onSaveInstanceState和onRestoreInstanceState触发的时机

先看图：

Android中的activity通过onSaveInstanceState()方法保存activity的状态，在onCreate或者onRestoreInstanceState方法恢复状态，如果是通过onRestart方法前端运行的话，只会执行onSaveinstanceState方法。直接看代码：

 package com.hebaijun.savestate;

 import android.app.Activity;
 import android.os.Bundle;
 import android.util.Log;

 public class SaveStateActivity extends Activity {
     public static final String TAG = "SaveStateActivity";

     @Override
     public void onCreate(Bundle savedInstanceState) {
         if (savedInstanceState!=null) {
             Log.v(TAG, savedInstanceState.getString("data"));
         }
         super.onCreate(savedInstanceState);
         setContentView(R.layout.main);
         Log.v(TAG, "onCreate");
     }

     @Override
     protected void onRestoreInstanceState(Bundle savedInstanceState) {
         Log.v(TAG, savedInstanceState.getString("data"));
         super.onRestoreInstanceState(savedInstanceState);
         Log.v(TAG, "onRestoreInstanceState");
     }

     @Override
     protected void onSaveInstanceState(Bundle outState) {
         outState.putString("data", "example");
         super.onSaveInstanceState(outState);
         Log.v(TAG, "onSaveInstanceState");
     }

     @Override
     protected void onPause() {
         super.onPause();
         Log.v(TAG, "onPause");
     }

     @Override
     protected void onResume() {
         super.onResume();
         Log.v(TAG, "onResume");
     }

     @Override
     protected void onStart() {
         super.onStart();
         Log.v(TAG, "onStart");
     }

     @Override
     protected void onRestart() {
         super.onRestart();
         Log.v(TAG, "onRestart");
     }
 }

main.xml

 xml version="1.0" encoding="utf-8"?>
 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:layout_width="fill_parent"
     android:layout_height="fill_parent"
     android:orientation="vertical" >

     <EditText
         android:id="@+id/editText1"
         android:layout_width="match_parent"
         android:layout_height="wrap_content" >
     EditText>

 LinearLayout>

1.运行程序后我们按HOME键，然后我们重新进入程序，Log如图：

• 我们可以看出onSaveInstanceState方法在onPause方法前执行。Android 3.0之后，onSaveInstanceState在onPause之后，onStop之前调用。
  
• 程序恢复执行的时候调用了onRestart方法，并没有调用 onRestoreInstanceState方法。

2.运行程序后按BACK键，Log如图：

• 程序并没有调用onSaveInstanceState方法，因为按BACK键表明用户已经明确退出，所以不会执行保存状态的操作。就是说 onSaveInstanceState能不能被执行到是不确定的，要保存永久的数据话还是在onPause方法中用其他方式保存。
• 如果在edittext中输入字符的话，重新执行也不会重新出现。

onRestoreInstanceState的执行时机

    private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {

                activity.mCalled = false;
                if (r.isPersistable()) {
                    mInstrumentation.callActivityOnCreate(activity,r.state, r.persistentState);
                } else {
                    mInstrumentation.callActivityOnCreate(activity,r.state);
                }
                r.activity = activity;
                r.stopped = true;
                if (!r.activity.mFinished) {
                    activity.performStart();
                    r.stopped = false;
                }
                if (!r.activity.mFinished) {
                    if (r.isPersistable()) {
                        if (r.state != null || r.persistentState != null) {
                            mInstrumentation.callActivityOnRestoreInstanceState(activity,r.state, r.persistentState);
                        }
                    } else if (r.state != null) {
                        mInstrumentation.callActivityOnRestoreInstanceState(activity,r.state);
                    }
                }
                if (!r.activity.mFinished) {
                    activity.mCalled = false;
                    if (r.isPersistable()) {
                        mInstrumentation.callActivityOnPostCreate(activity,r.state, r.persistentState);
                    } else {
                        mInstrumentation.callActivityOnPostCreate(activity,r.state);
                    }

   }

从代码的逻辑看出，OnRestoreInstanceState在performStart之后，OnPostCreate之前调用，调用的条件是r.state != null 。r.state是AMS传过来的，也就是说OnRestoreInstanceState的调用取决于AMS是否想恢复activity。

除此之外，r.state也会被传入onCreate函数，所以，对activity状态恢复的工作，可以在onCreate中进行，也可以在OnRestoreInstanceState中进行。

从Activity中onCreate函数和OnRestoreInstanceState函数设计初衷看，onCreate函数用于恢复activity本身的状态信息，也包括其中的fragment信息；OnRestoreInstanceState函数用于恢复Activity中window和DecorView的状态。

    protected void onCreate(@Nullable Bundle savedInstanceState) {
        if (savedInstanceState != null) {
            Parcelable p = savedInstanceState.getParcelable(FRAGMENTS_TAG);
            mFragments.restoreAllState(p, mLastNonConfigurationInstances != null
                    ? mLastNonConfigurationInstances.fragments : null);
        }
        mFragments.dispatchCreate();
        getApplication().dispatchActivityCreated(this, savedInstanceState);
    }

    protected void onRestoreInstanceState(Bundle savedInstanceState) {
        if (mWindow != null) {
            Bundle windowState = savedInstanceState.getBundle(WINDOW_HIERARCHY_TAG);
            if (windowState != null) {
                mWindow.restoreHierarchyState(windowState);
            }
        }
    }