页面可见性 API

一些例子：

• 网站有图片轮播效果，只有在用户观看轮播的时候，才会自动展示下一张幻灯片。
• 显示信息仪表盘的应用程序不希望在页面不可见时轮询服务器进行更新。
• 页面想要检测是否正在渲染，以便可以准确的计算网页浏览量
• 当设备进入待机模式时，网站想要关闭设备声音（用户按下电源键关闭屏幕）

开发者在过去使用不完善的代理来检测页面的可见性。比如，通过监听 blur 和 focus 事件来了解页面是否处于活动状态，但是它并没有告诉你页面是对用户隐藏的。页面可见性 API 解决了这个问题。

在页面可见性 API 之外，用户代理会采取许多策略来减轻后台或隐藏选项卡对性能的影响。这些可能包括：

• 大多数浏览器不会调用被隐藏的标签页或<iframe>框架当中requestAnimationFrame()定义的回调函数，这会提升性能并且延长电池的使用寿命。
• 在后台标签页或不活跃的标签页中 setTimeout() 等定时器会受到一定的限制以提升性能。参见实际延时比设定值更久的原因。
• Budget-based background timeout throttling is now available in modern browsers (Firefox 58+, Chrome 57+), placing an additional limit on background timer CPU usage. This operates in a similar way across modern browsers, with the details being as follows:
  • In Firefox, windows in background tabs each have their own time budget in milliseconds — a max and a min value of +50 ms and -150 ms, respectively. Chrome is very similar except that the budget is specified in seconds.
  • Windows are subjected to throttling after 30 seconds, with the same throttling delay rules as specified for window timers (again, see Reasons for delays longer than specified). In Chrome, this value is 10 seconds.
  • Timer tasks are only permitted when the budget is non-negative.
  • Once a timer's code has finished running, the duration of time it took to execute is subtracted from its window's timeout budget.
  • The budget regenerates at a rate of 10 ms per second, in both Firefox and Chrome.

Some processes are exempt from this throttling behavior. In these cases, you can use the Page Visibility API to reduce the tabs' performance impact while they're hidden.

• Tabs which are playing audio are considered foreground and aren't throttled.
• Tabs running code that's using real-time network connections (WebSockets and WebRTC) go unthrottled in order to avoid closing these connections timing out and getting unexpectedly closed.
• IndexedDB processes are also left unthrottled in order to avoid timeouts.

看一个 在线的例子（带声音的视频）

在此例中，当你切换到另一个标签时视频会暂停，当你返回到当前标签时视频会再次播放，代码如下：

// 设置隐藏属性和改变可见属性的事件的名称
var hidden, visibilityChange;
if (typeof document.hidden !== "undefined") {
  // Opera 12.10 and Firefox 18 and later support
  hidden = "hidden";
  visibilityChange = "visibilitychange";
} else if (typeof document.msHidden !== "undefined") {
  hidden = "msHidden";
  visibilityChange = "msvisibilitychange";
} else if (typeof document.webkitHidden !== "undefined") {
  hidden = "webkitHidden";
  visibilityChange = "webkitvisibilitychange";
}

var videoElement = document.getElementById("videoElement");

// 如果页面是隐藏状态，则暂停视频
// 如果页面是展示状态，则播放视频
function handleVisibilityChange() {
  if (document[hidden]) {
    videoElement.pause();
  } else {
    videoElement.play();
  }
}

// 如果浏览器不支持 addEventListener 或 Page Visibility API 给出警告
if (
  typeof document.addEventListener === "undefined" ||
  typeof document[hidden] === "undefined"
) {
  console.log(
    "This demo requires a browser, such as Google Chrome or Firefox, that supports the Page Visibility API.",
  );
} else {
  // 处理页面可见属性的改变
  document.addEventListener(visibilityChange, handleVisibilityChange, false);

  // 当视频暂停，设置 title
  // This shows the paused
  videoElement.addEventListener(
    "pause",
    function () {
      document.title = "Paused";
    },
    false,
  );

  // 当视频播放，设置 title
  videoElement.addEventListener(
    "play",
    function () {
      document.title = "Playing";
    },
    false,
  );
}

Document.hidden 只读

如果页面处于被认为是对用户隐藏状态时返回 true，否则返回 false。

Document.visibilityState 只读

是一个用来展示文档当前的可见性的DOMString 。该属性的值为以下值之一：

• visible : 页面内容至少是部分可见。在实际中，这意味着页面是非最小化窗口的前景选项卡。
• hidden : 页面内容对用户不可见。在实际中，这意味着文档可以是一个后台标签，或是最小化窗口的一部分，或是在操作系统锁屏激活的状态下。
• prerender : 页面内容正在被预渲染且对用户是不可见的 (被 document.hidden 当做隐藏). 文档可能初始状态为 prerender，但绝不会从其他值转为该值。
• 注释：有的浏览器不支持此功能unloaded : 页面正在从内存中卸载。
• 注释：有的浏览器不支持此功能

Document.onvisibilitychange

EventListener 提供在visibilitychange 事件被触发时要调用的代码。

//startSimulation 和 pauseSimulation 在其他地方定义
function handleVisibilityChange() {
  if (document.hidden) {
    pauseSimulation();
  } else {
    startSimulation();
  }
}

document.addEventListener("visibilitychange", handleVisibilityChange, false);

• Description of the Page Visibility API on the IEBlog.
• Description of the Page Visibility API by Google