推送服务 API
对于每条推送消息，也可以给出如下说明:
TTL — 定义消息在删除和未发送之前应排队多长时间。
优先级 — 定义每个消息的优先级，推送服务只发送高优先级的消息，确保用户因为一些突发情况关机或者断电等。
主题 — 为推送消息提供一个主题名称，该名称将用相同的主题替换挂起的消息，这样，一旦设备处于活动状态，用户就不会收到过时的信息。
一旦按照上面的解释将消息发送到推送服务，该消息将处于挂起状态，直到发生以下情况之一:
1 设备上线
2 消息由于 TTL 而在队列上过期
推送服务传递消息时，浏览器将接收它，解密它，并在的 Service Worker 中分派一个 push 事件。这里最重要的是，即使 Web 页面没有打开，浏览器也可以执行你的 Service Worker。流程如下:
@推送消息到达浏览器，浏览器解密它
@浏览器唤醒 Service Worker
@push 事件被分发给 Service Worker
设置推送事件监听器的代码应该与用 JavaScript 编写的任何其他事件监听器类似：
[cc lang=”js”]self.addEventListener(‘push’, function(event) {
var promise = self.registration.showNotification(‘Push notification!’);
event.waitUntil(promise);
});[/cc]
在 Service Worker 中，event.waitUntil(promise)，告诉浏览器在该promse 未完成之前工作将一直进行中，如果它希望完成该工作，它不应该终止 Sercice Worker。
需要了解 Service Worker 的一点是，你没有 Service Worker 代码运行时长的控制权。浏览器决定何时将其唤醒以及何时终止它。
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显示通知， 浏览器必须后台有进程允许，否则不会接受 通知
On desktop the browser needs to be running since that is the process that receives the push messages. Some extensions, like hangouts force the browser to keep running even when the last tab is closed so for users with one such extension installed push will work all the time.
On Android, the browser does not need to be running since the entity in charge of receiving the messages is baked into Google Play Services.
浏览器 后台进程 区分
CHROME – Chrome runs as a background process by default even when all the windows are closed. As long as the background process is running, notifications will still be received. If the Chrome background process is not running, notifications will not be received.
FIREFOX – On Mac OS X, the process still exists even if windows are closed, and a notification can be received if all windows are closed (as long as there is still a dot in the dock showing Firefox is still running). On Windows, the process exits after all windows are closed so notifications cannot be received unless a Firefox window is still open.
SAFARI – Safari does not have to be running to receive notifications, as they are sent directly to the operating system. The user still has to sign up for Safari web notifications, but after that they will be received even when Safari is completely closed.