifecycle of a component or directive in order to initialize new instances, initiate change detection when needed, respond to updates during change detection, and clean up before deletion of instances.

Prerequisites

Before working with lifecycle hooks, you should have a basic understanding of the following:

• TypeScript programming.
• Angular app-design fundamentals, as described in Angular Concepts.

Responding to lifecycle events

You can respond to events in the lifecycle of a component or directive by implementing one or more of the lifecycle hook interfaces in the Angular core library. The hooks give you the opportunity to act on a component or directive instance at the appropriate moment, as Angular creates, updates, or destroys that instance.

Each interface defines the prototype for a single hook method, whose name is the interface name prefixed with ng. For example, the OnInit interface has a hook method named ngOnInit(). If you implement this method in your component or directive class, Angular calls it shortly after checking the input properties for that component or directive for the first time.

export class PeekABoo implements OnInit {
  constructor(private logger: LoggerService) { }

  // implement OnInit's `ngOnInit` method
  ngOnInit() { this.logIt(`OnInit`); }

  logIt(msg: string) {
    this.logger.log(`#${nextId++} ${msg}`);
  }
}

You don't have to implement all (or any) of the lifecycle hooks, just the ones you need.

Lifecycle event sequence

After your application instantiates a component or directive by calling its constructor, Angular calls the hook methods you have implemented at the appropriate point in the lifecycle of that instance.

Angular executes hook methods in the following sequence. You can use them to perform the following kinds of operations.

Hook method	Purpose	Timing
ngOnChanges()	Respond when Angular sets or resets data-bound input properties. The method receives a SimpleChanges object of current and previous property values. Note that this happens very frequently, so any operation you perform here impacts performance significantly. See details in Using change detection hooks in this document.	Called before ngOnInit() and whenever one or more data-bound input properties change.
ngOnInit()	Initialize the directive or component after Angular first displays the data-bound properties and sets the directive or component's input properties. See details in Initializing a component or directive in this document.	Called once, after the first ngOnChanges().
ngDoCheck()	Detect and act upon changes that Angular can't or won't detect on its own. See details and example in Defining custom change detection in this document.	Called immediately after ngOnChanges() on every change detection run, and immediately after ngOnInit() on the first run.
ngAfterContentInit()	Respond after Angular projects external content into the component's view, or into the view that a directive is in. See details and example in Responding to changes in content in this document.	Called once after the first ngDoCheck().
ngAfterContentChecked()	Respond after Angular checks the content projected into the directive or component. See details and example in Responding to projected content changes in this document.	Called after ngAfterContentInit() and every subsequent ngDoCheck().
ngAfterViewInit()	Respond after Angular initializes the component's views and child views, or the view that contains the directive. See details and example in Responding to view changes in this document.	Called once after the first ngAfterContentChecked().
ngAfterViewChecked()	Respond after Angular checks the component's views and child views, or the view that contains the directive.	Called after the ngAfterViewInit() and every subsequent ngAfterContentChecked().
ngOnDestroy()	Cleanup just before Angular destroys the directive or component. Unsubscribe Observables and detach event handlers to avoid memory leaks. See details in Cleaning up on instance destruction in this document.	Called immediately before Angular destroys the directive or component.

Lifecycle example set

The live example demonstrates the use of lifecycle hooks through a series of exercises presented as components under the control of the root AppComponent. In each case a parent component serves as a test rig for a child component that illustrates one or more of the lifecycle hook methods.

The following table lists the exercises with brief descriptions. The sample code is also used to illustrate specific tasks in the following sections.

Component	Description
Peek-a-boo	Demonstrates every lifecycle hook. Each hook method writes to the on-screen log.
Spy	Shows how you can use lifecycle hooks with a custom directive. The SpyDirective implements the ngOnInit() and ngOnDestroy() hooks, and uses them to watch and report when an element goes in or out of the current view.
OnChanges	Demonstrates how Angular calls the ngOnChanges() hook every time one of the component input properties changes, and shows how to interpret the changes object passed to the hook method.
DoCheck	Implements the ngDoCheck() method with custom change detection. Watch the hook post changes to a log to see how often Angular calls this hook.
AfterView	Shows what Angular means by a view. Demonstrates the ngAfterViewInit() and ngAfterViewChecked() hooks.
AfterContent	Shows how to project external content into a component and how to distinguish projected content from a component's view children. Demonstrates the ngAfterContentInit() and ngAfterContentChecked() hooks.
Counter	Demonstrates a combination of a component and a directive, each with its own hooks.

Initializing a component or directive

Use the ngOnInit() method to perform the following initialization tasks.

• Perform complex initializations outside of the constructor. Components should be cheap and safe to construct. You should not, for example, fetch data in a component constructor. You shouldn't worry that a new component will try to contact a remote server when created under test or before you decide to display it.

  An ngOnInit() is a good place for a component to fetch its initial data. For an example, see the Tour of Heroes tutorial.

  In Flaw: Constructor does Real Work, Misko Hevery, Angular team lead, explains why you should avoid complex constructor logic.

• Set up the component after Angular sets the input properties. Constructors should do no more than set the initial local variables to simple values.

  Keep in mind that a directive's data-bound input properties are not set until after construction. If you need to initialize the directive based on those properties, set them when ngOnInit() runs.

  The ngOnChanges() method is your first opportunity to access those properties. Angular calls ngOnChanges() before ngOnInit(), but also many times after that. It only calls ngOnInit() once.

Cleaning up on instance destruction

Put cleanup logic in ngOnDestroy(), the logic that must run before Angular destroys the directive.

This is the place to free resources that won't be garbage-collected automatically. You risk memory leaks if you neglect to do so.

• Unsubscribe from Observables and DOM events.
• Stop interval timers.
• Unregister all callbacks that the directive registered with global or application services.

The ngOnDestroy() method is also the time to notify another part of the application that the component is going away.

General examples

The following examples demonstrate the call sequence and relative frequency of the various lifecycle events, and how the hooks can be used separately or together for components and directives.

Sequence and frequency of all lifecycle events

To show how Angular calls the hooks in the expected order, the PeekABooComponent demonstrates all of the hooks in one component.

In practice you would rarely, if ever, implement all of the interfaces the way this demo does.

The following snapshot reflects the state of the log after the user clicked the Create... button and then the Destroy... button.

The sequence of log messages follows the prescribed hook calling order: OnChanges, OnInit, DoCheck (3x), AfterContentInit, AfterContentChecked (3x), AfterViewInit, AfterViewChecked (3x), and OnDestroy.

Notice that the log confirms that input properties (the name property in this case) have no assigned values at construction. The input properties are available to the onInit() method for further initialization.

Had the user clicked the Update Hero button, the log would show another OnChanges and two more triplets of DoCheck, AfterContentChecked and AfterViewChecked. Notice that these three hooks fire often, so it is important to keep their logic as lean as possible.

Use directives to watch the DOM

The Spy example demonstrates how you can use hook method for directives as well as components. The SpyDirective implements two hooks, ngOnInit() and ngOnDestroy(), in order to discover when a watched element is in the current view.

This template applies the SpyDirective to a <div> in the ngFor hero repeater managed by the parent SpyComponent.

The example does not perform any initialization or clean-up. It just tracks the appearance and disappearance of an element in the view by recording when the directive itself is instantiated and destroyed.

A spy directive like this can provide insight into a DOM object that you cannot change directly. You can't touch the implementation of a native <div>, or modify a third party component. You can, however watch these elements with a directive.

The directive defines ngOnInit() and ngOnDestroy() hooks that log messages to the parent via an injected LoggerService.

// Spy on any element to which it is applied.
// Usage: <div mySpy>...</div>
@Directive({selector: '[mySpy]'})
export class SpyDirective implements OnInit, OnDestroy {

  constructor(private logger: LoggerService) { }

  ngOnInit()    { this.logIt(`onInit`); }

  ngOnDestroy() { this.logIt(`onDestroy`); }

  private logIt(msg: string) {
    this.logger.log(`Spy #${nextId++} ${msg}`);
  }
}

You can apply the spy to any native or component element, and see that it is initialized and destroyed at the same time as that element. Here it is attached to the repeated hero <div>:

<div *ngFor="let hero of heroes" mySpy class="heroes">
  {{hero}}
</div>

Each spy's creation and destruction marks the appearance and disappearance of the attached hero <div> with an entry in the Hook Log as seen here:

Adding a hero results in a new hero <div>. The spy's ngOnInit() logs that event.

The Reset button clears the heroes list. Angular removes all hero <div> elements from the DOM and destroys their spy directives at the same time. The spy's ngOnDestroy() method reports its last moments.

Use component and directive hooks together

In this example, a CounterComponent uses the ngOnChanges() method to log a change every time the parent component increments its input counter property.

This example applies the SpyDirective from the previous example to the CounterComponent log, in order to watch the creation and destruction of log entries.

Using change detection hooks

Angular calls the ngOnChanges() method of a component or directive whenever it detects changes to the input properties. The onChanges example demonstrates this by monitoring the OnChanges() hook.

ngOnChanges(changes: SimpleChanges) {
  for (let propName in changes) {
    let chng = changes[propName];
    let cur  = JSON.stringify(chng.currentValue);
    let prev = JSON.stringify(chng.previousValue);
    this.changeLog.push(`${propName}: currentValue = ${cur}, previousValue = ${prev}`);
  }
}

The ngOnChanges() method takes an object that maps each changed property name to a SimpleChange object holding the current and previous property values. This hook iterates over the changed properties and logs them.

The example component, OnChangesComponent, has two input properties: hero and power.

@Input() hero: Hero;
@Input() power: string;

The host OnChangesParentComponent binds to them as follows.

<on-changes [hero]="hero" [power]="power"></on-changes>

Here's the sample in action as the user makes changes.

The log entries appear as the string value of the power property changes. Notice, however, that the ngOnChanges() method does not catch changes to hero.name. This is because Angular calls the hook only when the value of the input property changes. In this case, hero is the input property, and the value of the hero property is the reference to the hero object. The object reference did not change when the value of its own name property changed.

Responding to view changes

As Angular traverses the view hierarchy during change detection, it needs to be sure that a change in a child does not attempt to cause a change in its own parent. Such a change would not be rendered properly, because of how unidirectional data flow works.

If you need to make a change that inverts the expected data flow, you must trigger a new change detection cycle to allow that change to be rendered. The examples illustrate how to make such changes safely.

The AfterView sample explores the AfterViewInit() and AfterViewChecked() hooks that Angular calls after it creates a component's child views.

Here's a child view that displays a hero's name in an <input>:

@Component({
  selector: 'app-child-view',
  template: '<input [(ngModel)]="hero">'
})
export class ChildViewComponent {
  hero = 'Magneta';
}

The AfterViewComponent displays this child view within its template:

template: `
  <div>-- child view begins --</div>
    <app-child-view></app-child-view>
  <div>-- child view ends --</div>`

The following hooks take action based on changing values within the child view, which can only be reached by querying for the child view via the property decorated with @ViewChild.

export class AfterViewComponent implements  AfterViewChecked, AfterViewInit {
  private prevHero = '';

  // Query for a VIEW child of type `ChildViewComponent`
  @ViewChild(ChildViewComponent) viewChild: ChildViewComponent;

  ngAfterViewInit() {
    // viewChild is set after the view has been initialized
    this.logIt('AfterViewInit');
    this.doSomething();
  }

  ngAfterViewChecked() {
    // viewChild is updated after the view has been checked
    if (this.prevHero === this.viewChild.hero) {
      this.logIt('AfterViewChecked (no change)');
    } else {
      this.prevHero = this.viewChild.hero;
      this.logIt('AfterViewChecked');
      this.doSomething();
    }
  }
  // ...
}

Wait before updating the view

In this example, the doSomething() method updates the screen when the hero name exceeds 10 characters, but waits a tick before updating comment.

// This surrogate for real business logic sets the `comment`
private doSomething() {
  let c = this.viewChild.hero.length > 10 ? `That's a long name` : '';
  if (c !== this.comment) {
    // Wait a tick because the component's view has already been checked
    this.logger.tick_then(() => this.comment = c);
  }
}

Both the AfterViewInit() and AfterViewChecked() hooks fire after the component's view has been composed. If you modify the code so that the hook updates the component's data-bound comment property immediately, you can see that Angular throws an error.

The LoggerService.tick_then() statement postpones the log update for one turn of the browser's JavaScript cycle, which triggers a new change-detection cycle.

Write lean hook methods to avoid performance problems

When you run the AfterView sample, notice how frequently Angular calls AfterViewChecked()$emdash;often when there are no changes of interest. Be very careful about how much logic or computation you put into one of these methods.

Responding to projected content changes

Content projection is a way to import HTML content from outside the component and insert that content into the component's template in a designated spot. You can identify content projection in a template by looking for the following constructs.

• HTML between component element tags.
• The presence of <ng-content> tags in the component's template.

AngularJS developers know this technique as transclusion.

The AfterContent sample explores the AfterContentInit() and AfterContentChecked() hooks that Angular calls after Angular projects external content into the component.

Consider this variation on the previous AfterView example. This time, instead of including the child view within the template, it imports the content from the AfterContentComponent's parent. The following is the parent's template.

`<after-content>
   <app-child></app-child>
 </after-content>`

Notice that the <app-child> tag is tucked between the <after-content> tags. Never put content between a component's element tags unless you intend to project that content into the component.

Now look at the component's template.

template: `
  <div>-- projected content begins --</div>
    <ng-content></ng-content>
  <div>-- projected content ends --</div>`

The <ng-content> tag is a placeholder for the external content. It tells Angular where to insert that content. In this case, the projected content is the <app-child> from the parent.

Using AfterContent hooks

AfterContent hooks are similar to the AfterView hooks. The key difference is in the child component.

• The AfterView hooks concern ViewChildren, the child components whose element tags appear within the component's template.

• The AfterContent hooks concern ContentChildren, the child components that Angular projected into the component.

The following AfterContent hooks take action based on changing values in a content child, which can only be reached by querying for them via the property decorated with @ContentChild.

export class AfterContentComponent implements AfterContentChecked, AfterContentInit {
  private prevHero = '';
  comment = '';

  // Query for a CONTENT child of type `ChildComponent`
  @ContentChild(ChildComponent) contentChild: ChildComponent;

  ngAfterContentInit() {
    // contentChild is set after the content has been initialized
    this.logIt('AfterContentInit');
    this.doSomething();
  }

  ngAfterContentChecked() {
    // contentChild is updated after the content has been checked
    if (this.prevHero === this.contentChild.hero) {
      this.logIt('AfterContentChecked (no change)');
    } else {
      this.prevHero = this.contentChild.hero;
      this.logIt('AfterContentChecked');
      this.doSomething();
    }
  }
  // ...
}

delay the update to ensure proper rendering.

Angular calls both AfterContent hooks before calling either of the AfterView hooks. Angular completes composition of the projected content before finishing the composition of this component's view. There is a small window between the AfterContent... and AfterView... hooks that allows you to modify the host view.

Defining custom change detection

To monitor changes that occur where ngOnChanges() won't catch them, you can implement your own change check, as shown in the DoCheck example. This example shows how you can use the ngDoCheck() hook to detect and act upon changes that Angular doesn't catch on its own.

The DoCheck sample extends the OnChanges sample with the following ngDoCheck() hook:

ngDoCheck() {

  if (this.hero.name !== this.oldHeroName) {
    this.changeDetected = true;
    this.changeLog.push(`DoCheck: Hero name changed to "${this.hero.name}" from "${this.oldHeroName}"`);
    this.oldHeroName = this.hero.name;
  }

  if (this.power !== this.oldPower) {
    this.changeDetected = true;
    this.changeLog.push(`DoCheck: Power changed to "${this.power}" from "${this.oldPower}"`);
    this.oldPower = this.power;
  }

  if (this.changeDetected) {
      this.noChangeCount = 0;
  } else {
      // log that hook was called when there was no relevant change.
      let count = this.noChangeCount += 1;
      let noChangeMsg = `DoCheck called ${count}x when no change to hero or power`;
      if (count === 1) {
        // add new "no change" message
        this.changeLog.push(noChangeMsg);
      } else {
        // update last "no change" message
        this.changeLog[this.changeLog.length - 1] = noChangeMsg;
      }
  }

  this.changeDetected = false;
}

This code inspects certain values of interest, capturing and comparing their current state against previous values. It writes a special message to the log when there are no substantive changes to the hero or the power so you can see how often DoCheck() is called. The results are illuminating.

While the ngDoCheck() hook can detect when the hero's name has changed, it is very expensive. This hook is called with enormous frequency—after every change detection cycle no matter where the change occurred. It's called over twenty times in this example before the user can do anything.

Most of these initial checks are triggered by Angular's first rendering of unrelated data elsewhere on the page. Just moving the cursor into another <input> triggers a call. Relatively few calls reveal actual changes to pertinent data. If you use this hook, your implementation must be extremely lightweight or the user experience suffers.