Thursday, 31 May 2018

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

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) {;

  confirmMission(astronaut: string) {;
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:
import { Component }          from '@angular/core';

import { MissionService }     from './mission.service';

  selector: 'app-mission-control',
  template: `
    <h2>Mission Control</h2>
    <button (click)="announce()">Announce mission</button>
    <app-astronaut *ngFor="let astronaut of astronauts"
      <li *ngFor="let event of history">{{event}}</li>
  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) {
      astronaut => {
        this.history.push(`${astronaut} confirmed the mission`);

  announce() {
    let mission = this.missions[this.nextMission++];
    this.history.push(`Mission "${mission}" announced`);
    if (this.nextMission >= this.missions.length) { this.nextMission = 0; }
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:
import { Component, Input, OnDestroy } from '@angular/core';

import { MissionService } from './mission.service';
import { Subscription }   from 'rxjs';

  selector: 'app-astronaut',
  template: `
      {{astronaut}}: <strong>{{mission}}</strong>
        [disabled]="!announced || confirmed">
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;

  ngOnDestroy() {
    // prevent memory leak when component destroyed
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:

Wednesday, 30 May 2018

Angular - 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 switch from the local variable to the ViewChild technique is solely for the purpose of demonstration.
Here is the parent, CountdownViewChildParentComponent:
import { AfterViewInit, ViewChild } from '@angular/core';
import { Component }                from '@angular/core';
import { CountdownTimerComponent }  from './countdown-timer.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>
  styleUrls: ['../assets/demo.css']
export class CountdownViewChildParentComponent implements AfterViewInit {

  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(); }
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 @ViewChild property 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.

Angular - Parent interacts with 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.
import { Component, OnDestroy, OnInit } from '@angular/core';

  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.message = `Holding at T-${this.seconds} seconds`;

  private countDown() {
    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);
The CountdownLocalVarParentComponent that hosts the timer component is as follows:
import { Component }                from '@angular/core';
import { CountdownTimerComponent }  from './countdown-timer.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 { }
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.