前面介绍了异步任务的处理，这里来学一下Handler和Message机制吧。下面这个代码和之前的一样，都是从网上下载图片的。

package com.example.myasynctask;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;

import org.apache.http.HttpEntity;
import org.apache.http.HttpRequest;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.util.EntityUtils;

import android.os.AsyncTask;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.app.Activity;
import android.app.ProgressDialog;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;

public class MainActivity extends Activity {
	private Button button;
	private ImageView iv;
	private String path = "http://cdn.duitang.com/uploads/item/201310/17/20131017194234_eZfFU.thumb.600_0.png";
	private Handler handler = new Handler() {
		@Override
		public void handleMessage(Message msg) {
			Bitmap bm = null;
			byte[] data = (byte[])msg.obj;
			bm = BitmapFactory.decodeByteArray(data, 0, data.length);
			iv.setImageBitmap(bm);
		};

	};

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		button = (Button) this.findViewById(R.id.bt);
		iv = (ImageView) this.findViewById(R.id.iv);

		button.setOnClickListener(new OnClickListener() {
			@Override
			public void onClick(View v) {
				// TODO Auto-generated method stub
				new Thread(new myThread()).start();
			}
		});
	}

	private class myThread implements Runnable {
		@Override
		public void run() {
			HttpClient httpClient = new DefaultHttpClient();
			HttpGet httpGet = new HttpGet(path);
			InputStream inputStream = null;
			try {
				HttpResponse httpResponse = httpClient.execute(httpGet);
				if (httpResponse.getStatusLine().getStatusCode() == 200 ) {
					HttpEntity httpEntity = httpResponse.getEntity();
					byte[] data = EntityUtils.toByteArray(httpEntity);
					Message message = Message.obtain();
					message.obj = data;
					handler.sendMessage(message);
				}
				
			} catch (Exception e) {
				// TODO: handle exception
				e.printStackTrace();
			}
			
		}
	}

}

Looper维护一个消息队列（在loop()函数里面）

Message.obtain可回收的获取Message对象（这样就不用多次new对象）

Handler通过sendMessage来把Message进行消息队列的入队，并且把Message的target（也就是对应的Handler绑定为自己）

当消息队列循环到相应的Message的时候，会调用Message绑定的Handler的dispatchMessage函数，该函数会调用handleMessage函数，也就是用户实现的handleMessage函数。Handler可以绑定多个Message，通过Message.what来识别身份。

--------源码所得，2014/ 9/ 9 修改