angular-cn/aio/content/guide/inputs-outputs.md

# Sharing data between child and parent directives and components

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.

<div class="alert is-helpful">

See the <live-example></live-example> for a working example containing the code snippets in this guide.

</div>

Consider the following hierarchy:

```html
<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.


{@a input}

## Sending data to a child component

The `@Input()` decorator in a child component or directive signifies that the property can receive its value from its parent component.

<div class="lightbox">
  <img src="generated/images/guide/inputs-outputs/input.svg" alt="Input data flow diagram">
</div>

To use `@Input()`, you must configure the parent and child.

### Configuring the child component

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.

<code-example path="inputs-outputs/src/app/item-detail/item-detail.component.ts" region="use-input" header="src/app/item-detail/item-detail.component.ts"></code-example>


In this case, `@Input()` decorates the property <code class="no-auto-link">item</code>, 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:

<code-example path="inputs-outputs/src/app/item-detail/item-detail.component.html" region="property-in-template" header="src/app/item-detail/item-detail.component.html"></code-example>

### Configuring the parent component

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`.

1. Use the child's selector, here `<app-item-detail>`, as a directive within the
parent component template.

2. Use [property binding](guide/property-binding) to bind the `item` property in the child to the `currentItem` property of the parent.

<code-example path="inputs-outputs/src/app/app.component.html" region="input-parent" header="src/app/app.component.html"></code-example>

3. In the parent component class, designate a value for `currentItem`:

<code-example path="inputs-outputs/src/app/app.component.ts" region="parent-property" header="src/app/app.component.ts"></code-example>

With `@Input()`, Angular passes the value for `currentItem` to the child so that `item` renders as `Television`.

The following diagram shows this structure:

<div class="lightbox">
  <img src="generated/images/guide/inputs-outputs/input-diagram-target-source.svg" alt="Property binding diagram">
</div>

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.

### Watching for `@Input()` changes

To watch for changes on an `@Input()` property, you can use `OnChanges`, one of Angular's [lifecycle hooks](guide/lifecycle-hooks).
See the [`OnChanges`](guide/lifecycle-hooks#onchanges) section of the [Lifecycle Hooks](guide/lifecycle-hooks) guide for more details and examples.

{@a output}

## Sending data to a parent component

The `@Output()` decorator in a child component or directive allows data to flow from the child to the parent.

<div class="lightbox">
  <img src="generated/images/guide/inputs-outputs/output.svg" alt="Output diagram">
</div>

`@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.

### Configuring the child component

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.

1. Import `Output` and `EventEmitter` in the child component class:

  ```js
  import { Output, EventEmitter } from '@angular/core';

  ```

1. 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.

  <code-example path="inputs-outputs/src/app/item-output/item-output.component.ts" region="item-output" header="src/app/item-output/item-output.component.ts"></code-example>

  The different parts of the above declaration are as follows:

    * `@Output()`&mdash;a decorator function marking the property as a way for data to go from the child to the parent
    * `newItemEvent`&mdash;the name of the `@Output()`
    * `EventEmitter<string>`&mdash;the `@Output()`'s type
    * `new EventEmitter<string>()`&mdash;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](api/core/EventEmitter).

1. Create an `addNewItem()` method in the same component class:

  <code-example path="inputs-outputs/src/app/item-output/item-output.component.ts" region="item-output-class" header="src/app/item-output/item-output.component.ts"></code-example>

  The `addNewItem()` function uses the `@Output()`, `newItemEvent`, to raise an event with the value the user types into the `<input>`.

### Configuring the child's template

The child's template has two controls.
The first is an HTML `<input>` with a [template reference variable](guide/template-reference-variables) , `#newItem`, where the user types in an item name.
The `value` property of the `#newItem` variable stores what the user types into the `<input>`.

<code-example path="inputs-outputs/src/app/item-output/item-output.component.html" region="child-output" header="src/app/item-output/item-output.component.html"></code-example>

The second element is a `<button>` with a `click` [event binding](guide/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.

### Configuring the parent component

The `AppComponent` in this example features a list of `items` in an array and a method for adding more items to the array.

<code-example path="inputs-outputs/src/app/app.component.ts" region="add-new-item" header="src/app/app.component.ts"></code-example>

The `addItem()` method takes an argument in the form of a string and then adds that string to the `items` array.

### Configuring the parent's template

1. In the parent's template, bind the parent's method to the child's event.

1. Put the child selector, here `<app-item-output>`, within the parent component's template, `app.component.html`.

  <code-example path="inputs-outputs/src/app/app.component.html" region="output-parent" header="src/app/app.component.html"></code-example>

  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:

  ```html
    <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.


## Using `@Input()` and `@Output()` together

You can use `@Input()` and `@Output()` on the same child component as follows:

<code-example path="inputs-outputs/src/app/app.component.html" region="together" header="src/app/app.component.html"></code-example>

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.

<div class="lightbox">
  <img src="generated/images/guide/inputs-outputs/input-output-diagram.svg" alt="Input/Output diagram">
</div>

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](guide/two-way-binding).