How to detect when a view element is rendered in Angular?

问题

My setup is an Angular Material data table with clickable rows. When a row is clicked, its contents are shown inline in a textarea for editing. My only problem is, I try to move input focus to the shown textarea. I tried to do it using @ViewChild, but it's being populated later, when the click handler has already executed.

Some code, to illustrate:

app.component.ts:

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

@Component({ selector: 'app-root', templateUrl: './app.component.html' })
export class AppComponent {
  columns = ['id', 'def'];
  selected: string;
  ruleDef: string;
  @ViewChild('edit') editArea: ElementRef;
  selectRule(rule: Rule) {
    this.selected = rule.id;
    if (this.editArea) this.editArea.nativeElement.focus();
  }
}

interface Rule {id: string, def: string}

app.component.html:

<mat-table #table [dataSource]="dataSource">
  <ng-container matColumnDef="id">
    <mat-header-cell *matHeaderCellDef>Rule ID</mat-header-cell>
    <mat-cell *matCellDef="let element">{{element.id}}</mat-cell>
  </ng-container>
  <ng-container matColumnDef="def">
    <mat-header-cell *matHeaderCellDef>Rule Definition</mat-header-cell>
    <mat-cell *matCellDef="let element">
      <mat-form-field *ngIf="element.id === selected">
        <code><textarea matInput [(ngModel)]="ruleDef" #edit></textarea></code>
      </mat-form-field>
      <code *ngIf="element.id !== selected">{{element.def}}</code>
    </mat-cell>
  </ng-container>
  <mat-header-row *matHeaderRowDef="columns"></mat-header-row>
  <mat-row *matRowDef="let row; columns: columns;" (click)="selectRule(row)"></mat-row>
</mat-table>

In the selectRule() function, the editArea is either undefined, or points to a row selected previously. Obviously the selectRule() is a wrong place to put the focus change into, but I couldn't find an appropriate event handler in Angular

回答1:

You must wait for the zone to settle before setting focus.

It could be done the same way as is used in angular material:

import {take} from 'rxjs/operators/take';

constructor(private zone: NgZone) { }

selectRule(rule: Rule) {
  this.selected = rule.id;

  this.zone.onMicrotaskEmpty.asObservable().pipe(
      take(1)
    )
    .subscribe(() => {
      this.editArea.nativeElement.focus();
    });
}

Ng-run Example

来源：https://stackoverflow.com/questions/49036289/how-to-detect-when-a-view-element-is-rendered-in-angular