A common pattern in Angular is sharing data between a parent component and one or more child components. You can implement this pattern by using the @Input()
and @Output()
directives.
See the live example for a working example containing the code snippets in this guide.
Consider the following hierarchy:
<parent-component> <child-component></child-component> </parent-component>
The <parent-component>
serves as the context for the <child-component>
.
@Input()
and @Output()
give a child component a way to communicate with its parent component. @Input()
allows a parent component to update data in the child component. Conversely, @Output()
allows the child to send data to a parent component.
The @Input()
decorator in a child component or directive signifies that the property can receive its value from its parent component.
To use @Input()
, you must configure the parent and child.
To use the @Input()
decorator in a child component class, first import Input
and then decorate the property with @Input()
, as in the following example.
import { Component, Input } from '@angular/core'; // First, import Input export class ItemDetailComponent { @Input() item: string; // decorate the property with @Input() }
In this case, @Input()
decorates the property item
, which has a type of string
, however, @Input()
properties can have any type, such as number
, string
, boolean
, or object
. The value for item
comes from the parent component.
Next, in the child component template, add the following:
<p> Today's item: {{item}} </p>
The next step is to bind the property in the parent component's template. In this example, the parent component template is app.component.html
.
Use the child's selector, here <app-item-detail>
, as a directive within the parent component template.
Use property binding to bind the item
property in the child to the currentItem
property of the parent.
<app-item-detail [item]="currentItem"></app-item-detail>
currentItem
:export class AppComponent { currentItem = 'Television'; }
With @Input()
, Angular passes the value for currentItem
to the child so that item
renders as Television
.
The following diagram shows this structure:
The target in the square brackets, []
, is the property you decorate with @Input()
in the child component. The binding source, the part to the right of the equal sign, is the data that the parent component passes to the nested component.
@Input()
changesTo watch for changes on an @Input()
property, you can use OnChanges
, one of Angular's lifecycle hooks. See the OnChanges
section of the Lifecycle Hooks guide for more details and examples.
The @Output()
decorator in a child component or directive allows data to flow from the child to the parent.
@Output()
marks a property in a child component as a doorway through which data can travel from the child to the parent.
The child component uses the @Output()
property to raise an event to notify the parent of the change. To raise an event, an @Output()
must have the type of EventEmitter
, which is a class in @angular/core
that you use to emit custom events.
The following example shows how to set up an @Output()
in a child component that pushes data from an HTML <input>
to an array in the parent component.
To use @Output()
, you must configure the parent and child.
The following example features an <input>
where a user can enter a value and click a <button>
that raises an event. The EventEmitter
then relays the data to the parent component.
Import Output
and EventEmitter
in the child component class:
import { Output, EventEmitter } from '@angular/core';
In the component class, decorate a property with @Output()
. The following example newItemEvent
@Output()
has a type of EventEmitter
, which means it's an event.
@Output() newItemEvent = new EventEmitter<string>();
The different parts of the above declaration are as follows:
@Output()
—a decorator function marking the property as a way for data to go from the child to the parentnewItemEvent
—the name of the @Output()
EventEmitter<string>
—the @Output()
's typenew EventEmitter<string>()
—tells Angular to create a new event emitter and that the data it emits is of type string.For more information on EventEmitter
, see the EventEmitter API documentation.
Create an addNewItem()
method in the same component class:
export class ItemOutputComponent { @Output() newItemEvent = new EventEmitter<string>(); addNewItem(value: string) { this.newItemEvent.emit(value); } }
The addNewItem()
function uses the @Output()
, newItemEvent
, to raise an event with the value the user types into the <input>
.
The child's template has two controls. The first is an HTML <input>
with a template reference variable , #newItem
, where the user types in an item name. The value
property of the #newItem
variable stores what the user types into the <input>
.
<label for="item-input">Add an item:</label> <input type="text" id="item-input" #newItem> <button (click)="addNewItem(newItem.value)">Add to parent's list</button>
The second element is a <button>
with a click
event binding.
The (click)
event is bound to the addNewItem()
method in the child component class. The addNewItem()
method takes as its argument the value of the #newItem.value
property.
The AppComponent
in this example features a list of items
in an array and a method for adding more items to the array.
export class AppComponent { items = ['item1', 'item2', 'item3', 'item4']; addItem(newItem: string) { this.items.push(newItem); } }
The addItem()
method takes an argument in the form of a string and then adds that string to the items
array.
In the parent's template, bind the parent's method to the child's event.
Put the child selector, here <app-item-output>
, within the parent component's template, app.component.html
.
<app-item-output (newItemEvent)="addItem($event)"></app-item-output>
The event binding, (newItemEvent)='addItem($event)'
, connects the event in the child, newItemEvent
, to the method in the parent, addItem()
.
The $event
contains the data that the user types into the <input>
in the child template UI.
To see the @Output()
working, you can add the following to the parent's template:
<ul> <li *ngFor="let item of items">{{item}}</li> </ul>
The *ngFor
iterates over the items in the items
array. When you enter a value in the child's <input>
and click the button, the child emits the event and the parent's addItem()
method pushes the value to the items
array and new item renders in the list.
@Input()
and @Output()
togetherYou can use @Input()
and @Output()
on the same child component as follows:
<app-input-output [item]="currentItem" (deleteRequest)="crossOffItem($event)"></app-input-output>
The target, item
, which is an @Input()
property in the child component class, receives its value from the parent's property, currentItem
. When you click delete, the child component raises an event, deleteRequest
, which is the argument for the parent's crossOffItem()
method.
The following diagram shows the different parts of the @Input()
and @Output()
on the <app-input-output>
child component.
The child selector is <app-input-output>
with item
and deleteRequest
being @Input()
and @Output()
properties in the child component class. The property currentItem
and the method crossOffItem()
are both in the parent component class.
To combine property and event bindings using the banana-in-a-box syntax, [()]
, see Two-way Binding.
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Licensed under the Creative Commons Attribution License 4.0.
https://v11.angular.io/guide/inputs-outputs