asyncTask异步任务：从网络下载图片

<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"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >
    <ImageView
        android:id="@+id/imageView1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true" />
    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="136dp"
        android:text="下载网络图片" />
</RelativeLayout>

package com.example.downloadimage01;
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.app.Activity;
import android.app.ProgressDialog;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.os.Bundle;
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 ImageView imageView ;
    private Button button ;   
    private ProgressDialog dialog ;
    //来自网络的图片
    private String image_path = "http://imgsrc.baidu.com/forum/pic/item/7c1ed21b0ef41bd51a5ac36451da81cb39db3d10.jpg" ;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        
        //添加弹出的对话框
        dialog = new ProgressDialog(this) ;
        dialog.setTitle("提示") ;
        dialog.setMessage("正在下载图片，请稍后···") ;
        
        imageView = (ImageView)findViewById(R.id.imageView1) ;
        button = (Button)findViewById(R.id.button1) ;
        button.setOnClickListener(new OnClickListener() {
            
            @Override
            public void onClick(View v) {
            //点击按钮时，执行异步任务的操作
                new DownTask().execute(image_path) ;
            }
        }) ;   //注意，这个地方的分号容易遗忘
    }
    /* 
     * 异步任务执行网络下载图片
     * */
    public class DownTask extends AsyncTask<String, Void, Bitmap> {
    //上面的方法中，第一个参数：网络图片的路径，第二个参数的包装类：进度的刻度，第三个参数：任务执行的返回结果
        @Override
        //在界面上显示进度条
        protected void onPreExecute() {
            dialog.show() ;
        };
        protected Bitmap doInBackground(String... params) {  //三个点，代表可变参数
            //使用网络链接类HttpClient类完成对网络数据的提取
            HttpClient httpClient = new DefaultHttpClient() ;
            HttpGet httpget = new HttpGet(params[0]) ;
            Bitmap bitmap = null ;
            try {
                HttpResponse httpResponse = httpClient.execute(httpget) ;
                if(httpResponse.getStatusLine().getStatusCode()==200){
                    HttpEntity httpEntity = httpResponse.getEntity() ;
                    byte[] data = EntityUtils.toByteArray(httpEntity);
                    bitmap = BitmapFactory.decodeByteArray(data, 0, data.length);
                }
            }  catch (Exception e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            return bitmap;
        }
            //主要是更新UI
            @Override
            protected void onPostExecute(Bitmap result) { 
                super.onPostExecute(result);
                imageView.setImageBitmap(result) ;//更新UI
                dialog.dismiss() ;                
            }        
    }
    @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;
    }
    
}