Android使用AsyncTask下载图片并显示进度条功能

public class AsyncTaskActivity2 extends Activity { 
 private Button btnDown;//图片框 
 private ImageView ivImage;//图片URL 
 private static String image_path = "http://www.wfu.edu.cn/page/main1406/images/nav2.jpg"; 
 //进度对话框 
 private ProgressDialog dialog; 
 @Override 
 protected void onCreate(Bundle savedInstanceState) { 
  super.onCreate(savedInstanceState); 
  setContentView(R.layout.asynctask_activity); 
  btnDown = (Button) findViewById(R.id.btnDown); 
  ivImage = (ImageView) findViewById(R.id.ivSinaImage); 
  //新建一个对话框，不显示 
  dialog = new ProgressDialog(this); 
  dialog.setTitle("提示"); 
  dialog.setMessage("正在下载，请稍后..."); 
  dialog.setProgressStyle(ProgressDialog.STYLE_HORIZONTAL);//带有水平滚动条 
  dialog.setCancelable(false);//不能 
  //单击按钮后执行AsyncTask，并传入图片URL 
  btnDown.setOnClickListener(new View.OnClickListener() { 
   @Override 
   public void onClick(View v) { 
    // 执行异步任务 
    new MyTask().execute(image_path);//启动AsyncTask,参数类型为异步类的第一个参数的类型 
   } 
  }); 
 } 
 //创建一个内部类，用来下载图片 
 public class MyTask extends AsyncTask<String, Integer, Bitmap> { 
  @Override 
  protected void onPreExecute() {//启动异步类后首先运行，可以设置 
   super.onPreExecute(); 
   dialog.show();//显示对话框 
  } 
   //第二步，一般进行复杂处理 
  @Override 
  protected Bitmap doInBackground(String... params) {//最重要，复杂处理，后台，接收execute传来的参数 
   Bitmap bitmap = null; //二进制图片 
   //字节数组输出流 
   ByteArrayOutputStream outputStream = new ByteArrayOutputStream(); 
   InputStream inputStream = null;//字节输入流 
   try { 
    HttpClient httpClient = new DefaultHttpClient();//创建一个Http客户端 
    HttpGet httpGet = new HttpGet(params[0]);//发送请求 
    HttpResponse httpResponse = httpClient.execute(httpGet);//返回 
    if (httpResponse.getStatusLine().getStatusCode() == 200) {//成功 
     inputStream = httpResponse.getEntity().getContent();//得到返回内容作为输入流 
     long file_length = httpResponse.getEntity().getContentLength();//文件长度 
     int len = 0; 
     byte[] data = new byte[1024];//每次读取的字节 
     int total_length = 0; 
     // 以字节的方式读取图片数据 
     while ((len = inputStream.read(data)) != -1) { 
      total_length += len; 
      // 计算进度 
      int values = (int) ((total_length / (float) file_length) * 100); 
      // 发布进度信息 
      publishProgress(values);//发布进度，AsyncTask第二个参数类型，触发onProgressUpdate更新进度条 
      outputStream.write(data, 0, len);//写入输出流 
     } 
     byte[] result=outputStream.toByteArray();//将字节数组输出流转换为字节数组 
     //生成二进制图片 
     bitmap=BitmapFactory.decodeByteArray(result, 0, result.length); 
    } 
   } catch (Exception e) { 
    e.printStackTrace(); 
   } finally { 
    try { 
     if (inputStream != null) { 
      inputStream.close(); 
     } 
    } catch (Exception e2) { 
    } 
   } 
   return bitmap; 
  } 
  //当有 publishProgress(value)发来的数据时触发,更新UI 
  @Override 
  protected void onProgressUpdate(Integer... values) { 
   super.onProgressUpdate(values); 
   // 设置进度对话框的进度值 
   dialog.setProgress(values[0]);//更新进度条，运行在UI 
  } 
  //第四步 
  @Override 
  protected void onPostExecute(Bitmap result) {//下载结束后，隐藏对话框 
   super.onPostExecute(result); 
   dialog.dismiss(); 
   ivImage.setImageBitmap(result); //更新UI，显示图片，运行在UI 
  } 
 } 
}