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Angular 2/4 @ContentChild and @ContentChildren Example

By Arvind Rai, September 13, 2017
This page will walk through Angular @ContentChild and @ContentChildren decorator example. They are used to fetch first or all elements from content DOM. @ContentChild gives first element matching the selector from the content DOM. @ContentChildren gives all elements of content DOM as QueryList. Contents queried by @ContentChild and @ContentChildren are set before ngAfterContentInit() is called. If we do any change in content DOM for the matching selector, that will be observed by @ContentChild and @ContentChildren and we will get updated value. As a selector for @ContentChild and @ContentChildren, we can pass directive, component or local template variable. By default @ContentChildren only selects direct children of content DOM and not all descendants. @ContentChildren has a metadata descendants and setting its value true, we can fetch all descendant elements. Here on this page we will provide @ContentChild and @ContentChildren example using directive, component and ElementRef . Now find the complete example step by step.

Technologies Used

Find the technologies being used in our example.
1. Angular 4.2.4
2. TypeScript 2.3.3
3. Node.js 6.10.1
4. Angular CLI 1.3.1
5. Angular Compiler CLI 4.2.4

Project Structure

Find the project structure of our demo application.
angular-demo
|
|--src
|   |
|   |--app 
|   |   |
|   |   |--app.module.ts
|   |   |--app.component.ts
|   |   |--book.directive.ts
|   |   |--writer.component.ts
|   |   |--favourite-books.component.ts
|   |   |--city.component.ts
|   |   |--address.component.ts
|   |   |--favourite-cities.component.ts
|   |   |--friend.component.ts
|   |   |--favourite-friends.component.ts
|   |   |--person.component.ts
|   |   |--person.component.html
|   |   
|   |--main.ts
|   |--index.html
|   |--styles.css
|
|--node_modules
|--package.json

@ContentChild and @ContentChildren

@ContentChild and @ContentChildren both are decorators. They are used to fetch single child element or all child elements from content DOM. Let us understand more about it.

@ContentChild

@ContentChild gives the first element or directive matching the selector from the content DOM. If new child element replaces the old one matching the selector in content DOM, then property will also be updated. @ContentChild has following metadata properties.
selector: Directive type or the name used for querying. Find the example when type is directive.
@ContentChild(BookDirective) book: BookDirective; 
read: This is optional metadata. It reads a different token from the queried element.

@ContentChildren

@ContentChildren is used to get QueryList of elements or directives from the content DOM. When there is change in content DOM, data in QueryList will also change. If child elements are added, we will get those new elements in QueryList. If child elements are removed, then those elements will be removed from the QueryList. The metadata properties of @ContentChildren are as follows.
selector: Directive type or the name used for querying. Find the example when type is directive.
@ContentChildren(BookDirective) topBooks: QueryList<BookDirective>
descendants: This is Boolean value. When it is true then direct children and other descendants will also be included. If the value is false then only direct children will be included. descendants is used as follows.
@ContentChildren(BookDirective, {descendants: true}) allBooks: QueryList<BookDirective>
The default value of descendants is false.
read: This is optional metadata. It reads a different token from the queried element.

Using AfterContentInit

AfterContentInit is a lifecycle hook that is called after directive content is fully initialized. It has a method ngAfterContentInit(). This method runs after angular loads external content into the component view. This method runs once after first ngDoCheck() method. Contents queried by @ContentChild and @ContentChildren are set before ngAfterContentInit() is called. AfterContentInit is used as given below.
@Component({
  selector: 'friend',
  template: ``
})
export class FriendComponent implements AfterContentInit {
	@ContentChild('name') nameRef: ElementRef;
	ngAfterContentInit() {
	   console.log(this.nameRef.nativeElement.innerHTML);
	}
}

Example 1: @ContentChild and @ContentChildren using Directive

Find the example of @ContentChild and @ContentChildren decorators using directive. First we will create a directive with selector as element name.
book.directive.ts
import { Directive, Input } from '@angular/core';

@Directive({
    selector: 'book'
})
export class BookDirective {
    @Input() bookId: string;
    @Input() bookName: string;
} 
In the above directive we have used two @Input() properties. <book> element can be used in any component. Now create a component to use @ContentChild decorator to query element of type <book>.
writer.component.ts
import { Component, ContentChild } from '@angular/core';
import { BookDirective } from './book.directive';

@Component({
  selector: 'writer',
  template: `
        Name: {{writerName}}
	<br/>Latest Book: {{book?.bookId}} - {{book?.bookName}} 
  `
})
export class WriterComponent {
	@ContentChild(BookDirective) book: BookDirective;
	writerName = 'Mahesh';
} 
Find the code snippet of person.component.html to use <writer> and <book> elements.
<writer>
  <book bookId="1" bookName="Java 8 Tutorials" *ngIf="latestBook"></book>
  <book bookId="2" bookName="Learning Angular 4" *ngIf="!latestBook"></book>
</writer>
<br/><button (click)="onChangeBook()">Change Book</button> 
Find the code snippet of person.component.ts.
latestBook = true;
onChangeBook() {
   this.latestBook = (this.latestBook === true)? false : true;
}
Now we will create the example of @ContentChildren using directive.
favourite-books.component.ts
import { Component, ContentChildren, QueryList } from '@angular/core';
import { BookDirective } from './book.directive';

@Component({
  selector: 'favourite-books',
  template: `
        <b>Top Favourite Books</b>
	<ng-template ngFor let-book [ngForOf]= "topBooks">
	   <br/>{{book.bookId}} - {{book.bookName}}
	</ng-template>
		
	<br/><b>All Favorite Books</b>
	<ng-template ngFor let-book [ngForOf]= "allBooks">
	   <br/>{{book.bookId}} - {{book.bookName}}
	</ng-template>	
  `
})
export class FavouriteBooksComponent {
    @ContentChildren(BookDirective) topBooks: QueryList<BookDirective>
    @ContentChildren(BookDirective, {descendants: true}) allBooks: QueryList<BookDirective>
} 
In the above component we are using @ContentChildren two times, one with default descendants and second with descendants with true value. Find the code snippet of person.component.html to use <favourite-books> and <book> elements.
<favourite-books>
  <book bookId="1" bookName="Hibernate 4 Tutorials"></book>
  <book bookId="2" bookName="Spring Boot Tutorials"></book>
  <favourite-books>
    <book bookId="3" bookName="Learning JavaScript"></book>
  </favourite-books>   
  <favourite-books *ngIf="showAllBook">
    <book bookId="4" bookName="Thymeleaf Tutorials"></book>
    <book bookId="5" bookName="Android Tutorials"></book>
  </favourite-books>   
</favourite-books>
<br/><button (click)="onShowAllBooks()" >
  <label *ngIf="!showAllBook">Show More</label>
  <label *ngIf="showAllBook">Show Less</label>
</button> 
Find the code snippet of person.component.ts.
showAllBook = false;
onShowAllBooks() {
   this.showAllBook = (this.showAllBook === true)? false : true;
} 
Now find the print screen of the output of @ContentChild and @ContentChildren decorators using directive.
Angular 2/4 @ContentChild and @ContentChildren Example

Example 2: @ContentChild and @ContentChildren using Component

Find the example of @ContentChild and @ContentChildren decorators using component. Here for child element we will create a component instead of directive.
city.component.ts
import { Component, Input } from '@angular/core';

@Component({
  selector: 'city',
  template: ``
})
export class CityComponent {
    @Input() cityId: string;	
    @Input() cityName: string;
} 
In the above component, we have used two @Input() properties. Now find the component that will use @ContentChild.
address.component.ts
import { Component, ContentChild } from '@angular/core';
import { CityComponent } from './city.component';

@Component({
  selector: 'address',
  template: `
        <b>{{title}}</b>
	<br/>City: {{city?.cityId}} - {{city?.cityName}} 
  `
})
export class AddressComponent {
	@ContentChild(CityComponent) city: CityComponent;
	title = 'Address';
} 
Find the code snippet of person.component.html to use <address> and <city> element.
<address>
  <city cityId="1" cityName="Varanasi" *ngIf="homeTown"></city>
  <city cityId="2" cityName="Noida" *ngIf="!homeTown"></city>
</address>
<br/><button (click)="onChangeCity()">Change City</button> 
Find the code snippet of person.component.ts.
homeTown = true;
onChangeCity() {
   this.homeTown = (this.homeTown === true)? false : true;
}
Now find the example for @ContentChildren using component.
favourite-cities.component.ts
import { Component, ContentChildren, QueryList } from '@angular/core';
import { CityComponent } from './city.component';

@Component({
  selector: 'favourite-cities',
  template: `
        <b>Top Favourite  Cities</b>
	<ng-template ngFor let-city [ngForOf]= "topCities">
	   <br/>{{city.cityId}} - {{city.cityName}}
	</ng-template>
		
	<br/><b>All Favourite Cities</b>
	<ng-template ngFor let-city [ngForOf]= "allCities">
	   <br/>{{city.cityId}} - {{city.cityName}}
	</ng-template>	
  `
})
export class FavouriteCitiesComponent {
    @ContentChildren(CityComponent) topCities: QueryList<CityComponent>
    @ContentChildren(CityComponent, {descendants: true}) allCities: QueryList<CityComponent>
} 
Find the code snippet of person.component.html to use <favourite-cities> and <city> element.
<favourite-cities>
  <city cityId="1" cityName="Noida"></city>
  <city cityId="2" cityName="Mumbai"></city>
  <favourite-cities>
    <city cityId="3" cityName="Gurugram"></city>
  </favourite-cities>   
  <favourite-cities *ngIf="showAllCity">
    <city cityId="4" cityName="New Delhi"></city>
    <city cityId="5" cityName="Bengaluru"></city>
  </favourite-cities>   
</favourite-cities>
<br/><button (click)="onShowAllCities()" >
  <label *ngIf="!showAllCity">Show More</label>
  <label *ngIf="showAllCity">Show Less</label>
</button> 
Find the code snippet of person.component.ts.
showAllCity = false;	
onShowAllCities() {
   this.showAllCity = (this.showAllCity === true)? false : true;
}
Now find the print screen of the output of @ContentChild and @ContentChildren decorators using component.
Angular 2/4 @ContentChild and @ContentChildren Example

Example 3: @ContentChild and @ContentChildren using ElementRef

Find the example of @ContentChild and @ContentChildren decorators using ElementRef. First find the component that will use @ContentChild with ElementRef.
friend.component.ts
import { Component, ContentChild, ElementRef, AfterContentInit } from '@angular/core';

@Component({
  selector: 'friend',
  template: `
	Friend Name: {{friendName}}
  `
})
export class FriendComponent implements AfterContentInit {
	@ContentChild('name') nameRef: ElementRef;
	
	get friendName(): String {
	   return this.nameRef.nativeElement.innerHTML;   
	}
	ngAfterContentInit() {
	   console.log(this.friendName);
	}
} 
In the above component name inside @ContentChild('name') is the local template variable of a HTML element. Find the code snippet of person.component.html to use <friend> with a <div> element.
<friend>
  <div #name *ngIf="bestFriend">Mahesh</div>
  <div #name *ngIf="!bestFriend">Krishna</div>
</friend>
<br/><button (click)="onChangeFriend()">Change Friend</button> 
Find the code snippet of person.component.ts.
bestFriend = true;   
onChangeFriend() {
   this.bestFriend = (this.bestFriend === true)? false : true;
}
Now find the example for @ContentChildren with ElementRef.
favourite-friends.component.ts
import { Component, ContentChildren, QueryList, ElementRef, AfterContentInit } from '@angular/core';

@Component({
  selector: 'favourite-friends',
  template: `
	<b>All Favourite Friends</b>
	<br/> {{allFriends}}	
  `
})
export class FavouriteFriendsComponent implements AfterContentInit {
        @ContentChildren('name') allFriendsRef: QueryList<ElementRef> ;
	
	get allFriends(): string {
	   return this.allFriendsRef ? this.allFriendsRef.map(f =>f.nativeElement.innerHTML).join(', ') : '';
	}
	ngAfterContentInit() {
	   console.log(this.allFriends);
	}	
} 
In the above component name inside @ContentChildren('name') is the local template variable of a HTML element. Find the code snippet of person.component.html to use <favourite-friends> with <div> element.
<favourite-friends>
  <div #name>Mohit</div>
  <div #name>Anup</div>
  <div #name *ngIf="showAllFriend">Nilesh</div>
  <div #name *ngIf="showAllFriend">Sravan</div>
</favourite-friends>
<br/><button (click)="onShowAllFriends()" >
  <label *ngIf="!showAllFriend">Show More</label>
  <label *ngIf="showAllFriend">Show Less</label>
</button> 
Find the code snippet of person.component.ts.
showAllFriend = false;
onShowAllFriends() {
   this.showAllFriend = (this.showAllFriend === true)? false : true;
}
Now find the print screen of the output of @ContentChild and @ContentChildren decorators using ElementRef.
Angular 2/4 @ContentChild and @ContentChildren Example

Other Components and Application Module used in Example

person.component.ts
import { Component } from '@angular/core';

@Component({
   selector: 'person-app',
   templateUrl: './person.component.html'
})
export class PersonComponent {
   latestBook = true;
   showAllBook = false;
   homeTown = true; 	   
   showAllCity = false;	
   bestFriend = true;    
   showAllFriend = false;
   onChangeBook() {
	   this.latestBook = (this.latestBook === true)? false : true;
   }
   onShowAllBooks() {
	   this.showAllBook = (this.showAllBook === true)? false : true;
   }     
   onChangeCity() {
	   this.homeTown = (this.homeTown === true)? false : true;
   }
   onShowAllCities() {
	   this.showAllCity = (this.showAllCity === true)? false : true;
   }
   onChangeFriend() {
	   this.bestFriend = (this.bestFriend === true)? false : true;
   }
   onShowAllFriends() {
	   this.showAllFriend = (this.showAllFriend === true)? false : true;
   }
} 
person.component.html
<h3>ContentChild using Directive</h3>
<writer>
  <book bookId="1" bookName="Java 8 Tutorials" *ngIf="latestBook"></book>
  <book bookId="2" bookName="Learning Angular 4" *ngIf="!latestBook"></book>
</writer>
<br/><button (click)="onChangeBook()">Change Book</button>

<h3>ContentChildren using Directive</h3>
<favourite-books>
	<book bookId="1" bookName="Hibernate 4 Tutorials"></book>
	<book bookId="2" bookName="Spring Boot Tutorials"></book>
	<favourite-books>
	   <book bookId="3" bookName="Learning JavaScript"></book>
	</favourite-books>   
	<favourite-books *ngIf="showAllBook">
	   <book bookId="4" bookName="Thymeleaf Tutorials"></book>
	   <book bookId="5" bookName="Android Tutorials"></book>
	</favourite-books>   
</favourite-books>
<br/><button (click)="onShowAllBooks()" >
  <label *ngIf="!showAllBook">Show More</label>
  <label *ngIf="showAllBook">Show Less</label>
</button>	

<h3>ContentChild using Component</h3>
<address>
  <city cityId="1" cityName="Varanasi" *ngIf="homeTown"></city>
  <city cityId="2" cityName="Noida" *ngIf="!homeTown"></city>
</address>
<br/><button (click)="onChangeCity()">Change City</button>

<h3>ContentChildren using Component</h3>
<favourite-cities>
	<city cityId="1" cityName="Noida"></city>
	<city cityId="2" cityName="Mumbai"></city>
	<favourite-cities>
	   <city cityId="3" cityName="Gurugram"></city>
	</favourite-cities>   
	<favourite-cities *ngIf="showAllCity">
	   <city cityId="4" cityName="New Delhi"></city>
	   <city cityId="5" cityName="Bengaluru"></city>
	</favourite-cities>   
</favourite-cities>
<br/><button (click)="onShowAllCities()" >
  <label *ngIf="!showAllCity">Show More</label>
  <label *ngIf="showAllCity">Show Less</label>
</button>	

<h3>ContentChild using ElementRef</h3>
<friend>
  <div #name *ngIf="bestFriend">Mahesh</div>
  <div #name *ngIf="!bestFriend">Krishna</div>
</friend>
<br/><button (click)="onChangeFriend()">Change Friend</button>

<h3>ContentChildren using ElementRef</h3>
<favourite-friends>
    <div #name>Mohit</div>
    <div #name>Anup</div>
    <div #name *ngIf="showAllFriend">Nilesh</div>
    <div #name *ngIf="showAllFriend">Sravan</div>
</favourite-friends>
<br/><button (click)="onShowAllFriends()" >
  <label *ngIf="!showAllFriend">Show More</label>
  <label *ngIf="showAllFriend">Show Less</label>
</button> 
app.component.ts
import { Component } from '@angular/core';

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

import { AppComponent }  from './app.component';
import { BookDirective }  from './book.directive';
import { WriterComponent }  from './writer.component';
import { FavouriteBooksComponent }  from './favourite-books.component';
import { CityComponent }  from './city.component';
import { AddressComponent }  from './address.component';
import { FavouriteCitiesComponent }  from './favourite-cities.component';
import { FriendComponent }  from './friend.component';
import { FavouriteFriendsComponent }  from './favourite-friends.component';
import { PersonComponent }  from './person.component';

@NgModule({
  imports: [     
        BrowserModule
  ],
  declarations: [
        AppComponent,
	BookDirective,
	WriterComponent,
	FavouriteBooksComponent,
	CityComponent,
	AddressComponent,
	FavouriteCitiesComponent,
	FriendComponent,
	FavouriteFriendsComponent,			
	PersonComponent
  ],
  providers: [

  ],
  bootstrap: [
        AppComponent
  ]
})
export class AppModule { } 

Run Application

To run the application, find following 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. Now access the URL http://localhost:4200

References

ContentChildDecorator
ContentChildrenDecorator

Download Source Code

POSTED BY
ARVIND RAI
ARVIND RAI
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