android universalimageloader 几点改进

Android 中加载图片的工作频繁且重复，找一款好的组件使用是很顺手的事情。开源框架 ImageLoader 用起来还不错，入门参考 http://blog.csdn.net/hhhccckkk/article/details/8898651

在项目中用了一段时间后，发现一些可以改进的地方：

（1）每次访问网络取图片，发现加载器总是会两次访问同一个地址，对于 GPRS 这样的蜗牛网速来说，这可不是什么好事。找找原因，起初以为是没有缓存在内存或SD卡，后来一一排除，原来是因为有段代码访问了两次，似乎开发者没有找到什么特别好的办法解决。本人尝试了几种办法，发现是可以改进的。

（2）我接受的方法是在 Application 中初始化加载器的时候要使用自己扩展的 ImageDecoder

protected void initImageCache() {
		/**
		 * 初始化图片加载
		 */
		Logger.d(TAG, "Initializing image loader.");
		File cacheDir = StorageUtils.getOwnCacheDirectory(getApplicationContext(), "myApplication/Cache");
		ImageLoaderConfiguration.Builder builder = new ImageLoaderConfiguration.Builder(getApplicationContext());
		builder.threadPoolSize(3); // 设置线程数量为3
		builder.threadPriority(Thread.NORM_PRIORITY - 1); // 设定线程等级比普通低一点
		builder.memoryCacheExtraOptions(200, 200); // 设定缓存在内存的图片大小最大为200x200
		builder.memoryCache(new UsingFreqLimitedMemoryCache(2 * 1024 * 1024));
		builder.discCache(new UnlimitedDiscCache(cacheDir)) ;
		builder.discCacheExtraOptions(displayMetrics.widthPixels, displayMetrics.heightPixels, CompressFormat.JPEG, 80, null);
		builder.denyCacheImageMultipleSizesInMemory(); // 拒绝缓存同一图片，有不同的大小
		builder.discCacheFileNameGenerator(new Md5FileNameGenerator());
		builder.imageDownloader(new MyImageLoader(getApplicationContext(),restClient));//new BaseImageDownloader(getApplicationContext()));//
		builder.imageDecoder(new MyImageDecoder(true));
		builder.enableLogging(); // 开启调试

		// 设置默认显示情况
		DisplayImageOptions.Builder displayImageOptionsBuilder = new DisplayImageOptions.Builder();
		displayImageOptionsBuilder.showImageForEmptyUri(R.drawable.house_icon_photo4); // 空uri的情况
		displayImageOptionsBuilder.cacheInMemory(true); // 缓存在内存
		displayImageOptionsBuilder.cacheOnDisc(true); // 缓存在磁盘
		displayImageOptionsBuilder.imageScaleType(ImageScaleType.IN_SAMPLE_POWER_OF_2);
		builder.defaultDisplayImageOptions(displayImageOptionsBuilder.build());
		ImageLoader.getInstance().init(builder.build());
		Logger.d(TAG, "Initialize image loader finished.");

	}

 （2）MyImageDecoder.java 扩展自 BaseImageDecoder

import java.io.IOException;
import java.io.InputStream;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory.Options;

import myapp.sdk.io.CloseShieldInputStream;
import com.nostra13.universalimageloader.core.assist.ImageSize;
import com.nostra13.universalimageloader.core.decode.BaseImageDecoder;
import com.nostra13.universalimageloader.core.decode.ImageDecodingInfo;
import com.nostra13.universalimageloader.utils.L;

public class MyImageDecoder  extends BaseImageDecoder{

	public MyImageDecoder(boolean loggingEnabled) {
		this.loggingEnabled = loggingEnabled;
	}
	@Override
	public Bitmap decode(ImageDecodingInfo decodingInfo) throws IOException {
		InputStream imageStream = getImageStream(decodingInfo);
		// inputStream mark
		imageStream.mark(imageStream.available()+1);  
		InputStream imageStreamClone = new CloseShieldInputStream(imageStream);
		ImageFileInfo imageInfo = defineImageSizeAndRotation(imageStreamClone, decodingInfo.getImageUri());
		Options decodingOptions = prepareDecodingOptions(imageInfo.imageSize, decodingInfo);
		// imageStream = getImageStream(decodingInfo);//michael remove double load from server
		// inputStream reset
		imageStream.reset();
		Bitmap decodedBitmap = decodeStream(imageStream, decodingOptions);
		if (decodedBitmap == null) {
			L.e(ERROR_CANT_DECODE_IMAGE, decodingInfo.getImageKey());
		} else {
			decodedBitmap = considerExactScaleAndOrientaiton(decodedBitmap, decodingInfo, imageInfo.exif.rotation, imageInfo.exif.flipHorizontal);
		}
		return decodedBitmap;
	}

}

 （3）CloseShieldInputStream.java 可以直接使用 apache common-io,但在 android 上为了克隆 InputStream 一点点功能，而引入整个 IO 包，100多KB，似乎不划算，所以单独复制了 IO 包中的几个文件：

import java.io.InputStream;

/***
 * Proxy stream that prevents the underlying input stream from being closed.
 * <p>
 * This class is typically used in cases where an input stream needs to be
 * passed to a component that wants to explicitly close the stream even if
 * more input would still be available to other components.
 *
 * @version $Id: CloseShieldInputStream.java 587913 2007-10-24 15:47:30Z niallp $
 * @since Commons IO 1.4
 */
public class CloseShieldInputStream extends ProxyInputStream {

    /***
     * Creates a proxy that shields the given input stream from being
     * closed.
     *
     * @param in underlying input stream
     */
    public CloseShieldInputStream(InputStream in) {
        super(in);
    }

    /***
     * Replaces the underlying input stream with a {@link ClosedInputStream}
     * sentinel. The original input stream will remain open, but this proxy
     * will appear closed.
     */
    public void close() {
        in = new ClosedInputStream();
    }

}

import java.io.FilterInputStream;
import java.io.IOException;
import java.io.InputStream;

/***
 * A Proxy stream which acts as expected, that is it passes the method 
 * calls on to the proxied stream and doesn't change which methods are 
 * being called. 
 * <p>
 * It is an alternative base class to FilterInputStream
 * to increase reusability, because FilterInputStream changes the 
 * methods being called, such as read(byte[]) to read(byte[], int, int).
 * 
 * @author Stephen Colebourne
 * @version $Id: ProxyInputStream.java 610010 2008-01-08 14:50:59Z niallp $
 */
public abstract class ProxyInputStream extends FilterInputStream {

    /***
     * Constructs a new ProxyInputStream.
     * 
     * @param proxy  the InputStream to delegate to
     */
    public ProxyInputStream(InputStream proxy) {
        super(proxy);
        // the proxy is stored in a protected superclass variable named 'in'
    }

    /***
     * Invokes the delegate's <code>read()</code> method.
     * @return the byte read or -1 if the end of stream
     * @throws IOException if an I/O error occurs
     */
    public int read() throws IOException {
        return in.read();
    }

    /***
     * Invokes the delegate's <code>read(byte[])</code> method.
     * @param bts the buffer to read the bytes into
     * @return the number of bytes read or -1 if the end of stream
     * @throws IOException if an I/O error occurs
     */
    public int read(byte[] bts) throws IOException {
        return in.read(bts);
    }

    /***
     * Invokes the delegate's <code>read(byte[], int, int)</code> method.
     * @param bts the buffer to read the bytes into
     * @param st The start offset
     * @param end The number of bytes to read
     * @return the number of bytes read or -1 if the end of stream
     * @throws IOException if an I/O error occurs
     */
    public int read(byte[] bts, int st, int end) throws IOException {
        return in.read(bts, st, end);
    }

    /***
     * Invokes the delegate's <code>skip(long)</code> method.
     * @param ln the number of bytes to skip
     * @return the number of bytes to skipped or -1 if the end of stream
     * @throws IOException if an I/O error occurs
     */
    public long skip(long ln) throws IOException {
        return in.skip(ln);
    }

    /***
     * Invokes the delegate's <code>available()</code> method.
     * @return the number of available bytes
     * @throws IOException if an I/O error occurs
     */
    public int available() throws IOException {
        return in.available();
    }

    /***
     * Invokes the delegate's <code>close()</code> method.
     * @throws IOException if an I/O error occurs
     */
    public void close() throws IOException {
        in.close();
    }

    /***
     * Invokes the delegate's <code>mark(int)</code> method.
     * @param idx read ahead limit
     */
    public synchronized void mark(int idx) {
        in.mark(idx);
    }

    /***
     * Invokes the delegate's <code>reset()</code> method.
     * @throws IOException if an I/O error occurs
     */
    public synchronized void reset() throws IOException {
        in.reset();
    }

    /***
     * Invokes the delegate's <code>markSupported()</code> method.
     * @return true if mark is supported, otherwise false
     */
    public boolean markSupported() {
        return in.markSupported();
    }

}

import java.io.InputStream;

/***
 * Closed input stream. This stream returns -1 to all attempts to read
 * something from the stream.
 * <p>
 * Typically uses of this class include testing for corner cases in methods
 * that accept input streams and acting as a sentinel value instead of a
 * <code>null</code> input stream.
 *
 * @version $Id: ClosedInputStream.java 601751 2007-12-06 14:55:45Z niallp $
 * @since Commons IO 1.4
 */
public class ClosedInputStream extends InputStream {
    
    /***
     * A singleton.
     */
    public static final ClosedInputStream CLOSED_INPUT_STREAM = new ClosedInputStream();

    /***
     * Returns -1 to indicate that the stream is closed.
     *
     * @return always -1
     */
    public int read() {
        return -1;
    }

}

（4）ImageLoader 会多次反复发送加载请求，对网络也是个灾难，在扩展的 MyImageLoader 中修改网络连接超时和读取网络超时的时间值

import android.content.Context;

import com.nostra13.universalimageloader.core.download.BaseImageDownloader;

public class MyImageLoader extends BaseImageDownloader {

	public MyImageLoader(Context context) {
		super(context,10000,60000);
	}
}