图片预取缓存策略是内存缓存（硬引用LruCache、软引用SoftReference<Bitmap>）、外部文件缓存（context.getCachedDir()），缓存中取不到的情况下再向服务端请求下载图片。同时缓存三张图片（当前预览的这张，前一张以及后一张）。
1.内存缓存
•//需要导入外部jar文件 android-support-v4.jar •import android.support.v4.util.LruCache; •//开辟8M硬缓存空间 •private final int hardCachedSize = 8*1024*1024; •//hard cache •private final LruCache<String, Bitmap> sHardBitmapCache = new LruCache<String, Bitmap>(hardCachedSize){ •@Override •public int sizeOf(String key, Bitmap value){ •return value.getRowBytes() * value.getHeight(); •} •@Override •protected void entryRemoved(boolean evicted, String key, Bitmap oldValue, Bitmap newValue){ •Log.v("tag", "hard cache is full , push to soft cache"); •//硬引用缓存区满，将一个最不经常使用的oldvalue推入到软引用缓存区 •sSoftBitmapCahe.put(key, new SoftReference<Bitmap>(oldValue)); •} •} •//软引用 •private static final int SOFT_CACHE_CAPACITY = 40; •private final static LinkedHashMap<String, SoftReference<Bitmap>> sSoftBitmapCache = •new LinkedHashMao<String, SoftReference<Bitmap>>(SOFT_CACHE_CAPACITY, 0.75f, true){ •@Override •public SoftReference<Bitmap> put(String key, SoftReference<Bitmap> value){ •return super.input(key, value); •} •@Override •protected boolean removeEldestEntry(LinkedHashMap.Entry<Stirng, SoftReference<Bitmap>> eldest){ •if(size() > SOFT_CACHE_CAPACITY){ •Log.v("tag", "Soft Reference limit , purge one"); •return true; •} •return false; •} •} •//缓存bitmap •public boolean putBitmap(String key, Bitmap bitmap){ •if(bitmap != null){ •synchronized(sHardBitmapCache){ •sHardBitmapCache.put(key, bitmap); •} •return true; •} •return false; •} •//从缓存中获取bitmap •public Bitmap getBitmap(String key){ •synchronized(sHardBitmapCache){ •final Bitmap bitmap = sHardBitmapCache.get(key); •if(bitmap != null) •return bitmap; •} •//硬引用缓存区间中读取失败，从软引用缓存区间读取 •synchronized(sSoftBitmapCache){ •SoftReference<Bitmap> bitmapReference = sSoftBtimapCache.get(key); •if(bitmapReference != null){ •final Bitmap bitmap2 = bitmapReference.get(); •if(bitmap2 != null) •return bitmap2; •else{ •Log.v("tag", "soft reference 已经被回收"); •sSoftBitmapCache.remove(key); •} •} •} •return null; •}
2.外部文件缓存
•private File mCacheDir = context.getCacheDir(); •private static final int MAX_CACHE_SIZE = 20 * 1024 * 1024; //20M •private final LruCache<String, Long> sFileCache = new LruCache<String, Long>(MAX_CACHE_SIZE){ •@Override •public int sizeOf(String key, Long value){ •return value.intValue(); •} •@Override •protected void entryRemoved(boolean evicted, String key, Long oldValue, Long newValue){ •File file = getFile(key); •if(file != null) •file.delete(); •} •} •private File getFile(String fileName) throws FileNotFoundException { •File file = new File(mCacheDir, fileName); •if(!file.exists() || !file.isFile()) •throw new FileNotFoundException("文件不存在或有同名文件夹"); •return file; •} •//缓存bitmap到外部存储 •public boolean putBitmap(String key, Bitmap bitmap){ •File file = getFile(key); •if(file != null){ •Log.v("tag", "文件已经存在"); •return true; •} •FileOutputStream fos = getOutputStream(key); •boolean saved = bitmap.compress(CompressFormat.JPEG, 100, fos); •fos.flush(); •fos.close(); •if(saved){ •synchronized(sFileCache){ •sFileCache.put(key, getFile(key).length()); •} •return true; •} •return false; •} •//根据key获取OutputStream •private FileOutputStream getOutputStream(String key){ •if(mCacheDir == null) •return null; •FileOutputStream fos = new FileOutputStream(mCacheDir.getAbsolutePath() + File.separator + key); •return fos; •} •//获取bitmap •private static BitmapFactory.Options sBitmapOptions; •static { •sBitmapOptions = new BitmapFactory.Options(); •sBitmapOptions.inPurgeable=true; //bitmap can be purged to disk •} •public Bitmap getBitmap(String key){ •File bitmapFile = getFile(key); •if(bitmapFile != null){ •Bitmap bitmap = BitmapFactory.decodeStream(new FileInputStream(bitmapFile), null, sBitmapOptions); •if(bitmap != null){ •//重新将其缓存至硬引用中 •... •} •} •}
3.从服务端下载图片
下载成功后调用1内存缓存的putBitmap()函数,缓存图片。
在外部文件缓存中也写入一份，调用2的putBitmap()函数.
4.预览图片的流程
1) 如果预览的图片在内存缓存区中，直接调用1的getBitmap()函数，获取bitmap数据(先在硬引用缓存区查找匹配，若硬引用区匹配失败，再去软引用区匹配)
2) 如果从内存缓存区读取失败，再从外部文件缓存中读取，调用2的getBitmap()函数
3) 如果从外部文件缓存中读取失败，则从服务端下载该图片，过程3.
5.生成key值
private static String generateKey(String fileId, int width, int height) { String ret = fileId + "_" + Integer.toString(width) + "x" + Integer.toString(height); return ret; }
- String key = generateKey(...)即可生成唯一的key值