Angular2的 ngOnInit 和 ngAfterViewInit 有什么区别？

我不能理解 ngOnInit和 ngAfterViewInit之间的区别。

我发现它们之间唯一的区别是 @ViewChild。根据以下代码，它们中的 elementRef.nativeElement是相同的。

我们应该使用 ngAfterViewInit的哪个场景？

@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'));
}
}

166484

小开

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.

小开

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.

小开

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',
[]
)
}
}