Android ImageLoader - load images sequencially in the background

本文介绍了一种简化Android中图片加载过程的自定义ImageLoader类，通过该类可以使用一行代码轻松加载图片到ImageView，并实现了缓存、取消加载等功能，极大地提高了开发效率。

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.

1 2	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.

1	`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<string , Bitmap> _urlToBitmap;
	private Queue<group> _queue;
	private DownloadThread _thread;
	private Bitmap _missing;
	private boolean _busy;
 
    /**
     * Constructor
     */
	private ImageLoader () {
		_urlToBitmap = new HashMap<string , Bitmap>();
		_queue = new LinkedList<group>();
		_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</group><group> 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</group><group>();
	}
 
	public void clearCache() {
		_urlToBitmap = new HashMap<string , Bitmap>();
	}
 
	public void cancel() {
		clearQueue();
		if ( _thread != null ) {
			_thread.disconnect();
			_thread = null;
		}
	}
 
	public void setMissingBitmap(Bitmap bitmap) {
		_missing = bitmap;
	}
 
	private void loadNext() {
		Iterator<group> 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();
			}
		}
	}
}