Handler机制详述1---Handler的简介和用法

1.Handler简介

handler是Android系统封装的用于线程更新UI，消息处理的机制。

［说明］

查看Android Framework源码可以看到，常见的Activity的生命周期onCreate(), onStart(), onResume(), onPause(), onStop(), onDestroy()都是通过handler发送不同Message，AMS（Activity Manager Service）通过Handler向ActivityThread发送消息，在ActivityThread中执行不同的Activity的周期。

2.Handler的用法

// Google文档的地址http://developer.android.com/reference/android/os/Handler.html// Google对Handler的介绍A Handler allows you to send and process Message and Runnable objects associated with a thread's MessageQueue. Each Handler instance is associated with a single thread and that thread's message queue. When you create a new Handler, it is bound to the thread / message queue of the thread that is creating it -- from that point on, it will deliver messages and runnables to that message queue and execute them as they come out of the message queue.

［说明］

查看Google文档可以看出，Handler允许我们绑定(associate with)一个线程的MessageQueue，并通过该handler向该线程发送Message或者Runnable对象，一个Handler绑定了一个线程以及这个线程的MessageQueue，当你创建一个Handler的时候，该Handler就会和Thread以及该Thread的MessageQueue绑定。

首先，下面的程序，在子线程中更新UI：

public class MainActivity extends Activity{TextView tv;@Overrideprotected void onCreate(Bundle savedInstanceState){super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);tv = (TextView) findViewById(R.id.tv);new Thread(){public void run(){try{Thread.sleep(1000);} catch (InterruptedException e){e.printStackTrace();}tv.setText("update UI");};}.start();}}

运行之后，程序奔溃，logcat中打印信息：

［说明］

上面信息的主要内容是，不允许在子线程中更新UI线程，Android自身设定的机制则是只能在子线程中处理耗时的操作，处理完毕之后，在UIThread中更新UI，而子线程是不能更新UI的。

Handler主要提供了下面API来实现子线程与UIThread之间的通信问题：

sendMessage
sendMessageDelayed
post(Runnable)
postDelayed(Runnable, long)

(1). 使用sendMessage进行线程通信：

public class MainActivity extends Activity{@SuppressLint("HandlerLeak")Handler handler = new Handler(){public void handleMessage(android.os.Message msg){if (msg.what == 0x1){tv.setText("update UI");}};};TextView tv;@Overrideprotected void onCreate(Bundle savedInstanceState){super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);tv = (TextView) findViewById(R.id.tv);new Thread(){public void run(){try{Thread.sleep(1000);} catch (InterruptedException e){e.printStackTrace();}handler.sendEmptyMessage(0x1);};}.start();}}

sendMessageDelayed(Message, long)是延迟long毫秒之后，再发送Message。
(2).post(Runnable)

public class MainActivity extends Activity{@SuppressLint("HandlerLeak")Handler handler = new Handler(){public void handleMessage(android.os.Message msg){};};TextView tv;@Overrideprotected void onCreate(Bundle savedInstanceState){super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);tv = (TextView) findViewById(R.id.tv);new Thread(){public void run(){try{Thread.sleep(1000);} catch (InterruptedException e){e.printStackTrace();}handler.post(new Runnable(){@Overridepublic void run(){tv.setText("update UI");}});};}.start();}}

下面利用Handler实现一个简单的图片轮换的功能，每隔1s循环替换ImageView显示的图片：

public class MainActivity extends Activity{Handler handler = new Handler();ImageView img;int[] images = // 存放循环播放的图片{ R.drawable.weather_0, R.drawable.weather_1, R.drawable.weather_2, R.drawable.weather_3, R.drawable.weather_4 };private int imgCount = images.length;private int index = 0;private long delayMillis = 1000L; // 每隔1s循环一次@Overrideprotected void onCreate(Bundle savedInstanceState){super.onCreate(savedInstanceState);setContentView(R.layout.activity_main);img = (ImageView) findViewById(R.id.img);handler.post(imgRunnable);}private Runnable imgRunnable = new Runnable(){public void run(){img.setBackgroundResource(images[index % imgCount]);index++;handler.postDelayed(imgRunnable, delayMillis);}};}

对应的xml文件中只有一个：

［注意］ 若要终止上面的循环，可以使用下面的方法：

handler.removeCallbacks(imgRunnable);

下面使用Handler演示消息截获的用法： ［说明］ Handler的创建有一个接受一个Handler.Callback参数的构造方法，此方法内部的handlerMessage(Message msg)方法有boolean类型的返回值，return true可以实现类似消息截获的效果：

public class SecondActivity extends Activity{@SuppressLint("HandlerLeak")Handler handler = new Handler(new Handler.Callback(){@Overridepublic boolean handleMessage(Message msg){if (msg.what == 0x1){Log.v(LOG, "返回true，消息被截获");return true;} else if (msg.what == 0x2){Log.v(LOG, "返回false，不截获");return false;}return false;}}){@Overridepublic void handleMessage(Message msg){Log.v(LOG, "收到消息....");}};Button btn1, btn2;private final String LOG = "SecondActivity";@Overrideprotected void onCreate(Bundle savedInstanceState){super.onCreate(savedInstanceState);setContentView(R.layout.activity_second);btn1 = (Button) findViewById(R.id.btn1);btn1.setOnClickListener(new OnClickListener(){@Overridepublic void onClick(View v){handler.sendEmptyMessage(0x1);}});btn2 = (Button) findViewById(R.id.btn2);btn2.setOnClickListener(new OnClickListener(){@Overridepublic void onClick(View v){handler.sendEmptyMessage(0x2);}});}}

界面效果如下：

点击“消息截获”按键，会向handler对象传递0x1的Message，在Handler.Callback的boolean handlerMessage()方法中返回true，此时该Message不会再被void handlerMessage()方法接收到：

相反，当点击“消息不截获”按键，会发送0x2消息，Handler.Callback的boolean handlerMessage()返回false，此时消息仍然会被void handlerMessage()方法所获的：

查看Handler的源码可以看到，消息的传递是通过dispatchMessage(Message msg)来发送的，而dispatchMessage函数如下：

当使用了Handler包含Handler.Callback参数的构造函数时，mCallback != null，此时会执行mCallback.handlerMessage(msg)方法，若此方法返回true，则直接执行return返回，不再执行下面的handlerMessage(msg)，若mCallback.handlerMessage(msg)返回false，mCallback.handlerMessage(msg)方法被调用之后，还会继续调用handlerMessage(msg)方法。

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