如何停止在 ngOnInit() 之前调用的 ngOnChanges

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"
    }
  }

}

Answer 2

One method I have found that works is based on the fact that the input values all have a previous value property. If the input has not previously been set then that value will be CD_INIT_VALUE (as a string). So you can make a condition that your block of code in ngOnChanges should only run if the previous value is not CD_INIT_VALUE . Here's an example for your case where you're testing ALL the input values:

ngOnChanges(changes: SimpleChanges) {
  let initialized: boolean = true;
  for (let prop in changes) {
    if (changes[prop].previousValue.toString() === 'CD_INIT_VALUE') {
      initialized = false;
      //we can break here since if any item is not initialized
      //we will say the inputs are NOT initialized
      break;
    }
  }

  if (initialized) {
    //code you want to execute
  }    
}

There are probably more elegant solutions but I've found this works. This is probably too late to help you but may help others as when I googled this I found this question. The solution was something I figured out from debugging.

Answer 3

In case it helps anyone here's an example of how I implemented the isFirstChange() solution suggested by @n00dl3 into my AngularJS app:

this.$onChanges = (changes) => {
    if (!changes.value.isFirstChange()) {
        // do stuff
    }
};

Answer 4

I guess this is not what the OP is looking for, but the following provides a way to get rid of the ngOnChanges() call without any modification in your component's code.

Instead of declaring your component with the appropriate selector in your template, you could instead have an ng-template:

<ng-template></ng-template>

And then spawn your component inside this ng-template using ViewContainerRef#createComponent method.

This way, your component will be instantiated without any input and ngOnChanges() won't be called before ngOnInit() anymore.