利用ActivityLifecycleCallbacks判断APP是否在后台运行

转自http://blog.csdn.net/lfdfhl/article/details/51045414

在开发的时候，我们时常有这样一个需求：判断APP是否在后台运行？ 

常见的方式是这样的： 



或者是这样的：



如上的这两种方式都可以满足我们的需求 。 

但是这么做效率高么？不！请注意，这里是一个大写的不！ 

这很像我们平时用瘟到死操作系统去打开任务管理器的感觉；或者写了一个看上去金光闪闪，牛叉哄哄的for循环挨个遍历一遍。 

我们可以不这么鲁莽么？我们可以换一种优雅的方式来实现这个功能么？我们可以用Android系统的API来实现么？可以！请注意，这里是一个大写的可以！ 

Android在SDK 14提供了一个Callback即ActivityLifecycleCallbacks，通过这个Callback可拿到App所有Activity的生命周期回调 


 

从这个接口里的几个方法名你大概猜到了：在这个回调中可以监测App中所有Activity的生命周期，它们都赤果果地站在你面前了。嗯哼，是不是有一种类似的感觉：公司妹子们的手机号都搞到手了？都到这份上了，下面该干什么了………. 

当然是看看这个回调怎么用，不要想歪了……. 

首先自定义一个Application在其onCreate( )方法中注册ActivityLifecycleCallbacks.

<code class="language-Java hljs java has-numbering" style="display: block; padding: 0px; color: inherit; box-sizing: border-box; font-family: 'Source Code Pro', monospace;font-size:undefined; white-space: pre; border-radius: 0px; word-wrap: normal; background: transparent;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">MyApplication</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">extends</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">Application</span> {</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> MyApplication mApplicationInstance;
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> ActivityLifecycleCallbacksImpl mActivityLifecycleCallbacksImpl;
<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onCreate</span>() {
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">super</span>.onCreate();
mApplicationInstance=<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> MyApplication();
mActivityLifecycleCallbacksImpl=<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> ActivityLifecycleCallbacksImpl();
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">this</span>.registerActivityLifecycleCallbacks(mActivityLifecycleCallbacksImpl);
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">static</span> MyApplication <span class="hljs-title" style="box-sizing: border-box;">getInstance</span>() {
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">if</span> (<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">null</span>==mApplicationInstance) {
mApplicationInstance=<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">new</span> MyApplication();
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">return</span> mApplicationInstance;
}
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">private</span> <span class="hljs-class" style="box-sizing: border-box;"><span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">class</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">ActivityLifecycleCallbacksImpl</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">implements</span> <span class="hljs-title" style="box-sizing: border-box; color: rgb(102, 0, 102);">ActivityLifecycleCallbacks</span>{</span>
<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onActivityCreated</span>(Activity activity, Bundle savedInstanceState) {
}
<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onActivityStarted</span>(Activity activity) {
}
<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onActivityResumed</span>(Activity activity) {
}
<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onActivityPaused</span>(Activity activity) {
}
<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onActivityStopped</span>(Activity activity) {
}
<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onActivitySaveInstanceState</span>(Activity activity, Bundle outState) {
}
<span class="hljs-annotation" style="color: rgb(155, 133, 157); box-sizing: border-box;">@Override</span>
<span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">public</span> <span class="hljs-keyword" style="color: rgb(0, 0, 136); box-sizing: border-box;">void</span> <span class="hljs-title" style="box-sizing: border-box;">onActivityDestroyed</span>(Activity activity) {
}
}
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这个时候我们就要考虑了：怎么判断APP是否运行在后台？ 

从Activity的生命周期我们可知： 

Activity调用onResume()，那么该Activity是可见的，即它在前台。 

Activity调用onPause()，那么该Activity就到了后台。 

顺着这个思路往下走，我们可以在ActivityLifecycleCallbacks中利用一个计数器activityCounter来判断是否有Activity运行在前台。 

1 在onActivityResumed()方法中使activityCounter+1 

2 在onActivityPaused()方法使activityCounter-1

所以： 

当activityCounter=0的时候我们可以判定APP运行在后台 

当activityCounter=1的时候我们可以判定APP运行在前台 

这么做是不是觉得挺nice的？嗯哼。 

这么做有瑕疵么？有遗漏的地方么？ 

我们来看这么一种常见的场景: 

假设存在两个界面Activity A和Activity B 

在Activity A启动后activityCounter=1;这个是没有疑问的。 

在某一时刻App需要从Activity A跳转到Activity B，此时这两个Activity的生命周期如下: 

A.onPause()->B.onCreate()->B.onStart()-> B.onResume()-> A.onStop() 

假若按照我们的处理方式: 

此时Activity A会执行到onPause()，那么在ActivityLifecycleCallbacks中使activityCounter-1，此时activityCounter=0;表示该APP运行在后台. 

这当然是错的：两个界面切换的过程中APP怎么可能是运行在后台的呢？ 

我们错在哪里了？！？ 

我们平常不就是习惯性地在onPause()和onResume()做操作么？ 

是的，平常那么做没啥做，因为那是单个的Activity，但是这里涉及到了两个Activity的跳转。我们错就错在惯性思维，错在了想当然。 

当我们冷静一下，回头再瞅瞅两个Activity跳转时两者的生命周期就可发现，只需很小的修改就可以纠正这个问题： 

1 在onActivityStarted()中对于activityCounter+1 

2 在onActivityStopped()中对于activityCounter-1 

这样就解决了刚才的问题。 

ActivityLifecycleCallbacks除了可以帮助我们判断App是否运行在后台？还能帮我们做什么呢？有时候项目是不是要求：退出应用时关闭所有的Activity呢？嗯哼，利用ActivityLifecycleCallbacks也可以做到.我们可以采用一个LinkedList管理应用中的界面 

1 在ActivityLifecycleCallbacks的onActivityCreated()将Activity添加到 LinkedList中 

2 在ActivityLifecycleCallbacks的onActivityDestroyed()将Activity从LinkedList中移除 

3 退出应用时销毁LinkedList中所有Activity 

在最前面我们也提到了ActivityLifecycleCallbacks是在API 14及其以上才有的。那么在Android4.0以下又可以怎么做呢？ 

其实不难，我们在应用的BaseActivity中按照该思路实现即可。 

备注说明： 

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