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Component Interactions

In this tutorial we will explore some of the ways and scenarios in which components interact with one another, passing data along as they interact. We will also explore some of the ways in which these data are passed.

Pass data from parent to child with input binding

In this example, the HeroChildComponent has two input properties, typically decorated with @Input decorations.

TypeScript Code:

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: string;
}

Live Demo:

It is just a code snippet explaining a particular concept and may not have any output

See the Pen Pass data from parent to child with input binding by w3resource (@w3resource) on CodePen.


In the above example, the second @Input aliases another child component property by name  `masterName` as `master`.

In the next example, 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.

TypeScript Code:

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';
}

Live Demo:

It is just a code snippet explaining a particular concept and may not have any output

See the Pen Pass data from parent to child with input binding 2 by w3resource (@w3resource) on CodePen.


Intercept input property changes with a setter

An input property setter can be used to intercept and act upon a value from the parent. For instance, in the example below, 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.

TypeScript Code:

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

Live Demo:

It is just a code snippet explaining a particular concept and may not have any output

See the Pen Intercept input property changes with a setter by w3resource (@w3resource) on CodePen.


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

TypeScript Code:

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 'Mr. IQ', '<no name set>', 'Bombasto'
  names = ['Mr. IQ', '   ', '  Bombasto  '];
}

Live Demo:

It is just a code snippet explaining a particular concept and may not have any output

See the Pen Intercept input property changes with a setter 2 by w3resource (@w3resource) on CodePen.


Intercept input property changes with ngOnChanges()

We can monitor, detect and act upon changes to the value of an input property with the ngOnChanges() method of the OnChanges lifecycle hook interface.

This approach is often preferred over the property setter when watching multiple, interacting input properties.

To illustrate, the VersionChildComponent detects changes to the major and minor input properties and composes a log message reporting these changes if any.

TypeScript Code:

import { Component, Input, OnChanges, SimpleChange } 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: number;
  @Input() minor: number;
  changeLog: string [] = [];
 
  ngOnChanges(changes: {[propKey: string]: SimpleChange}) {
    let log: string[] = [];
    for (let propName in changes) {
      let changedProp = changes[propName];
      let to = JSON.stringify(changedProp.currentValue);
      if (changedProp.isFirstChange()) {
        log.push(`Initial value of ${propName} set to ${to}`);
      } else {
        let from = JSON.stringify(changedProp.previousValue);
        log.push(`${propName} changed from ${from} to ${to}`);
      }
    }
    this.changeLog.push(log.join(', '));
  }
}

Live Demo:

It is just a code snippet explaining a particular concept and may not have any output

See the Pen Intercept input property changes with ngOnChanges() by w3resource (@w3resource) on CodePen.


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

TypeScript Code:

import { Component } from '@angular/core';
@Component({
  selector: 'app-version-parent',
  template: `
    <h2>Source code version</h2>
    <button (click)="newMinor()">New minor version</button>
    <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;
  }
}
 

Live Demo:

It is just a code snippet explaining a particular concept and may not have any output

See the Pen Intercept input property changes with ngOnChanges() 2 by w3resource (@w3resource) on CodePen.


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 decoration as seen in this VoterComponent:

TypeScript Code:

import { Component, EventEmitter, Input, Output } from '@angular/core'; 
@Component({
  selector: 'app-voter',
  template: `
    <h4>{{name}}</h4>
    <button (click)="vote(true)"  [disabled]="didVote">Agree</button>
    <button (click)="vote(false)" [disabled]="didVote">Disagree</button>
  `
})

export class VoterComponent {
  @Input() name: string;
  @Output() voted = new EventEmitter<boolean>();
  didVote = false;
 
  vote(agreed: boolean) {
    this.voted.emit(agreed);
    this.didVote = true;
  }
}

Live Demo:

It is just a code snippet explaining a particular concept and may not have any output

See the Pen Parent listens for child event by w3resource (@w3resource) on CodePen.


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.

TypeScript Code:

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 = ['Mr. IQ', 'Ms. Universe', 'Bombasto'];
 
  onVoted(agreed: boolean) {
    agreed ? this.agreed++ : this.disagreed++;
  }
}

Live Demo:

It is just a code snippet explaining a particular concept and may not have any output

See the Pen Parent listens for child event by w3resource (@w3resource) on CodePen.


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

Parent interacts with a child via local variable

A parent component cannot use data binding to read child properties or invoke child methods. You can 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. It has start and stop methods that control the clock and it displays a countdown status message in its own template.

TypeScript Code:

import { Component, OnDestroy, OnInit } from '@angular/core';
@Component({
  selector: 'app-countdown-timer',
  template: '<p>{{message}}</p>'
})
export class CountdownTimerComponent implements OnInit, OnDestroy {
  intervalId = 0;
  message = '';
  seconds = 11;
  clearTimer() { clearInterval(this.intervalId); }
  ngOnInit()    { this.start(); }
  ngOnDestroy() { this.clearTimer(); }
  start() { this.countDown(); }
  stop()  {
    this.clearTimer();
    this.message = `Holding at T-${this.seconds} seconds`;
  }
 
  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);
  }
}

Live Demo:

It is just a code snippet explaining a particular concept and may not have any output

See the Pen Parent interacts with child via local variable by w3resource (@w3resource) on CodePen.


The CountdownLocalVarParentComponent that hosts the timer component is as follows:

TypeScript Code:

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 (click)="timer.start()">Start</button>
  <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 { }

Live Demo:

It is just a code snippet explaining a particular concept and may not have any output

See the Pen Parent interacts with child via local variable by w3resource (@w3resource) on CodePen.


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

You can place a local variable, #timer, on the tag <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 we see the parent and child working together.

Parent calls an @ViewChild()

The local variable approach is simple and easy. 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 an instance of the parent component class must read or write child component values or must call child component 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 will change. 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:

TypeScript Code:

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 (click)="start()">Start</button>
  <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(); }
}

Live Demo:

It is just a code snippet explaining a particular concept and may not have any output

See the Pen Parent calls an @ViewChild() by w3resource (@w3resource) on CodePen.


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 via the @ViewChildproperty 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 afterAngular 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 app has to 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.

Parent and children communicate via a service

A parent component and its children share a service whose interface enables bi-directional 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.

TypeScript Code:

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);
  }
}

Live Demo:

It is just a code snippet explaining a particular concept and may not have any output

See the Pen Component interaction by w3resource (@w3resource) on CodePen.


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:

TypeScript Code:

import { Component }          from '@angular/core';
 
import { MissionService }     from './mission.service';
 
@Component({
  selector: 'app-mission-control',
  template: `
    <h2>Mission Control</h2>
    <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() {
    let mission = this.missions[this.nextMission++];
    this.missionService.announceMission(mission);
    this.history.push(`Mission "${mission}" announced`);
    if (this.nextMission >= this.missions.length) { this.nextMission = 0; }
  }
}

Live Demo:

It is just a code snippet explaining a particular concept and may not have any output

See the Pen component-interaction by w3resource (@w3resource) on CodePen.


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:

TypeScript Code:

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
        (click)="confirm()"
        [disabled]="!announced || confirmed">
        Confirm
      </button>
    </p>
  `
})
export class AstronautComponent implements OnDestroy {
  @Input() astronaut: string;
  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();
  }
}

Live Demo:

It is just a code snippet explaining a particular concept and may not have any output

See the Pen Component interactions by w3resource (@w3resource) on CodePen.


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 app because the lifetime of a AstronautComponent is the same as the lifetime of the app 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.

Previous: Template Syntax
Next: Component lifecycle hooks overview



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