What's the difference between ngOnInit and ngA

问题:

I can not understand what the difference between ngOnInit and ngAfterViewInit.

I found the only difference between them is @ViewChild. According to the following code, the elementRef.nativeElement in them are the same.

What scene should we use ngAfterViewInit?

@Component({
  selector: 'my-child-view',
  template: `
  <div id="my-child-view-id">{{hero}}</div>
  `
})
export class ChildViewComponent {
  @Input() hero: string = 'Jack';
}

//////////////////////
@Component({
  selector: 'after-view',
  template: `
    <div id="after-view-id">-- child view begins --</div>
      <my-child-view [hero]="heroName"></my-child-view>
    <div>-- child view ends --</div>`
   + `
    <p *ngIf="comment" class="comment">
      {{comment}}
    </p>
  `
})
export class AfterViewComponent implements AfterViewInit, OnInit {
  private prevHero = '';
  public heroName = 'Tom';
  public comment = '';

  // Query for a VIEW child of type `ChildViewComponent`
  @ViewChild(ChildViewComponent) viewChild: ChildViewComponent;

  constructor(private logger: LoggerService, private elementRef: ElementRef) {
  }

  ngOnInit(){
    console.log('OnInit');
    console.log(this.elementRef.nativeElement.querySelector('#my-child-view-id'));
    console.log(this.elementRef.nativeElement.querySelector('#after-view-id'));
    console.log(this.viewChild);
    console.log(this.elementRef.nativeElement.querySelector('p'));
  }

  ngAfterViewInit() {
    console.log('AfterViewInit');
    console.log(this.elementRef.nativeElement.querySelector('#my-child-view-id'));
    console.log(this.elementRef.nativeElement.querySelector('#after-view-id'));
    console.log(this.viewChild);
    console.log(this.elementRef.nativeElement.querySelector('p'));
  }
}

回答1:

ngOnInit() is called after ngOnChanges() was called the first time. ngOnChanges() is called every time inputs are updated by change detection.

ngAfterViewInit() is called after the view is initially rendered. This is why @ViewChild() depends on it. You can't access view members before they are rendered.

回答2:

ngOnInit() is called right after the directive's data-bound properties have been checked for the first time, and before any of its children have been checked. It is invoked only once when the directive is instantiated.

ngAfterViewInit() is called after a component's view, and its children's views, are created. Its a lifecycle hook that is called after a component's view has been fully initialized.

回答3:

Content is what is passed as children. View is the template of the current component.

The view is initialized before the content and ngAfterViewInit() is therefore called before ngAfterContentInit().

** ngAfterViewInit() is called when the bindings of the children directives (or components) have been checked for the first time. Hence its perfect for accessing and manipulating DOM with Angular 2 components. As @Günter Zöchbauer mentioned before is correct @ViewChild() hence runs fine inside it.

Example:

@Component({
    selector: 'widget-three',
    template: `<input #input1 type="text">`
})
export class WidgetThree{
    @ViewChild('input1') input1;

    constructor(private renderer:Renderer){}

    ngAfterViewInit(){
        this.renderer.invokeElementMethod(
            this.input1.nativeElement,
            'focus',
            []
        )
    }
}