两种保存状态的方法getSharedPreferences和onSaveInstanceState

虽然这些东西很简单有时候还真的让你搞混

@Override 
protected void onPause() { 
    super.onPause(); 
 
    SharedPreferences prefs = getSharedPreferences("X", MODE_PRIVATE); 
    Editor editor = prefs.edit(); 
    editor.putString("lastActivity", getClass().getName()); 
    editor.commit(); 
} 

public class Dispatcher extends Activity { 
 
    @Override 
    protected void onCreate(Bundle savedInstanceState) { 
        super.onCreate(savedInstanceState); 
 
        Class<?> activityClass; 
 
        try { 
            SharedPreferences prefs = getSharedPreferences("X", MODE_PRIVATE); 
            activityClass = Class.forName( 
                prefs.getString("lastActivity", Activity1.class.getName())); 
        } catch(ClassNotFoundException ex) { 
            activityClass = Activity1.class; 
        } 
 
        startActivity(new Intent(this, activityClass)); 
    } 
} 

上面的方法通常保存一个activity以便下次发动
2.

@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. 
  savedInstanceState.putBoolean("MyBoolean", true); 
  savedInstanceState.putDouble("myDouble", 1.9); 
  savedInstanceState.putInt("MyInt", 1); 
  savedInstanceState.putString("MyString", "Welcome back to Android"); 
  // etc. 
  super.onSaveInstanceState(savedInstanceState); 
} 
@Override 
public void onRestoreInstanceState(Bundle savedInstanceState) { 
  super.onRestoreInstanceState(savedInstanceState); 
  // Restore UI state from the savedInstanceState. 
  // This bundle has also been passed to onCreate. 
  boolean myBoolean = savedInstanceState.getBoolean("MyBoolean"); 
  double myDouble = savedInstanceState.getDouble("myDouble"); 
  int myInt = savedInstanceState.getInt("MyInt"); 
  String myString = savedInstanceState.getString("MyString"); 
} 

这个上次已经记过了用来旋转保存状态最好

对onSaveInstanceState的解释：
        为了获取activity被杀死前的状态，你应该为activity实现onSaveInstanceState() 方法。Android在activity有可能被销毁之前（即onPause() 调用之前）会调用此方法。它会将一个以名称-值对方式记录了activity动态状态的Bundle 对象传递给该方法。当activity再次启动时，这个Bundle会传递给onCreate()方法和随着onStart()方法调用的onRestoreInstanceState()，所以它们两个都可以恢复捕获的状态。
         与onPause()或先前讨论的其它方法不同，onSaveInstanceState() 和 onRestoreInstanceState() 并不是生命周期方法。它们并不是总会被调用。比如说，Android会在activity易于被系统销毁之前调用 onSaveInstanceState()，但用户动作（比如按下了BACK键）造成的销毁则不调用。在这种情况下，用户没打算再次回到这个activity，所以没有保存状态的必要。
         因为onSaveInstanceState()不是总被调用，所以你应该只用它来为activity保存一些临时的状态，而不能用来保存持久性数据。而是应该用onPause()来达到这个目的。


Google为何这样设计OnSharedPreferenceChangeListener
http://droidyue.com/blog/2014/11/29/why-onsharedpreferencechangelistener-was-not-called/