Wednesday, 30 May 2018

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.

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