ElementRef is undefined

问题:

I’m working in an angular 6 application. I have a textarea that I’d like to give focus to as soon as the page loads. I’m not able to do so.

Here is what the page looks like:

<div fxLayoutAlign="begin">
  <button mat-button color="primary" [routerLink]="['/Administration']">
    <i class="far fa-arrow-alt-circle-left"></i> Back to Administration
  </button>
</div>
    <div class="app-loading" *ngIf="_loading">
      <mat-spinner diameter=100></mat-spinner>
    </div>
    <div *ngIf="!_loading">

    <div class="knowledgeBody">

      <div *ngIf="_mode === 'view'">
        {{ _knowledgeText }}
        <div fxLayoutAlign="end">
          <button mat-button (click)="edit_clicked()" disabled="{{ _knowledgeText.trim().length === 0 }}">
            <i class="fas fa-edit"></i> Edit
          </button>
        </div>
      </div>

      <div *ngIf="_mode !== 'view'">
        <textarea matInput [(ngModel)]="_knowledgeText" cdkTextareaAutosize #knowledgeTextarea>
          {{ _knowledgeText }}
        </textarea>

        <div fxLayoutAlign="end">
          <button mat-button (click)="save_clicked()" disabled="{{ _knowledgeText.trim().length === 0 }}">
            <i class="fas fa-save"></i> Save
          </button>
        </div>
      </div>

  </div>

</div>

The textarea of interest is #knowledgeTextarea.

Here’s the component for the page:

@Component({
  selector: 'app-knowledge',
  templateUrl: './knowledge.component.html',
  styleUrls: ['./knowledge.component.scss']
})
export class KnowledgeComponent implements OnInit {

  private _mode : string;
  private _loading : boolean;
  private _knowledgeId : string;
  private _knowledgeText : string;
  private _description : string;
  private _groupPK : string;

  @ViewChild('knowledgeTextarea') knowledgeTextarea : ElementRef;

  constructor(private _activatedRoute : ActivatedRoute,
    private _watermelonService : WatermelonService,
    private _logService : LoggingService,
    private _dialog: MatDialog) {
      this._loading = true;
    }

  ngOnInit() {
…
    this.knowledgeTextarea.nativeElement.focus();
…
  }
…
}

When I run this, I get the following error:

Cannot read property ‘nativeElement’ of undefined.

I’ve read that this can happen when you reference an element on the page using the # identification (#knowledgeTextarea) inside an *ngIf block. So this…

  <div *ngIf="_mode !== 'view'">
    <textarea matInput [(ngModel)]="_knowledgeText" cdkTextareaAutosize #knowledgeTextarea>
      {{ _knowledgeText }}
    </textarea>
</div>

…will not work.

But then what is the alternative?

I tried this…

  <div *ngIf="_mode !== 'view'">
    <textarea matInput [(ngModel)]="_knowledgeText" cdkTextareaAutosize #knowledgeTextarea>
      {{ _knowledgeText }}
    </textarea>
    {{ knowledgeTextarea.focus() }}
</div>

…which worked but then I got a ExpressionChangedAfterItHasBeenCheckedError, which JB Nizet told me not to do here: Getting ExpressionChangedAfterItHasBeenCheckedError error.

If there is another way of doing this, please let me know. All I want is to be able to give the textarea focus when the page loads. Any way of accomplishing that would be greatly appreciated. Thanks.

回答1:

The issue is accessing the element before it was rendered. You must ensure that the content is rendered before it is being used. So instead of accessing it inside ngOnInit, you should do in ngAfterViewInit.

 @ViewChild('knowledgeTextarea') knowledgeTextarea: ElementRef;

  ngAfterViewInit() {
    this.knowledgeTextarea.nativeElement.focus();
  }

Working sample is here - https://stackblitz.com/edit/angular-jjyfwf