如何在Angular2中触发模型更改事件？

Question

I would like to build a drum machine in Angular2 similar to an existing Angular project.

The way I have structured the components is to have a SequencerComponent (parent) that emits an event (@Output) at a specified interval (BPM).

// sequencer.component.ts
@Output() metronome = new EventEmitter();

  onBeat(count: number) {
    if(this.isPlaying) {
      this.beat.one = count == 1;
      this.beat.two = count == 2;
      this.beat.three = count == 3;
      this.beat.four = count == 4;

      this.metronome.emit(this.beat);
    }
  }

PadsComponent (child) listens to this event (@Input) and should then play a sound if a PadComponent (grand-child) is set to active.

// pads.component.ts
export class PadsComponent {
  @Input() hit: any = {};
}

// pads.component.html
<pad [class.active]="hit.one" class="column pad"></pad>

<pad [class.active]="hit.two" class="column pad"></pad>

<pad [class.active]="hit.three" class="column pad"></pad>

<pad [class.active]="hit.four" class="column pad"></pad>

I can get the classes to change but obviously I want to do more than HTML attribute binding. Working example

How can I get the child and grand-child to run a function on model change? And is there a better, more idiomatic way to solve this problem?

I have tried using ngDoCheck but I haven't discovered how to use it in this use case.

Update : hit is being bound to the beat in sequencer.component.html like so:

<!-- sequencer.component.html -->
<div class="columns">
  <div class="column">
    <a class="button is-success" (click)="onPlay($event)" href="">Play</a>

    <a class="button is-danger" (click)="onStop($event)" href="">Stop</a>
  </div>
</div>

<pads [hit]="beat"></pads>

Answer 1

Instead of @Input() hit: any = {}; you can have a getter and a setter for hit :

private _hit;  // define a private variable _hit

@Input()
set hit(hit:any){  
    this._hit = hit;  // set the private variable and do whatever you want after each change
    playSomeSound(hit);
}
get hit(){  
    return this._hit;
}

Your template code is still the same,

<pad [class.active]="hit.one" class="column pad"></pad>
...

check this plunk for an example.

Answer 2

Here's an example of how to use ngDoCheck() to watch for model change and emit it:

ngDoCheck() {

  if (this.previousModel !== this.model) {
    this.modelChange.emit(this.model);
  }

  this.previousModel = this.model;
}