`MatSnackBar` is a service for displaying snack-bar notifications.

<!-- example(snack-bar-overview) -->

### Opening a snack-bar
A snack-bar can contain either a string message or a given component.
```ts
// Simple message.
let snackBarRef = snackBar.open('Message archived');

// Simple message with an action.
let snackBarRef = snackBar.open('Message archived', 'Undo');

// Load the given component into the snack-bar.
let snackBarRef = snackbar.openFromComponent(MessageArchivedComponent);
```

In either case, a `MatSnackBarRef` is returned. This can be used to dismiss the snack-bar or to
receive notification of when the snack-bar is dismissed. For simple messages with an action, the
`MatSnackBarRef` exposes an observable for when the action is triggered.
If you want to close a custom snack-bar that was opened via `openFromComponent`, from within the
component itself, you can inject the `MatSnackBarRef`.

```ts
snackBarRef.afterDismissed().subscribe(() => {
  console.log('The snack-bar was dismissed');
});


snackBarRef.onAction().subscribe(() => {
  console.log('The snack-bar action was triggered!');
});

snackBarRef.dismiss();
```

### Dismissal
A snack-bar can be dismissed manually by calling the `dismiss` method on the `MatSnackBarRef`
returned from the call to `open`.

Only one snack-bar can ever be opened at one time. If a new snackbar is opened while a previous
message is still showing, the older message will be automatically dismissed.

A snack-bar can also be given a duration via the optional configuration object:
```ts
snackbar.open('Message archived', 'Undo', {
  duration: 3000
});
```

### Sharing data with a custom snack-bar
You can share data with the custom snack-bar, that you opened via the `openFromComponent` method,
by passing it through the `data` property.

```ts
snackbar.openFromComponent(MessageArchivedComponent, {
  data: 'some data'
});
```

To access the data in your component, you have to use the `MAT_SNACK_BAR_DATA` injection token:

```ts
import {Component, Inject} from '@angular/core';
import {MAT_SNACK_BAR_DATA} from '@angular/material/snack-bar';

@Component({
  selector: 'your-snack-bar',
  template: 'passed in {{ data }}',
})
export class MessageArchivedComponent {
  constructor(@Inject(MAT_SNACK_BAR_DATA) public data: any) { }
}
```

### Setting the global configuration defaults
If you want to override the default snack bar options, you can do so using the
`MAT_SNACK_BAR_DEFAULT_OPTIONS` injection token.

```ts
@NgModule({
  providers: [
    {provide: MAT_SNACK_BAR_DEFAULT_OPTIONS, useValue: {duration: 2500}}
  ]
})
```

### Accessibility
Snack-bar messages are announced via an `aria-live` region. By default, the `polite` setting is
used. While `polite` is recommended, this can be customized by setting the `politeness` property of
the `MatSnackBarConfig`.

Focus is not, and should not be, moved to the snack-bar element. Moving the focus would be
disruptive to a user in the middle of a workflow. It is recommended that, for any action offered
in the snack-bar, the application offer the user an alternative way to perform the action.
Alternative interactions are typically keyboard shortcuts or menu options. When the action is
performed in this way, the snack-bar should be dismissed.

Snack-bars that have an action available should not be given a `duration`, as to accomodate
screen-reader users that want to navigate to the snack-bar element to activate the action. If the
user has manually moved their focus within the snackbar, focus should be placed somewhere sensible
based on the application context when the snack-bar is dismissed.

Don't use "Dismiss" as a snack-bar-action, instead preferring to use a `duration` when there is
no additional action associated with the notification.