警告提示框(Alert)

警告框组件通过提供一些灵活的预定义消息，为常见的用户动作提供常见的上下反馈消息和提示。

示例

警报是一组颜色控件（共八个颜色样式），可用于任何长度的文本，以及可选的关闭按钮，系统提供8个可用的正确的样式（如，.alert-success），如你还可以使用jQuery插件方法来解除内联元素。

This is a primary alert—check it out!

This is a secondary alert—check it out!

This is a success alert—check it out!

This is a danger alert—check it out!

This is a warning alert—check it out!

This is a info alert—check it out!

This is a light alert—check it out!

This is a dark alert—check it out!

<div class="alert alert-primary" role="alert">
  This is a primary alert—check it out!
</div>
<div class="alert alert-secondary" role="alert">
  This is a secondary alert—check it out!
</div>
<div class="alert alert-success" role="alert">
  This is a success alert—check it out!
</div>
<div class="alert alert-danger" role="alert">
  This is a danger alert—check it out!
</div>
<div class="alert alert-warning" role="alert">
  This is a warning alert—check it out!
</div>
<div class="alert alert-info" role="alert">
  This is a info alert—check it out!
</div>
<div class="alert alert-light" role="alert">
  This is a light alert—check it out!
</div>
<div class="alert alert-dark" role="alert">
  This is a dark alert—check it out!
</div>

网页中传达辅助技术及其背后的意义

使用颜色添加意义只提供一个视觉指示，这不会传达给需要辅助技术（如盲人、听力障碍者）的用户，也就决定了诸如屏幕阅读器并不能读出颜色本身的意义。一般推荐确保由颜色表示的信息从内容本身（例如可见文本）中显而易见，或者通过替代方法，例如隐藏在.sr-only该类中的附加文本来创造更多的辅助传达技术。

.sr-only，全称 screen reader only,意为:(仅供)屏幕阅读器,这个 class 主要用于增强 accessbility(可访问性),有时候 UI 上会出现一些仅供视觉识别的元素，比如说“菜单按钮”，只有视力正常的人才能清楚辨识这些元素的作用。而残障人士，比如弱势或盲人是不可能知道这些视觉识别元素是什么的。他们上网使用的是屏幕阅读器，也就是 screen reader（sr），屏幕阅读器需要找到能辨识的文本说明然后“读”出来给用户听。问题是图形元素怎么可能“读出来”呢？因此我们还要写上这些元素的文本说明，但是又不需要展示给普通用户看到，于是加上 sr-only 的意义就在于能保证屏幕阅读器正确读取且不会影响 UI 的视觉呈现（译者补充）。

链接颜色

使用 .alert-link 类可以为带颜色的警告文本框中的链接加上合适的颜色（BootStrap已经内置了相应的颜色解决方案，会自动对应有一个优化后的链接颜色方案-译者注）。

This is a primary alert with an example link. Give it a click if you like.

This is a secondary alert with an example link. Give it a click if you like.

This is a success alert with an example link. Give it a click if you like.

This is a danger alert with an example link. Give it a click if you like.

This is a warning alert with an example link. Give it a click if you like.

This is a info alert with an example link. Give it a click if you like.

This is a light alert with an example link. Give it a click if you like.

This is a dark alert with an example link. Give it a click if you like.

<div class="alert alert-primary" role="alert">
  This is a primary alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>
<div class="alert alert-secondary" role="alert">
  This is a secondary alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>
<div class="alert alert-success" role="alert">
  This is a success alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>
<div class="alert alert-danger" role="alert">
  This is a danger alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>
<div class="alert alert-warning" role="alert">
  This is a warning alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>
<div class="alert alert-info" role="alert">
  This is a info alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>
<div class="alert alert-light" role="alert">
  This is a light alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>
<div class="alert alert-dark" role="alert">
  This is a dark alert with <a href="#" class="alert-link">an example link</a>. Give it a click if you like.
</div>

额外附加内容

警报还可以包含其他HTML元素，如标、段落和分隔符。

Well done!

Aww yeah, you successfully read this important alert message. This example text is going to run a bit longer so that you can see how spacing within an alert works with this kind of content.

Whenever you need to, be sure to use margin utilities to keep things nice and tidy.

<div class="alert alert-success" role="alert">
  <h4 class="alert-heading">Well done!</h4>
  <p>Aww yeah, you successfully read this important alert message. This example text is going to run a bit longer so that you can see how spacing within an alert works with this kind of content.</p>
  <hr>
  <p class="mb-0">Whenever you need to, be sure to use margin utilities to keep things nice and tidy.</p>
</div>

关闭警告（小贴士效果）

使用.alert结合JavaScript，可以实现警报效果，贴在页面上，并可以自由关闭，其要点如下：

确保你正确加载了.alert警报组件，或者是编译后的 Bootstrap JavaScript代码组。
如果你要自行编译JavaScript组件，请将必须的 util.js包括进去。
可以在右上角定义一个.close关闭按钮效果，则需要在容器中引用 .alert-dismissible 类。
警告按钮上要增加data-dismiss="alert" 触发 JavaScript 动作，同时使用<button>元素，以确保在所有设备上都能获得正确的行为响应。
要在关闭警报时生成警报提示，请确保添加.fade和.show样式。

实际效果展示：

Holy guacamole! You should check in on some of those fields below.

<div class="alert alert-warning alert-dismissible fade show" role="alert">
  <strong>Holy guacamole!</strong> You should check in on some of those fields below.
  <button type="button" class="close" data-dismiss="alert" aria-label="Close">
    <span aria-hidden="true">&times;</span>
  </button>
</div>

JavaScript行为

触发

使用JavaScript插件解除警报:

$(".alert").alert()

或者使用 警报控件中的data 属性，如下所示:

<button type="button" class="close" data-dismiss="alert" aria-label="Close">
  <span aria-hidden="true">&times;</span>
</button>

请注意，关闭警报会将其从DOM中移除。

方法

方法	描述
`$().alert()`	Makes an alert listen for click events on descendant elements which have the `data-dismiss="alert"` attribute. (Not necessary when using the data-api’s auto-initialization.)
`$().alert('close')`	Closes an alert by removing it from the DOM. If the `.fade` and `.show` classes are present on the element, the alert will fade out before it is removed.
`$().alert('dispose')`	Destroys an element’s alert.

$(".alert").alert('close')

事件

Bootstrap警报插件提供额外的事件，可以直接取得调用方法和函式。

事件	描述
`close.bs.alert`	This event fires immediately when the `close` instance method is called.
`closed.bs.alert`	This event is fired when the alert has been closed (will wait for CSS transitions to complete).

$('#myAlert').on('closed.bs.alert', function () {
  // do something…
})