安卓Handler和Message

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activity_main.xml

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:text="下载网络图片" />

    <ImageView
        android:id="@+id/imageView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:src="@drawable/ic_launcher" />

</RelativeLayout>

MainActivity

package com.example.android_asytask_download;

import java.io.ByteArrayOutputStream;
import java.io.InputStream;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
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.R.integer;
import android.app.Activity;
import android.app.ProgressDialog;
import android.content.Entity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.ImageView;

public class MainActivity extends Activity {
    private Button button;
    private ImageView imageView;
    private ProgressDialog progressDialog;
    private final int IS_END = 1;
    private String image_path = "http://a.hiphotos.baidu.com/image/w%3D2048/sign=7610c70c5143fbf2c52ca1238446cb80/d4628535e5dde71120a3a1f2a5efce1b9d166145.jpg";
    private Handler handler = new Handler() {
        @Override
        public void handleMessage(android.os.Message msg) {
            byte[] data = (byte[]) msg.obj;
            Bitmap bitmap = BitmapFactory.decodeByteArray(data, 0, data.length);
            imageView.setImageBitmap(bitmap);
            if(msg.what==IS_END)
            {
                progressDialog.dismiss();
            }
        };
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        button = (Button) this.findViewById(R.id.button1);
        imageView = (ImageView) this.findViewById(R.id.imageView1);
        progressDialog = new ProgressDialog(this);
        progressDialog.setTitle("提示");
        progressDialog.setMessage("正在下载，请稍后...");
        progressDialog.setCancelable(false);// 点击不会失去焦点，直到下载结束

        button.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View arg0) {
                // TODO Auto-generated method stub
                new Thread(new MyThread()).start();
                progressDialog.show();
            }
        });
    }

    public class MyThread implements Runnable {
        @Override
        public void run() {
            // 线程耗时部分的主体操作
            HttpClient httpClient = new DefaultHttpClient();
            HttpGet httpGet = new HttpGet(image_path);
            try {
                HttpResponse httpResponse = httpClient.execute(httpGet);// 执行http操作
                if (httpResponse.getStatusLine().getStatusCode() == 200) {
                    byte[] data = EntityUtils.toByteArray(httpResponse
                            .getEntity());
                    // 一般实例化消息对象不用new Message()，而是通过Message.obtain()
                    Message message = Message.obtain();
                    message.obj = data;
                    message.what = IS_END;
                    handler.sendMessage(message);// handler对象发送消息
                }
            } catch (Exception e) {
                // TODO: handle exception
            }
        }
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

}

1.Handler对象负责发送消息和处理消息

2.Message消息对象实例化使用Message message = Message.obtain();