Android ImageLoader - load images sequencially in the background

"A few days ago I started to learn android… and it’s been a fairly smooth transition from flash. Although I have to say, as flash developers we’re just spoiled. We take for granted all the background stuff flash does for us to make coding that much easier.

One of those things is Loading images. in flash we have the Loader class which makes loading images very easy.

var loader:Loader = new Loader();

loader.load(new URLRequest("myimage.jpg"));

In java/android it takes a few more lines

HttpURLConnection conn = (HttpURLConnection) new URL("myimage.jpg").openConnection();

conn.setDoInput(true);

conn.connect();

InputStream inStream = conn.getInputStream();

Bitmap bitmap = BitmapFactory.decodeStream(inStream);

inStream.close();

conn.disconnect();

The code above runs in the same thread so you’ll lock up your UI until the image has finish loading. Adding the Threading code adds quite a bit more code. So I decided to create an ImageLoader class to make my life just a little easier. Now I can load an image to an ImageView with one line of code.

ImageLoader.getInstance().load(myImageView, "myimage.jpg", true);

The last parameter tells the ImageLoader class to cache that the bitmap.

The ImageLoader class loads images sequentially so you don’t slow your mobile device down to a crawl. To cancel Loading simply call clearQueue() and to clear the cache call clearCache()

Here’s the class

package com.wumedia.net;

import java.io.InputStream;

import java.net.HttpURLConnection;

import java.net.URL;

import java.util.HashMap;

import java.util.Iterator;

import java.util.LinkedList;

import java.util.Queue;

import android.graphics.Bitmap;

import android.graphics.BitmapFactory;

import android.os.Handler;

import android.widget.ImageView;

public class ImageLoader {

static private ImageLoader _instance;

static public ImageLoader getInstance() {

if (_instance == null) {

_instance = new ImageLoader();

}

return _instance;

}

private HashMap _urlToBitmap;

private Queue _queue;

private DownloadThread _thread;

private Bitmap _missing;

private boolean _busy;

/**

* Constructor

*/

private ImageLoader () {

_urlToBitmap = new HashMap();

_queue = new LinkedList();

_busy = false;

}

public Bitmap get(String url) {

return _urlToBitmap.get(url);

}

public void load(ImageView image, String url) {

load(image, url, false);

}

public void load(ImageView image, String url, boolean cache) {

if (_urlToBitmap.get(url) != null) {

if(image!=null) {

image.setImageBitmap(_urlToBitmap.get(url));

}

} else {

image.setImageBitmap(null);

queue(image, url, cache);

}

}

public void queue(ImageView image, String url, boolean cache) {

Iterator it = _queue.iterator();

if (image!=null) {

while (it.hasNext()) {

if (it.next().image.equals(image)) {

it.remove();

break;

}

}

} else if (url!=null) {

while (it.hasNext()) {

if (it.next().url.equals(url)) {

it.remove();

break;

}

}

}

_queue.add(new Group(image, url, null, cache));

loadNext();

}

public void clearQueue() {

_queue = new LinkedList();

}

public void clearCache() {

_urlToBitmap = new HashMap();

}

public void cancel() {

clearQueue();

if ( _thread != null ) {

_thread.disconnect();

_thread = null;

}

}

public void setMissingBitmap(Bitmap bitmap) {

_missing = bitmap;

}

private void loadNext() {

Iterator it = _queue.iterator();

if (!_busy && it.hasNext() ) {

_busy = true;

Group group = it.next();

it.remove();

// double check image availability

if (_urlToBitmap.get(group.url) != null) {

if (group.image!=null) {

group.image.setImageBitmap(_urlToBitmap.get(group.url));

}

_busy = false;

loadNext();

} else {

_thread = new DownloadThread(group);

_thread.start();

}

}

}

private void onLoad() {

if (_thread != null) {

Group group = _thread.group;

if (group.bitmap != null) {

if (group.cache) {

_urlToBitmap.put(group.url, group.bitmap);

}

if (group.image != null) {

group.image.setImageBitmap(group.bitmap);

}

} else if (_missing != null) {

if (group.image != null) {

group.image.setImageBitmap(_missing);

}

}

}

_thread = null;

_busy = false;

loadNext();

}

private class Group {

public Group(ImageView image, String url, Bitmap bitmap, boolean cache) {

this.image = image;

this.url = url;

this.bitmap = bitmap;

this.cache = cache;

}

public ImageView image;

public String url;

public Bitmap bitmap;

public boolean cache;

}

private class DownloadThread extends Thread {

final Handler threadHandler = new Handler();

final Runnable threadCallback = new Runnable() {

public void run() {

onLoad();

}

};

private HttpURLConnection _conn;

public Group group;

public DownloadThread(Group group) {

this.group = group;

}

@Override

public void run() {

InputStream inStream = null;

_conn = null;

try {

_conn = (HttpURLConnection) new URL(group.url).openConnection();

_conn.setDoInput(true);

_conn.connect();

inStream = _conn.getInputStream();

group.bitmap = BitmapFactory.decodeStream(inStream);

inStream.close();

_conn.disconnect();

inStream = null;

_conn = null;

} catch (Exception ex) {

// nothing

}

if (inStream != null) {

try {

inStream.close();

} catch (Exception ex) {}

}

disconnect();

inStream = null;

_conn = null;

threadHandler.post(threadCallback);

}

public void disconnect() {

if (_conn != null) {

_conn.disconnect();

}

}

}

}

" wu-media.com