Android: class to download remote resources and implemente Observer pattern and a simple extension of ImageView that use it.

attrs.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>
	<declare-styleable name="ImageViewExtended">
		<attr name="href" format="string"/>
	</declare-styleable>
</resources>

ImageViewExtended.java

package com.example.android;

import android.content.Context;
import android.widget.ImageView;
import android.util.AttributeSet;
import android.content.res.TypedArray;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.view.animation.AnimationUtils;
import android.view.animation.Animation;

import java.net.URL;
import java.io.File;
import java.net.URLEncoder;


/**
 * Declare in the layout this class like
 *
 * <com.example.android.ImageViewExtended/>
 *
 * In order to use the "href" attribute you have to declare its namespace like this
 * 	xmlns:ms="http://schemas.android.com/apk/res/com.example.android"
 *
 */
public class ImageViewExtended extends ImageView implements RemoteResourcesDownloader.RemoteResourcesObserver {
	private static final String TAG = "ImageViewExtended";
	private Context mContext;
	private String mHref;
	private Animation mAnimation;

	public ImageViewExtended(Context context) {
		this(context, null, 0);
	}

	public ImageViewExtended(Context context, AttributeSet attrs) {
		super(context, attrs);

		mContext = context;

		init(context, attrs, 0);
	}

	public ImageViewExtended(Context context, AttributeSet attrs, int defStyle) {
		super(context, attrs, defStyle);

		mContext = context;

		init(context, attrs, defStyle);
	}

	private void init(Context context, AttributeSet attrs, int defStyle) {
		TypedArray a = context.obtainStyledAttributes(attrs, R.styleable.ImageViewExtended, defStyle, 0);

		// this is useless for now since we have only one attribute
		final int N = a.getIndexCount();
		for (int i = 0; i < N; i++) {
			//android.util.Log.i(TAG, "N=" + N);
			int attr = a.getIndex(i);
			switch (attr) {
				case R.styleable.ImageViewExtended_href:
					//android.util.Log.i(TAG, "ImageViewExtended_href");
					setImageHref(a.getString(attr));
					break;
				default:
					android.util.Log.i(TAG, "default");
			}
		}

		a.recycle();
	}

	private void startSpinner() {
		mAnimation = AnimationUtils.loadAnimation(mContext, R.anim.clockwise_rotation);
		setImageResource(R.drawable.spinner_black_48);
		startAnimation(mAnimation);
	}

	private void stopSpinner() {
		mAnimation.cancel();
		mAnimation.reset();
	}

	private String getLocalFilePath(String filename, boolean inSDCard) {
		// http://stackoverflow.com/questions/3082325/my-cache-folder-in-android-is-null-do-i-have-to-create-it
		String cacheDirName;
		// get (if exists) the SD Card mount path and create (if doesn't exists) the directory
		if (inSDCard && android.os.Environment.getExternalStorageState().equals(android.os.Environment.MEDIA_MOUNTED)) {
			cacheDirName = android.os.Environment.getExternalStorageDirectory() + "/myApp/";
			File cacheDir = new File(cacheDirName);
			if (!cacheDir.exists()) {
				cacheDir.mkdirs();
			}
		} else {
			cacheDirName = mContext.getCacheDir().toString();
		}

		// "/data/com.example.android.myApp/cache/" + "meow.png"
		String localFileName = cacheDirName + filename;

		return localFileName;
	}

	private void setImageBroken() {
		setImageResource(R.drawable.broken_image);
	}

	/**
	 * This sets the href attribute and downloads the image if necessary.
	 *
	 * This implementations get the URL, find the last path component and look
	 * in the local filesystem if a file exists with the same name; if not call
	 * the RemoteResourcesDownloader and set a default image.
         * TODO: if mHref was already set with different value notify the downloader
         * to remove this instance from the observer for the old resource.
	 */
	public void setImageHref(String href) {
		mHref = href;

		// imageURL = "http://www.dominio.com/miao/bau/meow.png"
		final URL imageURL;
		try {
			imageURL = new URL(href);
		} catch(java.net.MalformedURLException e) {
			/*
			 * If the href is malformed what to do? we simply set a broken image
			 * and return.
			 */
			e.printStackTrace();
			setImageBroken();
			return;
		}
		// ["miao", "bau", "meow.png"]
		String[] components = imageURL.getFile().split("/");
		// "meow.png"
		String filename = "";
		try {
			filename = URLEncoder.encode(components[components.length - 1], "UTF-8");
		} catch (java.io.UnsupportedEncodingException e) {
			filename = components[components.length - 1];
		}

		String localFileName = getLocalFilePath(filename, false);

		final File localFile = new File(localFileName);

		if (!localFile.exists()) {
			startSpinner();

			RemoteResourcesDownloader.getInstance().askForResource(this, imageURL, localFile);
		} else {
			setBitmapFromFilename(localFileName);
		}
	}

	protected void setBitmapFromFilename(String localFileName) {
		Bitmap b = BitmapFactory.decodeFile(localFileName);
		super.setImageBitmap(b);
	}

	/**
	 * When the downloader has finished this instance will be advised and
	 * recall setImageHref() so to find the file locally and load the Bitmap.
	 */
	public void notifyEndDownload() {

		/*
		 * This will be called from a thread different from the UI thread and can cause
		 * crash of application. Run instead in the UIThread using post().
		 */
		post(new Runnable() {
			public void run() {
				stopSpinner();
				setImageHref(mHref);
			}
		});
	}
}

RemoteResourcesDownloader.java

package com.example.android;

import java.net.URL;
import java.util.HashMap;
import java.util.List;
import java.util.ArrayList;
import java.net.HttpURLConnection;
import java.io.InputStream;
import java.io.FileOutputStream;
import java.io.File;


/**
 * This class downloads a resource from remote server and then
 * notify some registered observers.
 *
 * The resource is downloaded in the filesystem with the same name.
 *
 * EXAMPLE OF USAGE
 * <pre>
 * RemoteResourcesDownloader.getInstance().askForResource(this,
 * 	new URL("http://www.example.com/random/resource"),
 * 	new File("/data/com.example.android/cache/"));
 * </pre>
 *
 * TODO: make the request grouped by some parameter. (imagine that a bunch of
 * images need to be donwloaded in order to show sponsors on a map and that we want
 * to refresh the map only when all the images are retrieved).
 * TODO: POST requests handling.
 * TODO: Timeout of connection.
 * TODO: return something like an InputStream for situation with streaming data (imagine
 * the observer needs to parse a JSON without saving on the filesystem).
 */
public class RemoteResourcesDownloader {
	private static RemoteResourcesDownloader instance = null;

	private RemoteResourcesDownloader() {
	}

	public interface RemoteResourcesObserver {
		public void notifyEndDownload();
	}

	/**
	 * This variable contains the pair URL-observers.
	 */
	private HashMap<URL, List<RemoteResourcesObserver>> mObservers = new HashMap();

	/**
	 * This class only downloads the resource in a background thread.
	 */
	private class DownloadTask extends Thread {
		private URL mURL;
		private File mDestination;

		public DownloadTask(URL url, File destination) {
			mDestination = destination;
			mURL = url;
		}

		@Override
		public void run() {
			/**
			 * If some error happens what to do? is it possible to atomize the write of the file.
			 */
			try {
				HttpURLConnection conn = (HttpURLConnection)mURL.openConnection();

				InputStream is = conn.getInputStream();
				FileOutputStream f = new FileOutputStream(mDestination);

				byte[] buffer = new byte[1024];
				int len = 0;
				while ((len = is.read(buffer)) > 0) {
					f.write(buffer, 0, len);
				}
			} catch(Exception e) {
				e.printStackTrace();
			}
			notifyObserver(mURL);
		}
	}

	public void notifyObserver(URL url) {
		List<RemoteResourcesObserver> observers = mObservers.get(url);

		for (RemoteResourcesObserver o : observers) {
			o.notifyEndDownload();
		}

		mObservers.remove(url);
	}

	synchronized public void askForResource(RemoteResourcesObserver object, URL url, File destination) {
		List<RemoteResourcesObserver> observers = new ArrayList();
		if (!mObservers.containsKey(url)) {
			new DownloadTask(url, destination).start();
		} else {
			observers = mObservers.get(url);
		}

		observers.add(object);
		mObservers.put(url, observers);
	}

	static public RemoteResourcesDownloader getInstance() {
		if (instance == null) {
			instance = new RemoteResourcesDownloader();
		}

		return instance;
	}
}