snack-bar 概览
MatSnackBar 是一个用来显示快餐栏通知的服务。
MatSnackBar is a service for displaying snack-bar notifications.
快餐栏可以包含一个字符串消息或指定的组件。
A snack-bar can contain either a string message or a given component.
// 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);无论哪种形式，都会返回一个 MatSnackBarRef。它可以用来关闭快餐栏或在快餐栏关闭时接收通知。
对于只有一个操作的简单消息，当该动作被触发时，MatSnackBarRef 会暴露出一个 Observable。
如果你要关闭一个用 openFromComponent 打开的自定义快餐栏，可以在该组件中注入一个 MatSnackBarRef。
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.
snackBarRef.afterDismissed().subscribe(() => {
console.log('The snack-bar was dismissed');
});
snackBarRef.onAction().subscribe(() => {
console.log('The snack-bar action was triggered!');
});
snackBarRef.dismiss();
可以调用由 open 调用返回的 MatSnackBarRef 中的 dismiss 方法来手动关闭快餐栏。
A snack-bar can be dismissed manually by calling the dismiss method on the MatSnackBarRef
returned from the call to open.
同一时刻只能打开一个快餐栏。如果在显示前一个消息时打开一个新的快餐栏，老的消息就会自动关闭。
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.
快餐栏还可以通过一个可选的配置对象来指定持续时间：
A snack-bar can also be given a duration via the optional configuration object:
snackbar.open('Message archived', 'Undo', {
duration: 3000
});
你可以传入 data 属性，来与 openFromComponent 打开的自定义快餐栏共享数据。
You can share data with the custom snack-bar, that you opened via the openFromComponent method,
by passing it through the data property.
snackbar.openFromComponent(MessageArchivedComponent, {
data: 'some data'
});要在组件中访问该数据，可以使用依赖注入令牌 MAT_SNACK_BAR_DATA：
To access the data in your component, you have to use the MAT_SNACK_BAR_DATA injection token:
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: string) { }
}
如果你要覆盖快餐栏的默认选项，可以使用 MAT_SNACK_BAR_DEFAULT_OPTIONS 令牌。
If you want to override the default snack bar options, you can do so using the
MAT_SNACK_BAR_DEFAULT_OPTIONS injection token.
@NgModule({
providers: [
{provide: MAT_SNACK_BAR_DEFAULT_OPTIONS, useValue: {duration: 2500}}
]
})
快餐栏消息会通过 aria-live 进行声明。默认情况下，它会设置为 polite。虽然建议使用 polite，但是也可以通过设置 MatSnackBarConfig 的 politeness 属性进行定制。
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.
焦点不会也不应该自动移到快餐栏元素上，否则会打断用户的工作流。建议的方式是，对于快餐栏中提供的任何操作， 都应该为用户提供一种替代途径来触发 —— 通常会用键盘快捷键或菜单项。当用这些方式执行完之后，应该自动关闭快餐栏。
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 offers 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.
具有可用动作的快餐栏不应该指定持续时间（duration），以支持那些希望导航到快餐栏中进行操作的屏幕阅读器用户。
如果用户手动把焦点移到了快餐栏中，那么当快餐栏关闭时，焦点应该根据应用上下文移到某些有意义的元素上。
Snack-bars that have an action available should not be given a duration, as to accommodate
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.
当没有与该通知有关的操作时，不要把 "关闭" 作为快餐栏的标准操作，而应该优先使用 duration 来自动关闭它。
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.