Android消息处理Handler,Message,Looper

MessageQueue

消息队列,存放消息的地方.每个线程只能拥有一个MessageQueue

Looper

Adnroid系统中的Looper负责管理线程的MessageQueue.除了主线程外,创建的线程默认是没有Looper和MessageQueue,创建一个Looper会同时创建一个MessageQueue,可以使用Looper.prepare()创建MessageQueue,Looper.loop()进入消息循环,Looper.release()释放资源.

	class LooperThread extends Thread {
		public Handler mHandler;

		@Override
		public void run() {
			// 创建消息队列
			Looper.prepare();
			mHandler = new Handler() {
				public void handleMessage(android.os.Message msg) {
					// process incoming messages here
				}
			};
			// 进入消息循环
			Looper.loop();
			super.run();
			
			//释放资源，使用Looper.release方法。
		}
 	}

Message

消息对象,MessageQueue里存放的对象,可以调用removeMessages()时，将Message从Message Queue中删除和通过Handler对象的obtainMessage()获取一个Message实例,可以在线程中使用Handler对象的sendEmptyMessage()或者sendMessage()来传递Bundle对象到Handler,对Handler类提供handlerMessage(Message msg)判断,通过msg.what来区分每一条信息

Handler

Handler的作用是把消息加入特定的Looper消息队列中,并分发和处理该消息队列中的消息.如另一个线程怎样把消息放入主(UI)线程的消息队列,可以通过Handler对象,通过调用Handler主线程的sendMessage接口,把消息队列放入主线程的消息队列,并在该Handler的handleMessage()来处理消息

下面是模拟多线程并发的例子其中有用到当所有线程执行完毕更新UI需要Handler

/** Called when the activity is first created. */

	private String URL = "http://www.baidu.com";
	private int NUM = 100;

	private Button myButton;
	private EditText mEditTextURL, mEditTextNUM;
	private ProgressDialog pd;

	private static int count = 0;

	Handler myHandler = new Handler() {
		public void handleMessage(android.os.Message msg) {
			if (msg.what == 0)
				pd.dismiss();
			super.handleMessage(msg);
		};
	};

	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);

		mEditTextURL = (EditText) findViewById(R.id.myEditTextURL);
		mEditTextNUM = (EditText) findViewById(R.id.myEditTextNUM);

		myButton = (Button) findViewById(R.id.myButton);
		myButton.setOnClickListener(new View.OnClickListener() {

			@Override 
			public void onClick(View v) {
				// TODO Auto-generated method stub
				if (!mEditTextURL.getText().toString().trim().equals(""))
					URL = mEditTextURL.getText().toString().trim();
				if (!mEditTextNUM.getText().toString().trim().equals(""))
					NUM = Integer.parseInt(mEditTextNUM.getText().toString()
							.trim());

				pd = ProgressDialog.show(HttpTestActivity.this, "xxx","xxx...");

				outFile();
				for (int i = 0; i < NUM; i++) {<blockquote><blockquote><p>
					count++;
					new Thread(new Handl(i)).start();
				}

			}
		});
	}

	public void outFile() {
		PrintStream stream = null;
		try {
			File f = new File(Environment.getExternalStorageDirectory(),
					"xxx.txt");
			if (!f.exists())
				f.createNewFile();
			stream = new PrintStream(f);
			System.setOut(stream);
		} catch (Exception e) {
			// TODO: handle exception
			e.printStackTrace();
		}
	}

	class Handl implements Runnable {
		private int No;

		public Handl(int no) {
			this.No = no;
		}

		@Override
		public void run() {
			// TODO Auto-generated method stub
			Thread.currentThread().setName(No + "");
			HttpClient httpClient = new DefaultHttpClient();
			// 设置请求超时
			httpClient.getParams().setParameter(
					CoreConnectionPNames.CONNECTION_TIMEOUT, 5000);
			// 设置读取超时
			httpClient.getParams().setParameter(
					CoreConnectionPNames.SO_TIMEOUT, 3000);
			HttpGet get = new HttpGet(URL);
			String strResult = "";

			try {
				HttpResponse response = httpClient.execute(get);
				if (response.getStatusLine().getStatusCode() == HttpStatus.SC_OK) {
					strResult = EntityUtils.toString(response.getEntity());
					System.out.println("ThreadID:"
							+ Thread.currentThread().getName());
					System.out.println(strResult);
				} else {
					System.out.println(Thread.currentThread().getName()
							+ "请求失败");
				}


				
				Thread.sleep((long) Math.random() * 1000);
			} catch (InterruptedException e) {
				// TODO: handle exception
				e.printStackTrace();
			} catch (IOException e) {
				// TODO: handle exception
				e.printStackTrace();
			} catch (Exception e) {
				// TODO: handle exception
				e.printStackTrace();
			} finally {
				synchronized (this) {
					count--;
				}
			}

			Message message = new Message();
			message.what = count; 
			myHandler.sendMessage(message);
		}
</p></blockquote></blockquote>

消息的处理者，handler负责将需要传递的信息封装成Message，通过调用handler对象的obtainMessage()来实现； 将消息传递给Looper，这是通过handler对象的sendMessage()来实现的。继而由Looper将Message放入MessageQueue中。 当Looper对象看到MessageQueue中含有Message，就将其广播出去。该handler对象收到该消息后，调用相应的handler对象的handleMessage()方法 对其进行处理。

更多参考:

倒计时程序

线程之间的消息传递