Handler介绍

　　下面举个例子：

　　在main.xml文件中：

 1 <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
 2     xmlns:tools="http://schemas.android.com/tools"
 3     android:layout_width="match_parent"
 4     android:layout_height="match_parent"
 5     android:orientation="vertical">
 6
 7     <ImageView
 8         android:id="@+id/imageView1"
 9         android:layout_width="fill_parent"
10         android:layout_height="0dp"
11          android:layout_weight="7"/>
12
13     <Button
14         android:id="@+id/button1"
15         android:layout_width="fill_parent"
16         android:layout_height="0dp"
17         android:background="@drawable/shape_button"
18         android:layout_weight="1"
19         android:text="下载图片" />
20 </LinearLayout>

　　在MainActivity.java文件中:

package com.example.handler1;

import java.io.IOException;

import org.apache.http.HttpEntity;
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.Bundle;
import android.os.Handler;
import android.os.Message;
import android.app.Activity;
import android.app.ProgressDialog;
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 String image_path = "http://p0.so.qhimg.com/t01aa653c8cd80777a5.jpg";
    private Message message;
    private Button button;
    private ProgressDialog dialog;
    private ImageView imageView;
    private Handler handler = new Handler(){
        @Override
        public void handleMessage(Message msg) {
            // TODO 自动生成的方法存根
            super.handleMessage(msg);
            byte[] data = (byte[]) msg.obj;
            imageView.setImageBitmap(BitmapFactory.decodeByteArray(data, 0,data.length));
            if(1 == msg.what){
                dialog.dismiss();
            }
        }
    };
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        dialog = new ProgressDialog(MainActivity.this);
        dialog.setTitle("提示");
        dialog.setMessage("正在下载,请稍候...");
        button = (Button) findViewById(R.id.button1);
        button.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO 自动生成的方法存根
                dialog.show();
                dialog.setCancelable(false);
                new Thread(new MyThread()).start();
            }
        });

    imageView = (ImageView) findViewById(R.id.imageView1);

    }

    @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;
    }

    private class MyThread implements Runnable{

        @Override
        public void run() {
            // TODO 自动生成的方法存根

            HttpClient client = new DefaultHttpClient();
            HttpGet get = new HttpGet(image_path);
            try {
                HttpResponse response = client.execute(get);
                if(response.getStatusLine().getStatusCode() == 200){
                    HttpEntity entity = response.getEntity();
                    byte[] data = EntityUtils.toByteArray(entity);
                    message = Message.obtain();
                    message.obj = data;
                    message.what = 1;
                    handler.sendMessage(message);
                }

            } catch (Exception e) {
                // TODO: handle exception
            }
        }

    }
}

　　在AndroidManifest.xml文件中配置下网络权限:

　　<uses-permission android:name="android.permission.INTERNET"/>

　　运行结果: