Android基础#36：Android返回键与Home键区别，以及Application的registerActivityLifecycleCallbacks

1. 内容简介：

在Android系统中，activity是由framework层创建的，而且，由framework管理这activity的生命周期。了解app中各个Actiivty的生命周期，对于开发Android应用程序至关重要。

那么，当activty进行切换的时候，如何知道相关activity的状态（生命周期相关函数的调用）呢？可以通过App的registerActivityLifecycleCallbacks来实现。

当按下Android返回键或Home键，activity生命周期是如何执行的呢？

2. Android返回键与Home键对activity生命周期的影响：

在Android中，当按下Home键，默认情况下stop前台的actiity，即activity设置成onstop，而不是ondestory。如果再次启动该activity不是调用onCreate，而是调用onSavedInstanceState方法，保持上次Activity的状态则是从onRestart开始->onStart->onResume;

而当按下back键则不同，back键默认finish前台的activity，即activity的状态为onDestory为止，再次启动该activity则从onCreate开始，不会调用onSavedInstanceState方法。

3. Application的registerActivityLifecycleCallbacks：注册Activity生命周期回调函数

功能：通过注册Application的registerActivityLifecycleCallbacks，可以监听Acitivty的生命周期相关函数的调用情况。

代码如下：

package com.test.myapplication5;import android.app.Activity;import android.app.Application;import android.os.Bundle;import android.util.Log;public class MyApp extends Application {    private static final String TAG = "MainApplication";    private static MyApp appInstance;    public static Application getAppInstance() { return appInstance;    }    @Override    public void onCreate() { super.onCreate(); Log.i(TAG, "===onCreate======"); appInstance = this; registActivityLife();    }    private void registActivityLife() { this.registerActivityLifecycleCallbacks(new Application.ActivityLifecycleCallbacks() {     @Override     public void onActivityCreated(Activity activity, Bundle bundle) {  Log.i(TAG, "[registActivityLife] onActivityCreated activity " + activity.getLocalClassName());     }     @Override     public void onActivityStarted(Activity activity) {  Log.i(TAG, "[registActivityLife] onActivityStarted activity " + activity.getLocalClassName());     }     @Override     public void onActivityResumed(Activity activity) {  Log.i(TAG, "[registActivityLife] onActivityResumed activity " + activity.getLocalClassName());     }     @Override     public void onActivityPaused(Activity activity) {  Log.i(TAG, "[registActivityLife] onActivityPaused activity " + activity.getLocalClassName());     }     @Override     public void onActivityStopped(Activity activity) {  Log.i(TAG, "[registActivityLife] onActivityStopped activity " + activity.getLocalClassName());     }     @Override     public void onActivitySaveInstanceState(Activity activity, Bundle bundle) {  Log.i(TAG, "[registActivityLife] onActivitySaveInstanceState activity " + activity.getLocalClassName());     }     @Override     public void onActivityDestroyed(Activity activity) {  Log.i(TAG, "[registActivityLife] onActivityDestroyed activity " + activity.getLocalClassName());     } });    }}

运行结果：

28755-28755/com.test.myapplication5 I/MainApplication: [registActivityLife] onActivityCreated activity MainActivity28755-28755/com.test.myapplication5 I/MainApplication: [registActivityLife] onActivityStarted activity MainActivity28755-28755/com.test.myapplication5 I/MainApplication: [registActivityLife] onActivityResumed activity MainActivity28755-28755/com.test.myapplication5 I/MainApplication: [registActivityLife] onActivityPaused activity MainActivity28755-28755/com.test.myapplication5 I/MainApplication: [registActivityLife] onActivityCreated activity activity.MyBrowserActivity228755-28755/com.test.myapplication5 I/MainApplication: [registActivityLife] onActivityStarted activity activity.MyBrowserActivity228755-28755/com.test.myapplication5 I/MainApplication: [registActivityLife] onActivityResumed activity activity.MyBrowserActivity228755-28755/com.test.myapplication5 I/MainApplication: [registActivityLife] onActivityStopped activity MainActivity28755-28755/com.test.myapplication5 I/MainApplication: [registActivityLife] onActivitySaveInstanceState activity MainActivity

相关activity的代码略。