angular - how to stop ngOnChanges Called before ngOnInit()

Question

In my angular application, i came up with a situation where ngOnchanges should only be called when the inputs are bound to changes. so, is there a way to stop the execution of ngOnChanges before ngOnInit. Is there a way to accomplish this. Thanks in Advance.

Answer 1

You cannot prevent this behavior, but you can:

Use a Subject :

class Foo implements OnChanges,OnInit,OnDestroy{

  onChanges = new Subject<SimpleChanges>();

  ngOnInit(){
    this.onChanges.subscribe((data:SimpleChanges)=>{
      // only when inited
    });
  }
  ngOnDestroy(){
    this.onChanges.complete();
  }

  ngOnChanges(changes:SimpleChanges){
    this.onChanges.next(changes);
  }

}

Use a boolean property:

class Foo implements OnChanges,OnInit{

  initialized=false;

  ngOnInit(){
    // do stuff
    this.initialized = true;
  }

  ngOnChanges(changes:SimpleChanges){
    if(this.initialized){
      // do stuff when ngOnInit has been called
    }
  }

}

Use the SimpleChanges API

You can also check the SimpleChange.isFirstChange() method :

isFirstChange() : boolean Check whether the new value is the first value assigned.

class Foo implements OnChanges,OnInit{

  @Input()
  bar:any;

  ngOnInit(){
    // do stuff
  }

  ngOnChanges(changes:SimpleChanges){
    if(!changes["bar"].isFirstChange()){
      // do stuff if this is not the initialization of "bar"
    }
  }

}