问题出在事件“visibilitychange”的行为上。

触发: - 当我切换到浏览器窗口内的不同选项卡时。

• 当我点击浏览器窗口的最小化/恢复按钮时。

(没关系)

未触发: - 当我使用 ALT+TAB 切换到不同的窗口/程序时。

• 当我切换到不同的窗口/程序时单击任务栏。

(这应该触发，因为就像最小化时一样，窗口的可见性可能会改变)

W3 页面可见性 API 文档:http://www.w3.org/TR/page-visibility/

规范表中没有关于 ALT+TAB/程序切换的“页面可见性”定义。我猜它在操作系统和浏览器之间有一些关系。

已测试

• 浏览器: Chrome 40.0.2214.115 m/Firefox 36.0.1/Internet Explorer 11.0.9600.17107
• 操作系统:Windows 8.1

是否有解决此问题的解决方法？实现相当简单，我使用 jQuery 监听“visibilitychange”事件，然后在其回调中检查“document.visibilityState”的值，但问题是事件没有按预期触发。

$(document).on('visibilitychange', function() {

    if(document.visibilityState == 'hidden') {
        // page is hidden
    } else {
        // page is visible
    }
});

这也可以在没有 jQuery 的情况下完成，但是 ALT+TAB 和任务栏切换隐藏/显示预期行为仍然缺失:

if(document.addEventListener){
    document.addEventListener("visibilitychange", function() {
        // check for page visibility
    });
}

我也试过 ifvisible.js 模块 ( https://github.com/serkanyersen/ifvisible.js )，但行为是一样的。

ifvisible.on('blur', function() {
    // page is hidden
});

ifvisible.on('focus', function() {
    // page is visible
});

我还没有在其他浏览器中测试过，因为如果我不能让它在 Windows 上的 Chrome 中工作，我真的不关心其他浏览器。

有什么帮助或建议吗？

更新

我尝试为事件名称使用不同的 vendor 前缀(visibilitychange、webkitvisibilitychange、mozvisibilitychange、msvisibilitychange)，但是当我切换到任务栏中的不同程序或 ALT+ 时，事件仍然没有触发TAB，或者即使我使用覆盖整个屏幕的 Windows 键打开 Windows 中的开始菜单。

我可以在 Chrome、Firefox 和 Internet Explorer 中重现完全相同的问题。

更新 #2

Here's我为这个问题写的一篇综述文章和一个纯 Javascript 解决方法来解决遇到的问题。

更新 #3

编辑以包含来源博客文章的副本。 (见接受的答案)

最佳答案

Here's我为这个问题写的一篇综述文章和一个纯 JavaScript 的变通方法来解决遇到的问题。

编辑以包含来源博客文章的副本:

In any kind of javascript application we develop there may be a feature or any change in the application which reacts according to the current user visibility state, this could be to pause a playing video when the user ALT+TABs to a different window, tracking stats about how the users interact with our application, how often does him switch to a different tab, how long does it take him to return and a lot of performance improvements that can benefit from this kind of API.

The Page Visibility API provides us with two top-level attributes: document.hidden (boolean) and document.visibilityState (which could be any of these strings: “hidden”, “visible”, “prerender”, “unloaded”). This would not be not good enough without an event we could listen to though, that’s why the API also provides the useful visibilitychange event.

So, here’s a basic example on how we could take action on a visibility change:

function handleVisibilityChange() {
  if(document.hidden) {
    // the page is hidden
  } else {
    // the page is visible
  }
}

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

We could also check for document.visibilityState value.

Dealing with vendor issues George Berkeley by John Smibert

Some of the implementations on some browsers still need that the attributes or even the event name is vendor-prefixed, this means we may need to listen to the msvisibilitychange event or check for the document.webkitHidden or the document.mozHidden attributes. In order to do so, we should check if any vendor-prefixed attribute is set, and once we know which one is the one used in the current browser (only if there’s the need for a prefix), we can name the event and attributes properly.

Here’s an example approach on how to handle these prefixes:

var browserPrefixes = ['moz', 'ms', 'o', 'webkit'];

// get the correct attribute name
function getHiddenPropertyName(prefix) {
  return (prefix ? prefix + 'Hidden' : 'hidden');
}

// get the correct event name
function getVisibilityEvent(prefix) {
  return (prefix ? prefix : '') + 'visibilitychange';
}

// get current browser vendor prefix
function getBrowserPrefix() {
  for (var i = 0; i < browserPrefixes.length; i++) {
    if(getHiddenPropertyName(browserPrefixes[i]) in document) {
      // return vendor prefix
      return browserPrefixes[i];
    }
  }

  // no vendor prefix needed
  return null;
}

// bind and handle events
var browserPrefix = getBrowserPrefix();

function handleVisibilityChange() {
  if(document[getHiddenPropertyName(browserPrefix )]) {
    // the page is hidden
    console.log('hidden');
  } else {
    // the page is visible
    console.log('visible');
  }
}

document.addEventListener(getVisibilityEvent(browserPrefix), handleVisibilityChange, false);

Other issues There is a challenging issue around the “Page Visibility” definition: how to determine if the application is visible or not if the window focus is lost for another window, but not the actual visibility on the screen? what about different kinds of visibility lost, like ALT+TAB, WIN/MAC key (start menu / dash), taskbar/dock actions, WIN+L (lock screen), window minimize, window close, tab switching. What about the behaviour on mobile devices?

There’s lots of ways in which we may lose or gain visibility and a lot of possible interactions between the browser and the OS, therefore I don’t think there’s a proper and complete “visible page” definition in the W3C spec. This is the definition we get for the document.hidden attribute:

HIDDEN ATTRIBUTE On getting, the hidden attribute MUST return true if the Document contained by the top level browsing context (root window in the browser’s viewport) [HTML5] is not visible at all. The attribute MUST return false if the Document contained by the top level browsing context is at least partially visible on at least one screen.

If the defaultView of the Document is null, on getting, the hidden attribute MUST return true.

To accommodate accessibility tools that are typically full screen but still show a view of the page, when applicable, this attribute MAY return false when the User Agent is not minimized but is fully obscured by other applications.

I’ve found several inconsistencies on when the event is actually fired, for example (Chrome 41.0.2272.101 m, on Windows 8.1) the event is not fired when I ALT+TAB to a different window/program nor when I ALT+TAB again to return, but it IS fired if I CTRL+TAB and then CTRL+SHIFT+TAB to switch between browser tabs. It’s also fired when I click on the minimize button, but it’s not fired if the window is not maximized and I click my editor window which is behing the browser window. So the behaviour of this API and it’s different implementations are still obscure.

A workaround for this, is to compensate taking advantage of the better implemented focus and blur events, and making a custom approach to the whole “Page Visibility” issue using an internal flag to prevent multiple executions, this is what I’ve come up with:

var browserPrefixes = ['moz', 'ms', 'o', 'webkit'],
    isVisible = true; // internal flag, defaults to true

// get the correct attribute name
function getHiddenPropertyName(prefix) {
  return (prefix ? prefix + 'Hidden' : 'hidden');
}

// get the correct event name
function getVisibilityEvent(prefix) {
  return (prefix ? prefix : '') + 'visibilitychange';
}

// get current browser vendor prefix
function getBrowserPrefix() {
  for (var i = 0; i < browserPrefixes.length; i++) {
    if(getHiddenPropertyName(browserPrefixes[i]) in document) {
      // return vendor prefix
      return browserPrefixes[i];
    }
  }

  // no vendor prefix needed
  return null;
}

// bind and handle events
var browserPrefix = getBrowserPrefix(),
    hiddenPropertyName = getHiddenPropertyName(browserPrefix),
    visibilityEventName = getVisibilityEvent(browserPrefix);

function onVisible() {
  // prevent double execution
  if(isVisible) {
    return;
  }
 
  // change flag value
  isVisible = true;
  console.log('visible}

function onHidden() {
  // prevent double execution
  if(!isVisible) {
    return;
  }

  // change flag value
  isVisible = false;
  console.log('hidden}

function handleVisibilityChange(forcedFlag) {
  // forcedFlag is a boolean when this event handler is triggered by a
  // focus or blur eventotherwise it's an Event object
  if(typeof forcedFlag === "boolean") {
    if(forcedFlag) {
      return onVisible();
    }

    return onHidden();
  }

  if(document[hiddenPropertyName]) {
    return onHidden();
  }

  return onVisible();
}

document.addEventListener(visibilityEventName, handleVisibilityChange, false);

// extra event listeners for better behaviour
document.addEventListener('focus', function() {
  handleVisibilityChange(true);
}, false);

document.addEventListener('blur', function() {
  handleVisibilityChange(false);
}, false);

window.addEventListener('focus', function() {
    handleVisibilityChange(true);
}, false);

window.addEventListener('blur', function() {
  handleVisibilityChange(false);
}, false);

I welcome any feedback on this workaround. Some other great sources for ideas on this subject:

Using the Page Visibility API Using PC Hardware more efficiently in HTML5: New Web Performance APIs, Part 2 Introduction to the Page Visibility API Conclusion The technologies of the web are continuously evolving, we’re still recovering from a dark past where tables where the markup king, where semantics didn’t mattered, and they weren’t any standards around how a browser should render a page.

It’s important we push these new standards forward, but sometimes our development requirements make us still need to adapt to these kind of transitions, by handling vendor prefixes, testing in different browsers and differents OSs or depend on third-party tools to properly identify this differences.

We can only hope for a future where the W3C specifications are strictly revised, strictly implemented by the browser developer teams, and maybe one day we will have a common standard for all of us to work with.

As for the Page Visibility API let’s just kinda cite George Berkeley and say that:

“being visible” is being perceived.

关于javascript - 使用 ALT+TAB 切换程序/窗口或单击任务栏时不会触发 visibilitychange 事件，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/28993157/