detect change of nested property for component input

Question

In a simplified way I have an Angular2 Component and an input object like this:

class MyObject{
 Prop1:string;
 Prop2:Number;
}  

@Component() 
export class         


        

Answer 1

You can use observables to support notification of subscribers. Angular itself doesn't provide support for observing changes of internal object state.

class MyObject{
  constructor() {
    this.prop1Change$ = new Observable(observer => 
        this._prop1Observer = observer).share(); // share() allows multiple subscribers

    this.prop2Change$ = new Observable(observer =>
        this._prop2Observer = observer).share();
        console.debug(this._prop2Observer);
  }

  prop1Change$: Observable;
  private _prop1Observer: Observer;
  _prop1:string;
  get prop1():string { return this._prop1 };
  set prop1(value:string) {
    this._prop1 = value;
    this._prop1Observer && this._prop1Observer.next(value);
  }

  prop1Change$: Observable;
  private _prop2Observer: Observer;
  _prop2:Number;
  get prop2():number { return this._prop2 };
  set prop2(value:number) {
    this._prop2 = value;
    console.debug(this);
    this._prop2Observer && this._prop2Observer.next(value);
  }
}

This code could be shortened by using Subject but Observable should be favored over Subject.

@Component() 
export class MyComponent{
  @Input() myObject: MyObject;

  count2:number;

  DoSomethingIfProp1Change(){
    console.log(myObject.prop1);
  }

  ngOnChanges(changes: {[propName: string]: SimpleChange}) {
    console.log('changes');
    if(changes['myObject']) {
      if(this.prop2Subscription) {
        this.prop2Subscription.unsubscribe();
      }
      this.prop2Subscription = this.myObject.prop2Change$.subscribe(value => {
        this.count2 = value;
        console.log('count2: ' + value);
      });
      // same for `prop2`
    }
  }
}

Plunker example

See also Delegation: EventEmitter or Observable in Angular2