angular-docs-cn/aio/content/guide/event-binding.md

# Event binding

Event binding allows you to listen for and respond to user actions such as keystrokes, mouse movements, clicks, and touches.

<div class="alert is-helpful">

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

</div>

## Binding to events

To bind to an event you use the Angular event binding syntax.
This syntax consists of a target event name within parentheses to the left of an equal sign, and a quoted template statement to the right.
In the following example, the target event name is `click` and the template statement is `onSave()`.

<code-example language="html" header="Event binding syntax">
&lt;button (click)="onSave()"&gt;Save&lt;/button&gt;
</code-example>

The event binding listens for the button's click events and calls the component's `onSave()` method whenever a click occurs.

<div class="lightbox">
  <img src='generated/images/guide/template-syntax/syntax-diagram.svg' alt="Syntax diagram">
</div>

## Binding to passive events

Angular also supports passive event listeners. For example, you can use the following steps to make a scroll event passive.

1. Create a file `zone-flags.ts` under `src` directory.
2. Add the following line into this file.

```
(window as any)['__zone_symbol__PASSIVE_EVENTS'] = ['scroll'];
```

3. In the `src/polyfills.ts` file, before importing zone.js, import the newly created `zone-flags`.

```
import './zone-flags';
import 'zone.js';  // Included with Angular CLI.
```

After those steps, if you add event listeners for the `scroll` event, the listeners will be `passive`.

## Custom events with `EventEmitter`

[Directives](guide/built-in-directives) typically raise custom events with an Angular [EventEmitter](api/core/EventEmitter) as follows.

1. The directive creates an `EventEmitter` and exposes it as a property.
1. The directive then calls `EventEmitter.emit(data)` to emit an event, passing in message data, which can be anything.
1. Parent directives listen for the event by binding to this property and accessing the data through the `$event` object.

Consider an `ItemDetailComponent` that presents item information and responds to user actions.
Although the `ItemDetailComponent` has a delete button, it doesn't contain the functionality to delete the hero.
It can only raise an event reporting the user's delete request.


<code-example path="event-binding/src/app/item-detail/item-detail.component.html" header="src/app/item-detail/item-detail.component.html (template)" region="line-through"></code-example>

The component defines a `deleteRequest` property that returns an `EventEmitter`.
When the user clicks **Delete**, the component invokes the `delete()` method, telling the `EventEmitter` to emit an `Item` object.

<code-example path="event-binding/src/app/item-detail/item-detail.component.ts" header="src/app/item-detail/item-detail.component.ts (deleteRequest)" region="deleteRequest"></code-example>

The hosting parent component binds to the `deleteRequest` event of the `ItemDetailComponent` as follows.

<code-example path="event-binding/src/app/app.component.html" header="src/app/app.component.html (event-binding-to-component)" region="event-binding-to-component"></code-example>

When the `deleteRequest` event fires, Angular calls the parent component's `deleteItem()` method with the item.

### Determining an event target

To determine an event target, Angular checks if the name of the target event matches an event property of a known directive.
In the following example, Angular checks to see if `myClick` is an event on the custom `ClickDirective`.

<code-example path="event-binding/src/app/app.component.html" region="custom-directive" header="src/app/app.component.html"></code-example>

If the target event name, `myClick` fails to match an element event or an output property of `ClickDirective`, Angular reports an "unknown directive" error.

## What's next

For more information on how event binding works, see [How event binding works](guide/event-binding-concepts).