Android 2.2 API Demos -- PersistentState(Activity)

首先看API Demo中的示例，PersistentState Activity。

1. 在PersistentState Activity运行窗口的第一个文本框中追加输入"It will be saved."，在第二个文本框中输入"It will be lost."并且光标选择"be saved."。

2. 点击back按钮或者以其它方式退出这个界面。

3. 当再次进入这个界面时发现，在第一个文本框中添加的文字和对文字的选择没有改变，而第二个文本框中新输入的文字消失了。

从上面的示例可以看出当我们离开这个Activity之后，有些信息被保存了。当我们回到这个Activity时，被保存的信息能再次加载进来。

从代码中可以看出，

Java代码

package com.example.android.apis.app;

import com.example.android.apis.R;

import android.app.Activity;
import android.content.SharedPreferences;
import android.os.Bundle;
import android.widget.EditText;
import android.widget.TextView;

public class PersistentState extends Activity
{
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        // Be sure to call the super class.
        super.onCreate(savedInstanceState);

        setContentView(R.layout.save_restore_state);

        // Set message to be appropriate for this screen.
        ((TextView)findViewById(R.id.msg)).setText(R.string.persistent_msg);

        // Retrieve the EditText widget whose state we will save.
        mSaved = (EditText)findViewById(R.id.saved);
    }

    @Override
    protected void onResume() {
        super.onResume();

        SharedPreferences prefs = getPreferences(0); 
        String restoredText = prefs.getString("text", null);
        if (restoredText != null) {
            mSaved.setText(restoredText, TextView.BufferType.EDITABLE);

            int selectionStart = prefs.getInt("selection-start", -1);
            int selectionEnd = prefs.getInt("selection-end", -1);
            if (selectionStart != -1 && selectionEnd != -1) {
                mSaved.setSelection(selectionStart, selectionEnd);
            }
        }
    }

    @Override
    protected void onPause() {
        super.onPause();

        SharedPreferences.Editor editor = getPreferences(0).edit();
        editor.putString("text", mSaved.getText().toString());
        editor.putInt("selection-start", mSaved.getSelectionStart());
        editor.putInt("selection-end", mSaved.getSelectionEnd());
        editor.commit();
    }

    private EditText mSaved;
}

当我们点击back按钮离开PersistentState Activity时，触发了onPause()方法，在onPause()方法里我们对第一个文本框的状态（其中的文本和对文本的选择）进行了保存。

当我们再次进入这个Activity时，触发了onResume()方法，在onResume()里我们将保存的信息重新加载到第一个文本框中。

通过onPause和onResume方法，我们可以对Activity的状态改变进行相应，执行一定的操作。其实，Android中Activity的状态改变不止create，pause和resume，相应的方法也不止onCreate，onPause和onResume。

在Android系统中，Activity是通过activity stack来管理的。当一个新的activity被创建，它就被置于栈顶并处于running状态。

Activity有四种状态：

1.处于屏幕最前端的Activity（栈顶）为active或者running状态；
2.失去了焦点但是可见的Activity为paused状态，例如被透明或者非全屏Activity遮盖；
3.被完全掩盖的Activity为stopped状态；
4.当Activity处于paused或者stopped状态时由于系统需要内存被杀死或者执行了finish()方法的Activity将处于destroyed状态。

参考Activity的生命周期图，

相关方法描述如下：

Method	Description	Killable?	Next
onCreate()	Called when the activity is first created. This is where you should do all of your normal static set up: create views, bind data to lists, etc. This method also provides you with a Bundle containing the activity's previously frozen state, if there was one.Always followed by onStart().	No	onStart()
	onRestart()	Called after your activity has been stopped, prior to it being started again.Always followed by onStart()	No	onStart()
onStart()	Called when the activity is becoming visible to the user.Followed by onResume() if the activity comes to the foreground, or onStop() if it becomes hidden.	No	onResume() or onStop()
	onResume()	Called when the activity will start interacting with the user. At this point your activity is at the top of the activity stack, with user input going to it.Always followed by onPause().	No	onPause()
onPause()	Called when the system is about to start resuming a previous activity. This is typically used to commit unsaved changes to persistent data, stop animations and other things that may be consuming CPU, etc. Implementations of this method must be very quick because the next activity will not be resumed until this method returns.Followed by either onResume() if the activity returns back to the front, or onStop() if it becomes invisible to the user.	Yes	onResume() or onStop()
onStop()	Called when the activity is no longer visible to the user, because another activity has been resumed and is covering this one. This may happen either because a new activity is being started, an existing one is being brought in front of this one, or this one is being destroyed.Followed by either onRestart() if this activity is coming back to interact with the user, or onDestroy() if this activity is going away.	Yes	onRestart() or onDestroy()
onDestroy()	The final call you receive before your activity is destroyed. This can happen either because the activity is finishing (someone called finish() on it, or because the system is temporarily destroying this instance of the activity to save space. You can distinguish between these two scenarios with the isFinishing() method.	Yes	nothing