Android开发3.1:Activity详细介绍

详细了解Activity生命周期，根据观察Logcat理解执行过程状态

  版本：Android4.3 API18  整理测试笔记：liuxinming 

  创建Android Project

  

什么是Activity?

Android四大组件之一

Activity是Android程序的表现层，程序的每一个显示屏幕就是一个Activity

Activity类的继承关系

相关概述

一个程序一般由多个Activity组成，各activities之间关系很松散，它们之间没有直接的关联。必须有一个activity被指定为主activity，它是程序启动时首先显示的界面。每个activity都可以随意启动其它的activity。每当一个activity被启动，则前一个activity就被停止。一个程序中的所有启动的activity都被放在一个栈中，所以被停止的activity并没有销毁，而在存于棧中。新启动的activity先被存放于栈中，然后获得输入焦点。在当前活动的activity上点返回键，它被从棧中取出，然后销毁，然后上一个activity被恢复。

    当一个activity因为新的activity启动而被停止时，它会收到状态变化的通知，这样的变化有多个，每个都会引起系统调用一个相应的回调方法以通知activity，这些回调方法被统称为“生命周期回调方法”。这些回调方法分别在Activity被创建、停止、恢复、销毁时被调用。

如何创建Activity

1、从类activity继承

2、实现”生命周期回调方法“

两个最重要的方法

1、onCreate()

       这个是必须实现的函数，在其中做初始化工作。（注：必须在此函数中调用setContentView()函数设置Activity的界面）

2、onPause()

      这个虽然很重要，但不是必须实现的。此函数在用户离开Activity时被调用（这一般并不表示Activity要被销毁了）。在这个函数中，你一般需要提交那些需要保存状态的数据--因为用户可能不再返回这个Activity了。

其他方法(视情况实现)

3、onStart() 
      当Activity对用户可见的时候被调用。
4、onResume()  
       当Activity开始和用户交互的时候被调用。
5、onStop() 
      当Activity不在对用户可见的时候被调用。
6、onDestroy() 
      当被销毁的时候调用

看案例代码

[java] view plaincopy

1. package com.example.catlog;  
2.   
3. import android.os.Bundle;  
4. import android.app.Activity;  
5. import android.util.Log;  
6. import android.view.Menu;  
7.   
8. public class MainActivity extends Activity {  
9.     private String tag = "MainActivity";  
10.     @Override  
11.     protected void onCreate(Bundle savedInstanceState) {  
12.         super.onCreate(savedInstanceState);  
13.         setContentView(R.layout.activity_main);  
14.          Log.d(tag, "In the onStart() event");    
15.     }  
16.     @Override    
17.     public void onStart() {      
18.         super.onStart();      
19.         Log.d(tag, "In the onStart() event");      
20.     }      
21.         
22.     public void onRestart() {      
23.         super.onRestart();      
24.         Log.d(tag, "In the onRestart() event");      
25.     }      
26.       
27.     public void onResume() {      
28.         super.onResume();      
29.         Log.d(tag, "In the onResume() event");      
30.     }      
31.       
32.     public void onPause() {      
33.         super.onPause();      
34.         Log.d(tag, "In the onPause() event");      
35.     }      
36.       
37.     public void onStop() {      
38.         super.onStop();      
39.         Log.d(tag, "In the onStop() event");      
40.     }      
41.       
42.     public void onDestroy() {      
43.         super.onDestroy();      
44.         Log.d(tag, "In the onDestroy() event");      
45.     }      
46.     @Override  
47.     public boolean onCreateOptionsMenu(Menu menu) {  
48.         // Inflate the menu; this adds items to the action bar if it is present.  
49.         getMenuInflater().inflate(R.menu.main, menu);  
50.         return true;  
51.     }  
52.   
53. }  

观察Logcat

① 启动Hello Word项目 ，我们可以看到如上图 Logcat 日志

    首先onCreate()创建一个页面布局

    其次onStart()对用户可见时调用

            onResume()当和用户交互时调用

② 当没有动作时，虚拟机界面变黑，调用如下函数

     onPause()  在用户离开时被调用，但并不表示要被销毁【暂停】

     onStop() 当Activity不在被用户可见时调用 【停止】

③ 当滑动打开界面时，调用日志如下
     onRestart() 重新把Activity加载进来，呈现给用户。

     onStart() 对用户可见时调用

     onResume() 当和用户交互时调用

    ============================下面动作是继续执行暂停，停止，注意：：：多了一个销毁（因为这次我把程序关闭了）

    onDestroy() 当销毁 Activity时调用

结语

Android系统把ActivityLife这个Activity先杀死（具体顺序是先暂停、后关闭再销毁），然后再启动（具体顺序是先创建，后启动再恢复）。通过这个例子我们也清楚的看到了，是Android系统而不是程序员在控制Activity的生命周期。

生命周期

    

关于更详细，更多API以及类介绍参考API手册：
http://android.toolib.net/reference/android/app/Activity.html

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