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Angular @ViewChildren

By Arvind Rai, March 03, 2019
Angular @ViewChildren Decorator is used to get the QueryList of elements or directives from the view DOM. When a new child element is added or removed, the QueryList will be updated and its changes function will emit new value. @ViewChildren sets the data before ngAfterViewInit callback. @ViewChildren has following metadata properties.
selector: Selector for querying.
read: It is used to read different token from the queried elements.
@ViewChildren supports following selector.
1. A Component class
@ViewChildren(WriterComponent) 
writers: QueryList<WriterComponent>; 
2. A Directive Class
@ViewChildren(MessageDirective)
private msgList: QueryList<MessageDirective>; 
3. Template reference variable
@ViewChildren('bkWriter') 
writers: QueryList<WriterComponent>;	
@ViewChildren('pname') 
persons: QueryList<ElementRef>; 
4. Using read metadata.
@ViewChildren(WriterComponent, { read: ElementRef })
writers: QueryList<ElementRef>; 
@ViewChildren(MessageDirective, {read: ViewContainerRef})
msgList: QueryList<ViewContainerRef>; 
Here on this page we will provide @ViewChildren complete example with Component, Directive and Template Reference Variable.

Technologies Used

Find the technologies being used in our example.
1. Angular 7.0.0
2. Angular CLI 7.0.3
3. TypeScript 3.1.1
4. Node.js 10.3.0
5. NPM 6.1.0
6. RxJS 6.3.3

@ViewChildren with Component

To use @ViewChildren with Component, we need to pass Component name or template reference variable as selector to @ViewChildren. Suppose we have a component WriterComponent. We can use QueryList of WriterComponent in any other component using @ViewChildren.
@ViewChildren(WriterComponent) 
writers1: QueryList<WriterComponent>;	
Find the code to use read metadata. Here we will read ElementRef from the queried elements.
@ViewChildren(WriterComponent, {read: ElementRef}) 
writers2: QueryList<ElementRef>; 
Here we will read ViewContainerRef from the queried elements.
@ViewChildren(WriterComponent, {read: ViewContainerRef}) 
writers3: QueryList<ViewContainerRef>; 
We can access QueryList inside ngAfterViewInit method because @ViewChildren sets the data before AfterViewInit. Now find the complete code.
vc-demo1.component.ts
import { Component, ViewChildren, AfterViewInit, ViewContainerRef, QueryList, ElementRef } from '@angular/core';
import { WriterComponent } from './writer.component';

@Component({
	selector: 'app-vc-demo1',
	template: `
        <h3>@ViewChildren + Component</h3>
	<div>
	  <writer name="Krishna" book="Angular Tutorials"></writer> <br/>
	  <writer name="Mahesh" book="Java Tutorials"></writer> <br/>
	  <writer name="Krishna" book="jQuery Tutorials"></writer> <br/>
	  <writer name="Bramha" book="Hibernate Tutorials" *ngIf="allWritersVisible"></writer> <br/>   
	  <writer name="Vishnu" book="Spring Tutorials" *ngIf="allWritersVisible"></writer>  
	</div>
	<button (click)="onShowAllWriters()" >
	  <label *ngIf="!allWritersVisible">Show More</label>
	  <label *ngIf="allWritersVisible">Show Less</label>
	</button>	
  `
})
export class VCOneDemoComponent implements AfterViewInit {
	@ViewChildren(WriterComponent)
	writers1: QueryList<WriterComponent>;

	@ViewChildren(WriterComponent, { read: ElementRef })
	writers2: QueryList<ElementRef>;

	@ViewChildren(WriterComponent, { read: ViewContainerRef })
	writers3: QueryList<ViewContainerRef>;

	allWritersVisible = false;

	ngAfterViewInit() {
		console.log('--- @ViewChildren + Component ---');
		this.writers1.changes.subscribe(list => {
			list.forEach(writer => console.log(writer.writerName + ' - ' + writer.bookName));
		});
		console.log(this.writers1.length);
		console.log("Result with ElementRef:");
		this.writers2.forEach(el => console.log(el));

		console.log("Result with ViewContainerRef:");
		this.writers3.forEach(vref => console.log(vref));
	}
	onShowAllWriters() {
		this.allWritersVisible = (this.allWritersVisible === true) ? false : true;
	}
} 
writer.component.ts
import { Component, Input } from '@angular/core';

@Component({
  selector: 'writer',
  template: `
       {{writerName}} - {{bookName}}
  `
})
export class WriterComponent {
  @Input('name') writerName: string;
  @Input('book') bookName: string;
} 
On click of button, the size of QueryList will be changed and this change can be observed using QueryList.changes.
We can also use template reference variable with @ViewChildren to obtain QueryList of Component. Suppose we have following HTML code.
<writer name="Krishna" book="Angular Tutorials" #bkWriter></writer> <br/>
<writer name="Mahesh" book="Java Tutorials" #bkWriter></writer> <br/>
<writer name="Krishna" book="jQuery Tutorials" #bkWriter></writer> <br/> 
In the above code we have used bkWriter as template reference variable for WriterComponent. Now we will query data as following.
@ViewChildren('bkWriter') 
writers1: QueryList<WriterComponent>; 

@ViewChildren with Directive

We will query Directive using @ViewChildren and embed a message using createEmbeddedView of ViewContainerRef class.
vc-demo2.component.ts
import { Component, ViewChild, ViewChildren, AfterViewInit, ViewContainerRef, TemplateRef, QueryList } from '@angular/core';
import { MessageDirective } from './message.directive';

@Component({
	selector: 'app-vc-demo2',
	template: `
	<h3>@ViewChildren + Directive</h3>
	<div cpMsg></div>
	<div cpMsg></div>
	<div cpMsg></div>	
		
	<ng-template #msgTemp>
          Hello World!
	</ng-template>
   `
})
export class VCTwoDemoComponent implements AfterViewInit {
	@ViewChildren(MessageDirective)
	private msgList: QueryList<MessageDirective>;

	@ViewChild('msgTemp')
	private msgTempRef: TemplateRef<any>;

	ngAfterViewInit() {
		console.log('--- @ViewChildren + Directive ---');
		console.log("this.msgList.length: " + this.msgList.length);
		this.msgList.forEach(messageDirective =>
			messageDirective.viewContainerRef.createEmbeddedView(this.msgTempRef));
	}
} 
We can also use read metadata to read ViewContainerRef directly from the directive.
@ViewChildren(MessageDirective, {read: ViewContainerRef})
private msgList: QueryList<ViewContainerRef>; 

ngAfterViewInit() {
   this.msgList.forEach(vcRef => vcRef.createEmbeddedView(this.msgTempRef));
} 
Find the Directive used in the example.
message.directive.ts
import { Directive, ViewContainerRef } from '@angular/core';

@Directive({
    selector: '[cpMsg]'
})
export class MessageDirective {
    constructor(public viewContainerRef: ViewContainerRef) { }
} 

@ViewChildren with ElementRef

We will use @ViewChildren with ElementRef here.
vc-demo3.component.ts
import { Component, ViewChildren, AfterViewInit, QueryList, ElementRef } from '@angular/core';

@Component({
	selector: 'app-vc-demo3',
	template: `
         <h3>@ViewChildren + ElementRef</h3>
         <div>
	   <div #pname>Mohit</div>
	   <div #pname>Anup</div>
	   <div #pname>Nilesh</div>
         </div>
   `
})
export class VCThreeDemoComponent implements AfterViewInit {
	@ViewChildren('pname')
	persons: QueryList<ElementRef>;

	ngAfterViewInit() {
		console.log('--- @ViewChildren + ElementRef ---');
		this.persons.forEach(el => console.log(el.nativeElement.innerHTML));
	}
} 
In the above code we have some <div> and we have assigned template reference variable to it and we are querying ElementRef.
Now find the other files used in the demo.
app.component.ts
import { Component } from '@angular/core';

@Component({
    selector: 'app-root',
    template: `
       <app-vc-demo1></app-vc-demo1>
       <app-vc-demo2></app-vc-demo2>       
       <app-vc-demo3></app-vc-demo3>        
   `
})
export class AppComponent {
} 
app.module.ts
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';

import { AppComponent }  from './app.component';
import { WriterComponent } from './writer.component';
import { VCOneDemoComponent } from './vc-demo1.component';
import { VCTwoDemoComponent } from './vc-demo2.component';
import { VCThreeDemoComponent } from './vc-demo3.component';
import { MessageDirective } from './message.directive';

@NgModule({
  imports: [     
      BrowserModule
  ],
  declarations: [
      AppComponent,
      WriterComponent,
      VCOneDemoComponent,
      VCTwoDemoComponent,  
      VCThreeDemoComponent,          
      MessageDirective
  ],
  providers: [ ],
  bootstrap: [
      AppComponent
  ]
})
export class AppModule { } 

@ViewChild vs @ViewChildren vs @ContentChild vs @ContentChildren

@ViewChild and @ViewChildren query the elements from view DOM and @ContentChild and @ContentChildren query the elements from content DOM.
1 @ViewChild queries a single element or directive. It will be first element or the Directive matching the selector from the view DOM. It is used as following.
@ViewChild(WriterComponent)
writer: WriterComponent;

@ViewChild(MessageDirective)
message: MessageDirective; 
2. @ViewChildren is used to get the QueryList of elements or directives from the view DOM. It is used as following.
@ViewChildren(WriterComponent) 
writers: QueryList<WriterComponent>;	

@ViewChildren(MessageDirective)
private msgList: QueryList<MessageDirective>; 
3. @ContentChild gives the first element or directive matching the selector from the content DOM. It is used as following.
@ContentChild(BookDirective) 
book: BookDirective; 
4. @ContentChildren is used to get QueryList of elements or directives from the content DOM. It is used as following.
@ContentChildren(BookDirective) 
books: QueryList<BookDirective> 

Run Application

To run the application, find the steps.
1. Download source code using download link given below on this page.
2. Use downloaded src in your Angular CLI application. To install Angular CLI, find the link.
3. Run ng serve using command prompt.
4. Access the URL http://localhost:4200
Find the print screen of the output.
Angular @ViewChildren

References

Angular Doc: ViewChildren
Angular 2/4 QueryList Example
Angular 2/4 @ContentChild and @ContentChildren Example

Download Source Code

POSTED BY
ARVIND RAI
ARVIND RAI
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