Property decorator that configures a content query.

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Description

Use to get the QueryList of elements or directives from the content 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.

Content queries are set before the ngAfterContentInit callback is called.

Does not retrieve elements or directives that are in other components' templates, since a component's template is always a black box to its ancestors.

Metadata Properties:

• selector - The directive type or the name used for querying.
• descendants - If true include all descendants of the element. If false then only query direct children of the element.
• 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.
• read - Used to read a different token from the queried elements.

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 @ContentChildren('cmp'))
• Any provider defined in the child component tree of the current component (e.g. @ContentChildren(SomeService) someService: SomeService)
• Any provider defined through a string token (e.g. @ContentChildren('someToken') someTokenVal: any)
• A TemplateRef (e.g. query <ng-template></ng-template> with @ContentChildren(TemplateRef) template;)

In addition, multiple string selectors can be separated with a comma (e.g. @ContentChildren('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

Here is a simple demonstration of how the ContentChildren decorator can be used.

import {AfterContentInit, ContentChildren, Directive, QueryList} from '@angular/core';

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

@Directive({selector: 'someDir'})
class SomeDir implements AfterContentInit {
  @ContentChildren(ChildDirective) contentChildren!: QueryList<ChildDirective>;

  ngAfterContentInit() {
    // contentChildren is set
  }
}

Tab-pane example

Here is a slightly more realistic example that shows how ContentChildren decorators can be used to implement a tab pane component.

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

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

@Component({
  selector: 'tab',
  template: `
    <div class="top-level">Top level panes: {{serializedPanes}}</div>
    <div class="nested">Arbitrary nested panes: {{serializedNestedPanes}}</div>
  `
})
export class Tab {
  @ContentChildren(Pane) topLevelPanes!: QueryList<Pane>;
  @ContentChildren(Pane, {descendants: true}) arbitraryNestedPanes!: QueryList<Pane>;

  get serializedPanes(): string {
    return this.topLevelPanes ? this.topLevelPanes.map(p => p.id).join(', ') : '';
  }
  get serializedNestedPanes(): string {
    return this.arbitraryNestedPanes ? this.arbitraryNestedPanes.map(p => p.id).join(', ') : '';
  }
}

@Component({
  selector: 'example-app',
  template: `
    <tab>
      <pane id="1"></pane>
      <pane id="2"></pane>
      <pane id="3" *ngIf="shouldShow">
        <tab>
          <pane id="3_1"></pane>
          <pane id="3_2"></pane>
        </tab>
      </pane>
    </tab>

    <button (click)="show()">Show 3</button>
  `,
})
export class ContentChildrenComp {
  shouldShow = false;

  show() {
    this.shouldShow = true;
  }
}