Activity生命周期

    Activity是Android应用中最常用的。Activity有其固有的状态和生命周期。但是Android系统对Activity状态切换的处理往往并不是我们所期望的那样，所以了解并掌握Activity的生命周期，对于开发高质量的Android应用是十分必要的。

 

 

    Android系统会将当前存在的Activity组织成一个栈。当显示新的Activity时，将其创建后放入栈顶。如果用户按了返回键，顶层的Activity就会被移走，下面的一个Activity就显示出来了。

 

    一个Activity一共有四种状态：

    Active： 激活状态。Activity处于栈顶，其可见并拥有焦点。当其他Activity被激活时，此Activity便会进入暂停状态。

    Paused：暂停状态。Activity可见，但没有焦点。通常是它前面还有一个透明的或者不是全屏的Activity。

    Stopped：停止状态。Activity完全不可见时，便处于此状态。
    Inactive： 未激活状态。Activity被关闭后的状态，它也会被从Activity栈中移除。

 

 

    对Activity在各种状态之间的切换，Android系统提供了完整的事件处理函数。

   

import android.app.Activity; import android.os.Bundle; public class MyActivity extends Activity { private String LOG_TAG = "MyApplication"; @Override public void onCreate(Bundle savedInstanceState) { // Initialize activity. super.onCreate(savedInstanceState); } // Called after onCreate has finished, use to restore UI state @Override public void onRestoreInstanceState(Bundle savedInstanceState) { // Restore UI state from the savedInstanceState. // This bundle has also been passed to onCreate. super.onRestoreInstanceState(savedInstanceState); } // Called before subsequent visible lifetimes // for an activity process. @Override public void onRestart() { // Load changes knowing that the activity has already // been visible within this process. super.onRestart(); } // Called at the start of the visible lifetime. @Override public void onStart() { // Apply any required UI change now that the Activity is visible. super.onStart(); } // Called at the start of the active lifetime. @Override public void onResume() { // Resume any paused UI updates, threads, or processes required // by the activity but suspended when it was inactive. super.onResume(); } // Called to save UI state changes at the // end of the active lifecycle. @Override public void onSaveInstanceState(Bundle savedInstanceState) { // Save UI state changes to the savedInstanceState. // This bundle will be passed to onCreate if the process is // killed and restarted. super.onSaveInstanceState(savedInstanceState); } // Called at the end of the active lifetime. @Override public void onPause() { // Suspend UI updates, threads, or CPU intensive processes // that don’t need to be updated when the Activity isn’t // the active foreground activity. super.onPause(); } // Called at the end of the visible lifetime. @Override public void onStop() { // Suspend remaining UI updates, threads, or processing // that aren’t required when the Activity isn’t visible. // Persist all edits or state changes // as after this call the process is likely to be killed. super.onStop(); } // Called at the end of the full lifetime. @Override public void onDestroy() { // Clean up any resources including ending threads, // closing database connections etc. super.onDestroy(); } }