使用AsyncTask下载图片并显示进度条

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
}

}
}