在Angular（Angular 4）中，无法使用@ViewChild访问另一个组件的ElementRef

Question

I was trying to access the child component using @ViewChild in Parent Component(AppComponent.ts)

      import { Component, ViewChild, ElementRef, AfterViewInit} from '@angular/core';
      import { HeaderComponent } from '../app/components/header/header.component';
      @Component({
        selector: 'app-root',
        templateUrl: './app.component.html',
        styleUrls: ['./app.component.less']
      })
      export class AppComponent implements AfterViewInit {
        title = 'app';
        @ViewChild('myHeader') myHeader: ElementRef;
        ngAfterViewInit() {
          this.myHeader.nativeElement.style.backgroundColor = 'red';
        }  
      }

Parent HTML(app.component.html)

<div>
    <app-header></app-header>
</div>

Now Child HeaderComponent is as below:

    import { Component, OnInit, ViewEncapsulation } from '@angular/core';

    @Component({
      selector: 'app-header',
      templateUrl: './header.component.html',
      styleUrls: ['./header.component.less']
    })
    export class HeaderComponent implements OnInit {
      constructor() {
      }
      ngOnInit() {

      }

    }

Header HTML(header.component.html)(Child)

<div #myHeader> hello</div>

In ngAfterViewInit function this.myHeader.nativeElement is giving error.

AppComponent_Host.html:1 ERROR TypeError: Cannot read property 'nativeElement' of undefined
at AppComponent.webpackJsonp.../../../../../src/app/app.component.ts.AppComponent.ngAfterViewInit (app.component.ts:12)
at callProviderLifecycles (core.es5.js:11189)
at callElementProvidersLifecycles (core.es5.js:11164)
at callLifecycleHooksChildrenFirst (core.es5.js:11148)
at checkAndUpdateView ....

Although when i tried to access any child defined in Parent HTML(app.component.html) its working like below:

<div #myHeader> <app-header></app-header></div>

But i want to access any ElementRef in header.component.html.I tired to have View Encapsulation also like below.

encapsulation: ViewEncapsulation.None,

in HeaderComponent but it also not working.Please let me know what else i am missing here.

Answer 1

When you are using Component as @ViewChild you can directly set their instance instead of ElementRef

@ViewChild(HeaderComponent) myHeader: HeaderComponent;

In case you are using ElementRef you should specify the selector in the HTML as below,

<div #myHeader> hello</div>

Answer 2

You can't access element reference from child components template directly. You can access HeaderComponent in your AppComponent, using header components reference you can access all public properties of HeaderComponent. In HeaderComponent you can access any element from HeaderComponent's template & expose that as public property.

import { Component, OnInit, ViewEncapsulation, ViewChild, ElementRef} from '@angular/core'; 

@Component({ selector: 'app-header', templateUrl: './header.component.html', 

styleUrls: ['./header.component.less'] 
    }) 
export class HeaderComponent implements OnInit { 
        @ViewChild('myHeader')
     public myHeader: ElementRef; 
        constructor() { } 
        ngOnInit() { } 
        }

  /*---- AppComponent----*/
    import { Component, ViewChild, ElementRef, AfterViewInit} from '@angular/core';
 import { HeaderComponent } from '../app/components/header/header.component';



@Component(
{ selector: 'app-root', 
templateUrl: './app.component.html', styleUrls: ['./app.component.less'] 
}) 
export class AppComponent implements AfterViewInit 
{ title = 'app'; 

@ViewChild(HeaderComponent) 
header: HeaderComponent;

 ngAfterViewInit() { 
this.header.myHeader.nativeElement.style.backgroundColor = 'red'; 
}

 }