android生命周期

上图就是android应用程序的一个生命周期，需要明确的一点是，在stoped阶段，没有任何代码执行！

通常情况下，不需要实现destroyed的方法来清理系统的垃圾以及其它操作，但是在下列情况时，要实现它：

　　activity includes background threads that you created during onCreate() or other other long-running resources that could potentially leak 　　memory if not properly closed, you should kill them during onDestroy(). 

例如：

@Override
public void onDestroy() {
    super.onDestroy();  // Always call the superclass
    
    // Stop method tracing that the activity started during onCreate()
    android.os.Debug.stopMethodTracing();
}

注意：

he system calls onDestroy() after it has already called onPause() and onStop() in all situations except one: when you call finish() from within the onCreate() method. In some cases, such as when your activity operates as a temporary decision maker to launch another activity, you might callfinish() from within onCreate() to destroy the activity. In this case, the system immediately callsonDestroy() without calling any of the other lifecycle methods.

 在onPause通常需要处理的是:

• Stop animations or other ongoing actions that could consume CPU.
• Commit unsaved changes, but only if users expect such changes to be permanently saved when they leave (such as a draft email).
• Release system resources, such as broadcast receivers, handles to sensors (like GPS), or any resources that may affect battery life while your activity is paused and the user does not need them.