Your app will often need a way to let users interact with controls that
change application state. For example, imagine that you have a template
that shows a blog title, and supports expanding the post to show the body.

If you add the
[`{{action}}`](http://emberjs.com/api/classes/Ember.Templates.helpers.html#method_action)
helper to an HTML element, when a user clicks the element, the named event
will be sent to the template's corresponding component or controller.

```app/templates/components/single-post.hbs
<h3 {{action "toggleBody"}}>{{title}}</h3>
{{#if isShowingBody}}
  <p>{{{body}}}</p>
{{/if}}
```

In the component or controller, you can then define what the action does within
the `actions` hook:

```app/components/single-post.js
export default Ember.Component.extend({
  actions: {
    toggleBody() {
      this.toggleProperty('isShowingBody');
    }
  }
});
```

## Action Parameters

You can optionally pass arguments to the action handler. Any values
passed to the `{{action}}` helper after the action name will be passed to
the handler as arguments.

For example, if the `post` argument was passed:

```handlebars
<p><button {{action "select" post}}>✓</button> {{post.title}}</p>
```

The `select` action handler would be called with a single argument
containing the post model:

```app/components/single-post.js
export default Ember.Component.extend({
  actions: {
    select(post) {
      console.log(post.get('title'));
    }
  }
});
```

## Specifying the Type of Event

By default, the
[`{{action}}`](http://emberjs.com/api/classes/Ember.Templates.helpers.html#method_action)
helper listens for click events and triggers the action when the user clicks
on the element.

You can specify an alternative event by using the `on` option.

```handlebars
<p>
  <button {{action "select" post on="mouseUp"}}>✓</button>
  {{post.title}}
</p>
```

You should use the <code>camelCased</code> event names, so two-word names like `keypress`
become `keyPress`.

## Allowing Modifier Keys

By default the `{{action}}` helper will ignore click events with
pressed modifier keys. You can supply an `allowedKeys` option
to specify which keys should not be ignored.

```handlebars
<button {{action "anActionName" allowedKeys="alt"}}>
  click me
</button>
```

This way the `{{action}}` will fire when clicking with the alt key
pressed down.

## Allowing Default Browser Action

By default, the `{{action}}` helper prevents the default browser action of the
DOM event. If you want to allow the browser action, you can stop Ember from
preventing it.

For example, if you have a normal link tag and want the link to bring the user
to another page in addition to triggering an ember action when clicked, you can
use `preventDefault=false`:

```handlebars
<a href="newPage.htm" {{action "logClick" preventDefault=false}}>Go</a>
```

Without `preventDefault=false`, if the user clicked on the link, Ember.js
will trigger the action, but the user will remain on the current page.

With `preventDefault=false`, if the user clicked on the link, Ember.js
will trigger the action *and* the user will be directed to the new page.

## Modifying the action's first parameter

If a `value` option for the
[`{{action}}`](http://emberjs.com/api/classes/Ember.Templates.helpers.html#method_action)
helper is specified, its value will be considered a property path that will
be read off of the first parameter of the action. This comes very handy with
event listeners and enables to work with one-way bindings.

```handlebars
<label>What's your favorite band?</label>
<input type="text" value={{favoriteBand}} onblur={{action "bandDidChange"}}/>
```

Let's assume we have an action handler that prints its first parameter:

```js
actions: {
  bandDidChange(newValue) {
    console.log(newValue);
  }
}
```

By default, the action handler receives the first parameter of the event
listener, the event object the browser passes to the handler, so
`bandDidChange` prints `Event {}`.

Using the `value` option modifies that behavior by extracting that property from
the event object:

```handlebars
<label>What's your favorite band?</label>
<input type="text" value={{favoriteBand}} onblur={{action "bandDidChange" value="target.value"}}/>
```

The `newValue` parameter thus becomes the `target.value` property of the event
object, which is the value of the input field the user typed. (e.g 'Foo Fighters')

## Attaching Actions to Non-Clickable Elements

Note that actions may be attached to any element of the DOM, but not all
respond to the `click` event. For example, if an action is attached to an `a`
link without an `href` attribute, or to a `div`, some browsers won't execute
the associated function. If it's really needed to define actions over such
elements, a CSS workaround exists to make them clickable, `cursor: pointer`.
For example:

```css
[data-ember-action] {
  cursor: pointer;
}
```
