Android Universal Image Loader 磁盘缓存分析

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前言

图片加载方面UIL的确很好用，所有有空就看了看UIL的源码，方面自己了解原理并能更熟练的使用它。

源码版本：Android-Universal-Image-Loader-1.9.5

项目地址:Universal-Image-Loader

注意点：此版本discCache过时，用diskCache替换

磁盘缓存分析

DisplayImageOptions开启磁盘缓存

options.cacheOnDisk(true);

在ImageLoaderConfiguration的里面配置磁盘缓存空间

config.diskCacheSize(50 * 1024 * 1024); // 50 MiBconfig.diskCache(new UnlimitedDiskCache(cacheDir)); // 你可以传入自己的磁盘缓存

1.磁盘缓存路径

如果使用的是自定义的缓存路径当然没啥问题，但是我觉得还是有必要知道默认缓存路径，不然我们怎么知道有没有缓存成功呢？

获取缓存目录

File file = ImageLoader.getInstance().getDiskCache().getDirectory();String filePath = file.getAbsolutePath();

获得结果为：
/storage/emulated/0/Android/data/com.nostra13.universalimageloader/cache/uil-images
即：位于SD卡的Android/data/应用程序包名/cache/uil-images文件夹下
例如：
/Android/data/com.baidu.netdisk/cache/uil-images

将该文件夹下的文件复制出来，后缀名加上.jpg即可看到图片。

创建DiskCache的源码部分
DefaultConfigurationFactory.java

    /**     * Creates default implementation of {@link DiskCache} depends on incoming parameters     */    public static DiskCache createDiskCache(Context context,            FileNameGenerator diskCacheFileNameGenerator,            long diskCacheSize, int diskCacheFileCount) {        File reserveCacheDir = createReserveDiskCacheDir(context);        if (diskCacheSize > 0 || diskCacheFileCount > 0) {            File individualCacheDir = StorageUtils.getIndividualCacheDirectory(context);            try {                return new LruDiskCache(individualCacheDir, reserveCacheDir,                 diskCacheFileNameGenerator, diskCacheSize,diskCacheFileCount);                //LruDiskCache 缓存满时优先删除最近最少使用的元素。            } catch (IOException e) {                L.e(e);                // continue and create unlimited cache            }        }        File cacheDir = StorageUtils.getCacheDirectory(context);        return new UnlimitedDiskCache(cacheDir, reserveCacheDir, diskCacheFileNameGenerator);    }    /** Creates reserve disk cache folder which will be used if primary disk cache folder becomes unavailable */    private static File createReserveDiskCacheDir(Context context) {        File cacheDir = StorageUtils.getCacheDirectory(context, false);        File individualDir = new File(cacheDir, "uil-images");//指定缓存目录        if (individualDir.exists() || individualDir.mkdir()) {            cacheDir = individualDir;        }        return cacheDir;    }

2.磁盘缓存时机

既然开启了磁盘缓存
即：cacheOnDisk = true;

在DisplayImageOptions.java 看看谁调用了

    public boolean isCacheOnDisk() {        return cacheOnDisk;    }

毫无疑问，加载图片之前需要用到
LoadAndDisplayImageTask.java

    private Bitmap tryLoadBitmap() throws TaskCancelledException {        Bitmap bitmap = null;        try {            File imageFile = configuration.diskCache.get(uri);            if (imageFile != null && imageFile.exists() && imageFile.length() > 0) {            //有缓存当然就直接用缓存啦~                L.d(LOG_LOAD_IMAGE_FROM_DISK_CACHE, memoryCacheKey);                loadedFrom = LoadedFrom.DISC_CACHE;                checkTaskNotActual();                bitmap = decodeImage(Scheme.FILE.wrap(imageFile.getAbsolutePath()));            }            if (bitmap == null || bitmap.getWidth() <= 0 || bitmap.getHeight() <= 0) {                L.d(LOG_LOAD_IMAGE_FROM_NETWORK, memoryCacheKey);                loadedFrom = LoadedFrom.NETWORK;                String imageUriForDecoding = uri;                if (options.isCacheOnDisk() && tryCacheImageOnDisk()) {                //判断是否需要磁盘缓存，如果是调用tryCacheImageOnDisk()                    imageFile = configuration.diskCache.get(uri);                    if (imageFile != null) {                        imageUriForDecoding = Scheme.FILE.wrap(imageFile.getAbsolutePath());                    }                }                checkTaskNotActual();                bitmap = decodeImage(imageUriForDecoding);                //如果imageUriForDecoding是磁盘的uri就从磁盘取流，否则网络取流                //decodeImage里面调用的是ImageDownloader.getStream()方法                if (bitmap == null || bitmap.getWidth() <= 0 || bitmap.getHeight() <= 0) {                    fireFailEvent(FailType.DECODING_ERROR, null);                }            }        } catch (IllegalStateException e) {            fireFailEvent(FailType.NETWORK_DENIED, null);        } catch (TaskCancelledException e) {            throw e;        } catch (IOException e) {            L.e(e);            fireFailEvent(FailType.IO_ERROR, e);        } catch (OutOfMemoryError e) {            L.e(e);            fireFailEvent(FailType.OUT_OF_MEMORY, e);        } catch (Throwable e) {            L.e(e);            fireFailEvent(FailType.UNKNOWN, e);        }        return bitmap;    }

tryCacheImageOnDisk()方法

    /** @return <b>true</b> - if image was downloaded successfully; <b>false</b> - otherwise */    private boolean tryCacheImageOnDisk() throws TaskCancelledException {        L.d(LOG_CACHE_IMAGE_ON_DISK, memoryCacheKey);        boolean loaded;        try {            loaded = downloadImage();//去下载图片            if (loaded) {            //下载成功                int width = configuration.maxImageWidthForDiskCache;                int height = configuration.maxImageHeightForDiskCache;                if (width > 0 || height > 0) {                    L.d(LOG_RESIZE_CACHED_IMAGE_FILE, memoryCacheKey);                    resizeAndSaveImage(width, height); // TODO : process boolean result                    //根据用户配置的宽高保存图片                }            }        } catch (IOException e) {            L.e(e);            loaded = false;        }        return loaded;    }

图片下载方法：downloadImage()

    private boolean downloadImage() throws IOException {        InputStream is = getDownloader().getStream(uri, options.getExtraForDownloader());        if (is == null) {            L.e(ERROR_NO_IMAGE_STREAM, memoryCacheKey);            return false;        } else {            try {                return configuration.diskCache.save(uri, is, this);                //下载完毕，存进diskCache            } finally {                IoUtils.closeSilently(is);            }        }    }

图片根据用户配置重存 resizeAndSaveImage(int maxWidth, int maxHeight)

    /** Decodes image file into Bitmap, resize it and save it back */    private boolean resizeAndSaveImage(int maxWidth, int maxHeight) throws IOException {        // Decode image file, compress and re-save it        boolean saved = false;        File targetFile = configuration.diskCache.get(uri);        //刚刚下载完不是存进去了吗？拿出来改造~        if (targetFile != null && targetFile.exists()) {            ImageSize targetImageSize = new ImageSize(maxWidth, maxHeight);            DisplayImageOptions specialOptions = new DisplayImageOptions.Builder().cloneFrom(options)                    .imageScaleType(ImageScaleType.IN_SAMPLE_INT).build();            ImageDecodingInfo decodingInfo = new ImageDecodingInfo(memoryCacheKey,                    Scheme.FILE.wrap(targetFile.getAbsolutePath()), uri, targetImageSize, ViewScaleType.FIT_INSIDE,                    getDownloader(), specialOptions);            Bitmap bmp = decoder.decode(decodingInfo);            if (bmp != null && configuration.processorForDiskCache != null) {                L.d(LOG_PROCESS_IMAGE_BEFORE_CACHE_ON_DISK, memoryCacheKey);                bmp = configuration.processorForDiskCache.process(bmp);                if (bmp == null) {                    L.e(ERROR_PROCESSOR_FOR_DISK_CACHE_NULL, memoryCacheKey);                }            }            if (bmp != null) {                saved = configuration.diskCache.save(uri, bmp);                //ok改造完再存回去                bmp.recycle();            }        }        return saved;    }

3磁盘缓存类型

LimitedAgeDiskCache和UnlimitedDiskCache
两个都是继承了BaseDiskCache

LimitedAgeDiskCache限制了缓存对象最长存活周期的磁盘缓存；
UnlimitedDiskCache与BaseDiskCache没啥区别只是命名更让人好理解。

    DiskCache diskCache;    if (limit > 0) {//limit表示Max file age (in seconds)        diskCache = new LimitedAgeDiskCache(cacheDir, limit);    } else {        diskCache = new UnlimitedDiskCache(cacheDir);    }    config.diskCache(diskCache);

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