这对我很管用。

例如，我的名为' My -component'的组件使用*ngIf="showMe"来显示 像这样:

<my-component [showMe]="showMe" *ngIf="showMe"></my-component>

因此，当组件初始化时，直到"showMe"为真，组件才会显示出来。因此，我的@ViewChild引用都没有定义。

这是我使用@ViewChildren和它返回的QueryList的地方。参见QueryList上的angular文章和@ViewChildren使用演示。

你可以使用@ViewChildren返回的QueryList，并使用rxjs订阅对引用项的任何更改，如下所示。@ViewChild没有这个能力。

import { Component, ViewChildren, ElementRef, OnChanges, QueryList, Input } from '@angular/core';
import 'rxjs/Rx';

@Component({
    selector: 'my-component',
    templateUrl: './my-component.component.html',
    styleUrls: ['./my-component.component.css']
})
export class MyComponent implements OnChanges {

  @ViewChildren('ref') ref: QueryList<any>; // this reference is just pointing to a template reference variable in the component html file (i.e. <div #ref></div> )
  @Input() showMe; // this is passed into my component from the parent as a    

  ngOnChanges () { // ngOnChanges is a component LifeCycle Hook that should run the following code when there is a change to the components view (like when the child elements appear in the DOM for example)
    if(showMe) // this if statement checks to see if the component has appeared becuase ngOnChanges may fire for other reasons
      this.ref.changes.subscribe( // subscribe to any changes to the ref which should change from undefined to an actual value once showMe is switched to true (which triggers *ngIf to show the component)
        (result) => {
          // console.log(result.first['_results'][0].nativeElement);                                         
          console.log(result.first.nativeElement);                                          

          // Do Stuff with referenced element here...   
        } 
      ); // end subscribe
    } // end if
  } // end onChanges 
} // end Class

希望这能帮助一些人节省一些时间和挫折。

这里有一些对我有用的东西。

@ViewChild('mapSearch', { read: ElementRef }) mapInput: ElementRef;

ngAfterViewInit() {
  interval(1000).pipe(
        switchMap(() => of(this.mapInput)),
        filter(response => response instanceof ElementRef),
        take(1))
        .subscribe((input: ElementRef) => {
          //do stuff
        });
}

所以我基本上每秒钟设置一次检查，直到*ngIf变成true，然后我做与ElementRef相关的事情。

这对我很管用。

例如，我的名为' My -component'的组件使用*ngIf="showMe"来显示 像这样:

<my-component [showMe]="showMe" *ngIf="showMe"></my-component>

因此，当组件初始化时，直到"showMe"为真，组件才会显示出来。因此，我的@ViewChild引用都没有定义。

这是我使用@ViewChildren和它返回的QueryList的地方。参见QueryList上的angular文章和@ViewChildren使用演示。

你可以使用@ViewChildren返回的QueryList，并使用rxjs订阅对引用项的任何更改，如下所示。@ViewChild没有这个能力。

import { Component, ViewChildren, ElementRef, OnChanges, QueryList, Input } from '@angular/core';
import 'rxjs/Rx';

@Component({
    selector: 'my-component',
    templateUrl: './my-component.component.html',
    styleUrls: ['./my-component.component.css']
})
export class MyComponent implements OnChanges {

  @ViewChildren('ref') ref: QueryList<any>; // this reference is just pointing to a template reference variable in the component html file (i.e. <div #ref></div> )
  @Input() showMe; // this is passed into my component from the parent as a    

  ngOnChanges () { // ngOnChanges is a component LifeCycle Hook that should run the following code when there is a change to the components view (like when the child elements appear in the DOM for example)
    if(showMe) // this if statement checks to see if the component has appeared becuase ngOnChanges may fire for other reasons
      this.ref.changes.subscribe( // subscribe to any changes to the ref which should change from undefined to an actual value once showMe is switched to true (which triggers *ngIf to show the component)
        (result) => {
          // console.log(result.first['_results'][0].nativeElement);                                         
          console.log(result.first.nativeElement);                                          

          // Do Stuff with referenced element here...   
        } 
      ); // end subscribe
    } // end if
  } // end onChanges 
} // end Class

希望这能帮助一些人节省一些时间和挫折。

前面提到的问题是ngIf导致视图未定义。答案是使用ViewChildren而不是ViewChild。我有类似的问题，我不想要一个网格显示，直到所有的参考数据已经加载。

html:

   <section class="well" *ngIf="LookupData != null">
       <h4 class="ra-well-title">Results</h4>
       <kendo-grid #searchGrid> </kendo-grid>
   </section>

组件代码

import { Component, ViewChildren, OnInit, AfterViewInit, QueryList  } from '@angular/core';
import { GridComponent } from '@progress/kendo-angular-grid';

export class SearchComponent implements OnInit, AfterViewInit
{
    //other code emitted for clarity

    @ViewChildren("searchGrid")
    public Grids: QueryList<GridComponent>

    private SearchGrid: GridComponent

    public ngAfterViewInit(): void
    {
        
        this.Grids.changes.subscribe((comps: QueryList <GridComponent>) =>
        {
            this.SearchGrid = comps.first;
        });

       
    }
}

这里我们使用了ViewChildren，你可以在它上面监听变化。在本例中，任何引用#searchGrid的子元素。

我也遇到过类似的问题，我想我应该发布这篇文章，以防别人犯同样的错误。首先，要考虑的一件事是AfterViewInit;你需要等待视图被初始化，然后才能访问@ViewChild。但是，我的@ViewChild仍然返回null。问题出在我的*ngIf。*ngIf指令杀死了我的控件组件，所以我不能引用它。

import { Component, ViewChild, OnInit, AfterViewInit } from 'angular2/core';
import { ControlsComponent } from './controls/controls.component';
import { SlideshowComponent } from './slideshow/slideshow.component';

@Component({
  selector: 'app',
  template: `
    <controls *ngIf="controlsOn"></controls>
    <slideshow (mousemove)="onMouseMove()"></slideshow>
  `,
  directives: [SlideshowComponent, ControlsComponent],
})
export class AppComponent {
  @ViewChild(ControlsComponent) controls: ControlsComponent;

  controlsOn: boolean = false;

  ngOnInit() {
    console.log('on init', this.controls);
    // this returns undefined
  }

  ngAfterViewInit() {
    console.log('on after view init', this.controls);
    // this returns null
  }

  onMouseMove(event) {
    this.controls.show();
    // throws an error because controls is null
  }
}

编辑 正如下面@Ashg提到的，一个解决方案是使用@ViewChildren而不是@ViewChild。

对我来说，使用ngAfterViewInit而不是ngOnInit修复了这个问题:

export class AppComponent implements OnInit {
  @ViewChild('video') video;
  ngOnInit(){
    // <-- in here video is undefined
  }
  public ngAfterViewInit()
  {
    console.log(this.video.nativeElement) // <-- you can access it here
  }
}