Example
This tutorial explains how to download Image using AsyncTask in Android. The example below download image while showing progress bar while during download.
Understanding Android AsyncTask
Async task enables you to implement MultiThreading without get Hands dirty into threads. AsyncTask enables proper and easy use of the UI thread. It allows performing background operations and passing the results on the UI thread. If you are doing something isolated related to UI, for example downloading data to present in a list, go ahead and use AsyncTask.- AsyncTasks should ideally be used for short operations (a few seconds at the most.)
- An asynchronous task is defined by 3 generic types, called Params, Progress and Result, and 4 steps, called
onPreExecute(),doInBackground(),onProgressUpdate()andonPostExecute(). - In
onPreExecute()you can define code, which need to be executed before background processing starts. - doInBackground have code which needs to be executed in background, here in
doInBackground()we can send results to multiple times to event thread by publishProgress() method, to notify background processing has been completed we can return results simply. onProgressUpdate()method receives progress updates fromdoInBackground()method, which is published viapublishProgress()method, and this method can use this progress update to update event threadonPostExecute()method handles results returned bydoInBackground()method.- The generic types used are
- Params, the type of the parameters sent to the task upon execution
- Progress, the type of the progress units published during the background computation.
- Result, the type of the result of the background computation.
- If an async task not using any types, then it can be marked as Void type.
- An running async task can be cancelled by calling
cancel(boolean)method.
Downloading image using Android AsyncTask
your .xml layout
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:orientation="vertical" >
<Button
android:id="@+id/downloadButton"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:text="Click Here to Download" />
<ImageView
android:id="@+id/imageView"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:contentDescription="Your image will appear here" />
</LinearLayout>
.java class
package com.javatechig.droid;
import java.io.InputStream;
import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.HttpStatus;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
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.util.Log;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;
public class ImageDownladerActivity extends Activity {
private ImageView downloadedImg;
private ProgressDialog simpleWaitDialog;
private String downloadUrl = "http://www.9ori.com/store/media/images/8ab579a656.jpg";
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.asynch);
Button imageDownloaderBtn = (Button) findViewById(R.id.downloadButton);
downloadedImg = (ImageView) findViewById(R.id.imageView);
imageDownloaderBtn.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View v) {
// TODO Auto-generated method stub
new ImageDownloader().execute(downloadUrl);
}
});
}
private class ImageDownloader extends AsyncTask {
@Override
protected Bitmap doInBackground(String... param) {
// TODO Auto-generated method stub
return downloadBitmap(param[0]);
}
@Override
protected void onPreExecute() {
Log.i("Async-Example", "onPreExecute Called");
simpleWaitDialog = ProgressDialog.show(ImageDownladerActivity.this,
"Wait", "Downloading Image");
}
@Override
protected void onPostExecute(Bitmap result) {
Log.i("Async-Example", "onPostExecute Called");
downloadedImg.setImageBitmap(result);
simpleWaitDialog.dismiss();
}
private Bitmap downloadBitmap(String url) {
// initilize the default HTTP client object
final DefaultHttpClient client = new DefaultHttpClient();
//forming a HttpGet request
final HttpGet getRequest = new HttpGet(url);
try {
HttpResponse response = client.execute(getRequest);
//check 200 OK for success
final int statusCode = response.getStatusLine().getStatusCode();
if (statusCode != HttpStatus.SC_OK) {
Log.w("ImageDownloader", "Error " + statusCode +
" while retrieving bitmap from " + url);
return null;
}
final 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
final Bitmap bitmap = BitmapFactory.decodeStream(inputStream);
return bitmap;
} 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("ImageDownloader", "Something went wrong while" +
" retrieving bitmap from " + url + e.toString());
}
return null;
}
}
}
Since there is currently no comment field for examples (or I haven't found it or I haven't permission for it) here is some comment about this:
This is a good example what can be done with AsyncTask.
However the example currently has problems with
- possible memory leaks
- app crash if there was a screen rotation shortly before the async task finished.
For details see:
- Pass Activity as WeakReference to avoid memory leaks
- http://stackoverflow.com/documentation/android/117/asynctask/5377/possible-problems-with-inner-async-tasks
- Avoid leaking Activities with AsyncTask
