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ViewChildren

decorator

Property decorator that configures a view query.

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Description

Use to get the QueryList of elements or directives from the view DOM. Any time a child element is added, removed, or moved, the query list will be updated, and the changes observable of the query list will emit a new value.

View queries are set before the ngAfterViewInit callback is called.

Metadata Properties:

  • selector - The directive type or the name used for querying.
  • read - Used to read a different token from the queried elements.
  • emitDistinctChangesOnly - The QueryList#changes observable will emit new values only if the QueryList result has changed. When false the changes observable might emit even if the QueryList has not changed. Note: * This config option is deprecated, it will be permanently set to true and removed in future versions of Angular.

The following selectors are supported.

  • Any class with the @Component or @Directive decorator
  • A template reference variable as a string (e.g. query <my-component #cmp></my-component> with @ViewChildren('cmp'))
  • Any provider defined in the child component tree of the current component (e.g. @ViewChildren(SomeService) someService!: SomeService)
  • Any provider defined through a string token (e.g. @ViewChildren('someToken') someTokenVal!: any)
  • A TemplateRef (e.g. query <ng-template></ng-template> with @ViewChildren(TemplateRef) template;)

In addition, multiple string selectors can be separated with a comma (e.g. @ViewChildren('cmp1,cmp2'))

The following values are supported by read:

  • Any class with the @Component or @Directive decorator
  • Any provider defined on the injector of the component that is matched by the selector of this query
  • Any provider defined through a string token (e.g. {provide: 'token', useValue: 'val'})
  • TemplateRef, ElementRef, and ViewContainerRef

Further information is available in the Usage Notes...

Options

Usage notes

import {AfterViewInit, Component, Directive, QueryList, ViewChildren} from '@angular/core';

@Directive({selector: 'child-directive'})
class ChildDirective {
}

@Component({selector: 'someCmp', templateUrl: 'someCmp.html'})
class SomeCmp implements AfterViewInit {
  @ViewChildren(ChildDirective) viewChildren!: QueryList<ChildDirective>;

  ngAfterViewInit() {
    // viewChildren is set
  }
}

Another example

import {AfterViewInit, Component, Directive, Input, QueryList, ViewChildren} from '@angular/core';

@Directive({selector: 'pane'})
export class Pane {
  @Input() id!: string;
}

@Component({
  selector: 'example-app',
  template: `
    <pane id="1"></pane>
    <pane id="2"></pane>
    <pane id="3" *ngIf="shouldShow"></pane>

    <button (click)="show()">Show 3</button>

    <div>panes: {{serializedPanes}}</div>
  `,
})
export class ViewChildrenComp implements AfterViewInit {
  @ViewChildren(Pane) panes!: QueryList<Pane>;
  serializedPanes: string = '';

  shouldShow = false;

  show() {
    this.shouldShow = true;
  }

  ngAfterViewInit() {
    this.calculateSerializedPanes();
    this.panes.changes.subscribe((r) => {
      this.calculateSerializedPanes();
    });
  }

  calculateSerializedPanes() {
    setTimeout(() => {
      this.serializedPanes = this.panes.map(p => p.id).join(', ');
    }, 0);
  }
}

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Licensed under the Creative Commons Attribution License 4.0.
https://angular.io/api/core/ViewChildren