Download Image using AsyncTask in Android

<li>AsyncTasks should ideally be used for short operations (a few seconds at the most.)</li>
<li>An asynchronous task is defined by 3 generic types, called Params, Progress and Result, and 4 steps, called <code>onPreExecute()</code>, <code>doInBackground()</code>, <code>onProgressUpdate()</code> and <code>onPostExecute()</code>.</li>
<li>In <code>onPreExecute()</code> you can define code, which need to be executed before background processing starts.</li>
<li>doInBackground have code which needs to be executed in background, here in <code>doInBackground()</code> 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.</li>
<li><code>onProgressUpdate()</code> method receives progress updates from <code>doInBackground()</code> method, which is published via <code>publishProgress()</code> method, and this method can use this progress update to update event thread</li>
<li><code>onPostExecute()</code> method handles results returned by <code>doInBackground()</code> method.</li>
<li>The generic types used are

<li>Params, the type of the parameters sent to the task upon execution</li>
<li>Progress, the type of the progress units published during the background computation.</li>
<li>Result, the type of the result of the background computation.</li>

<li>If an async task not using any types, then it can be marked as Void type.</li>
<li>An running async task can be cancelled by calling <code>cancel(boolean)</code> method.</li>

<?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>

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;
        }
    }
}