带有对话框(范例)的Android异步任务

AsyncTask可以正确，方便地使用UI线程。

AsyncTask被设计成一个围绕线程和Handler的helper类，不构成通用线程框架，理想情况下，AsyncTasks应该用于短操作（最多几秒）。如果需要保持线程长时间运行，强烈建议您使用java.util.concurrent pacakge提供的各种API，例如 Executor，ThreadPoolExecutor和FutureTask。

详细阅读http://developer.android.com/reference/android/os/AsyncTask.html

异步任务允许在后台处理长任务，从而释放UI线程。

LongTask.java


public class LongTask extends AsyncTask<Void, Void, Boolean>
{
 private Context ctx;
 private String TAG="LongTask.java";
 private String url;
 private ProgressDialog p;
 private String filename;
 private ImageView i;
 private Bitmap bitmap;
 /*
 Constructor
 */
 public LongTask(String fileurl,String filename,ImageView i,Context ctx)
 {
 Log.v(TAG,"Url Passed");
 this.url=fileurl;
 this.ctx=ctx;
 this.filename=filename;
 this.p=new ProgressDialog(ctx);
 this.i=i;
 this.bitmap=null;
 }

 /*
 Runs on the UI thread before doInBackground
 */
 @Override
 protected void onPreExecute() {
 super.onPreExecute();
 p.setMessage("Saving image to SD Card");
 p.setIndeterminate(false);
 p.setProgressStyle(ProgressDialog.STYLE_SPINNER);
 p.setCancelable(false);
 p.show();
 }

 /*
 This method to perform a computation on a background thread.
 */
 @Override
 protected Boolean doInBackground(Void... voids) {

 /*
 1. Instantiate file class and pass the directory and subdirectory
 SDCARD/PICTURES/Test
 */
 File location = new File(
 Environment
 .getExternalStoragePublicDirectory(Environment.DIRECTORY_PICTURES),
"Test");

 /*
 2. Create Directory Test with File object media
 */
 if (!location.exists()) {
 if (!location.mkdirs()) {
 Log.v(TAG,"Directory was not created");
 return null;
 }
 }
 /*
 3. Begin HTTP download
 */

 DefaultHttpClient client = new DefaultHttpClient();
 HttpGet getRequest = new HttpGet(url);
 try {
 HttpResponse response = client.execute(getRequest);
 //check 200 OK for success
 int statusCode = response.getStatusLine().getStatusCode();
 if (statusCode != HttpStatus.SC_OK) {
 Log.v("FIle download error","Error.Status.Code ->" + statusCode +" from url ->"+ url);
 return false;
 }

 HttpEntity entity = response.getEntity();
 if (entity != null) {
 InputStream inputStream = null;
 try {
 // getting contents from the stream
 inputStream = entity.getContent();
 // decoding stream data back into image Bitmap that android understands
 bitmap = BitmapFactory.decodeStream(inputStream);
 File thumburl = new File(location, filename+".jpg");
 try {
 File file = new File(thumburl.getPath());
 FileOutputStream fOut = new FileOutputStream(file);
 bitmap.compress(Bitmap.CompressFormat.JPEG, 60, fOut);
 fOut.flush();
 fOut.close();
 return true;
 }
 catch (Exception e)
 {
 e.printStackTrace();
 }
 }
 finally
 {
 if (inputStream != null)
 {
 inputStream.close();
 }
 entity.consumeContent();
 }
 }
 }
 catch (Exception e) {
 // You Could provide a more explicit error message for IOException
 getRequest.abort();
 Log.e(TAG,"Image download error->"+ e.toString());
 }
 return false;
 }

 /*
 Runs on the UI thread after doInBackground
 */
 @Override
 protected void onPostExecute(Boolean result) {
 super.onPostExecute(result);
 p.dismiss();
 if(result)
 {
 // Do something awesome here
 Toast.makeText(ctx,"Download complete", Toast.LENGTH_SHORT).show();
 i.setImageBitmap(bitmap);
 }
 else
 {
 Toast.makeText(ctx,"Download failed, network issue",Toast.LENGTH_SHORT).show();
 }
 }
}

MyActivty.java


public class MyActivity extends Activity {
 @Override
 public void onCreate(Bundle savedInstanceState) {
 super.onCreate(savedInstanceState);
 setContentView(R.layout.main);
 }

 public void GetFile(View view)
 {
 ImageView i=(ImageView)findViewById(R.id.imageView);
 String fileUrl="http://25.media.tumblr.com/d55a509993790027240311c9f611aaf8/tumblr_n0hpzpKEfE1st5lhmo1_1280.jpg";
 LongTask l=new LongTask(fileUrl,"my_large_download_test.jpg",i,MyActivity.this);
 l.execute();

 }
}

结果