Angular Component Interactions: Passing Data and Communication Methods

Last update on July 09 2024 13:10:57 (UTC/GMT +8 hours)

Angular - Component Interactions

In this tutorial, we will explore various ways and scenarios in which components interact with one another, passing data as they interact. We will also delve into different methods of data transmission between components.

Passing Data from Parent to Child with Input Binding

Components can pass data to their child components using input bindings. The child component defines input properties decorated with @Input decorator.

Example

HeroChildComponent with two input properties:

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;
}

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

HeroParentComponent nests the child HeroChildComponent inside an *ngFor repeater:

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';
}

Intercept Input Property Changes with a Setter

An input property setter can be used to intercept and act upon a value from the parent.

Example

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;
  }
}

NameParentComponent demonstrating name variations:

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 {
  names = ['Mr. IQ', '   ', '  Bombasto  '];
}

Intercept Input Property Changes with ngOnChanges()

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

Example

VersionChildComponent detects changes to the major and minor input properties:

TypeScript Code:

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: number;
  @Input() minor: number;
  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(' '));
  }
}

VersionParentComponent supplies the minor and major values:

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;
  }
}

Parent Listens for Child Event

The child component exposes an EventEmitter property to emit events when something happens, which the parent binds to and reacts to those events.

Example

VoterComponent emits an event when a button is clicked:

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;
  }
}

VoteTakerComponent binds an event handler to the child event:

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++;
  }
}

Parent Interacts with a Child via Local Variable

A parent component can read child properties or invoke child methods using a template reference variable.

Example

CountdownTimerComponent with start and stop methods:

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; }
        this.message = 'T-${this.seconds} seconds and counting';
      }
    }, 1000);
  }
}

CountdownLocalVarParentComponent hosts the timer component:

TypeScript Code:

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

@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 { }

Parent Calls an @ViewChild()

When the parent component class requires access to child component values or methods, use the ViewChild decorator.

Example

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">{{timerComponent.seconds}}</div>
    <app-countdown-timer></app-countdown-timer>
  `,
  styleUrls: ['../assets/demo.css']
})
export class CountdownViewChildParentComponent implements AfterViewInit {
  @ViewChild(CountdownTimerComponent)
  private timerComponent: CountdownTimerComponent;

  ngAfterViewInit() {
    setTimeout(() => this.timerComponent.start(), 0);
  }

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

Bidirectional Service Communication

For bidirectional communication, create a service to store and manage shared data between components.

Example

MissionService to communicate between components:

TypeScript Code:

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

@Injectable()
export class MissionService {
  private missionAnnouncedSource = new Subject<string>();
  private missionConfirmedSource = new Subject<string>();

  missionAnnounced$ = this.missionAnnouncedSource.asObservable();
  missionConfirmed$ = this.missionConfirmedSource.asObservable();

  announceMission(mission: string) {
    this.missionAnnouncedSource.next(mission);
  }

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

MissionControlComponent announces missions:

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>
  `,
  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; }
  }
}

AstronautComponent listens to the mission announcement and confirms:

TypeScript Code:

import { Component, Input, OnDestroy } from '@angular/core';
import { Subscription } from 'rxjs';
import { MissionService } from './mission.service';

@Component({
  selector: 'app-astronaut',
  template: `
    <p>
      {{astronaut}}: <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() {
    this.subscription.unsubscribe();
  }
}

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