图片预取缓存策略是内存缓存（硬引用LruCache、软引用SoftReference）、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
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//需要导入外部jar文件 android-support-v4.jar
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import android.support.v4.util.LruCache;
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//开辟8M硬缓存空间
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private final int hardCachedSize = 8*1024*1024;
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//hard cache
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private final LruCache sHardBitmapCache = new LruCache(hardCachedSize){
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@Override
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public int sizeOf(String key, Bitmap value){
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return value.getRowBytes() * value.getHeight();
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}
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@Override
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protected void entryRemoved(boolean evicted, String key, Bitmap oldValue, Bitmap newValue){
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Log.v("tag", "hard cache is full , push to soft cache");
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//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
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sSoftBitmapCahe.put(key, new SoftReference(oldValue));
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}
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}
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//软引用
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private static final int SOFT_CACHE_CAPACITY = 40;
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private final static LinkedHashMap> sSoftBitmapCache =
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new LinkedHashMao>(SOFT_CACHE_CAPACITY, 0.75f, true){
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@Override
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public SoftReference put(String key, SoftReference value){
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return super.input(key, value);
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}
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@Override
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protected boolean removeEldestEntry(LinkedHashMap.Entry> eldest){
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if(size() > SOFT_CACHE_CAPACITY){
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Log.v("tag", "Soft Reference limit , purge one");
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return true;
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}
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return false;
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}
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}
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//缓存bitmap
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public boolean putBitmap(String key, Bitmap bitmap){
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if(bitmap != null){
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synchronized(sHardBitmapCache){
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sHardBitmapCache.put(key, bitmap);
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}
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return true;
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}
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return false;
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}
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//从缓存中获取bitmap
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public Bitmap getBitmap(String key){
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synchronized(sHardBitmapCache){
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final Bitmap bitmap = sHardBitmapCache.get(key);
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if(bitmap != null)
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return bitmap;
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}
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//硬引用缓存区间中读取失败，从软引用缓存区间读取
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synchronized(sSoftBitmapCache){
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SoftReference bitmapReference = sSoftBtimapCache.get(key);
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if(bitmapReference != null){
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final Bitmap bitmap2 = bitmapReference.get();
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if(bitmap2 != null)
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return bitmap2;
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else{
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Log.v("tag", "soft reference 已经被回收");
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sSoftBitmapCache.remove(key);
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}
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}
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}
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return null;
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}
2.外部文件缓存
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private File mCacheDir = context.getCacheDir();
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private static final int MAX_CACHE_SIZE = 20 * 1024 * 1024; //20M
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private final LruCache sFileCache = new LruCache(MAX_CACHE_SIZE){
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@Override
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public int sizeOf(String key, Long value){
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return value.intValue();
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}
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@Override
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protected void entryRemoved(boolean evicted, String key, Long oldValue, Long newValue){
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File file = getFile(key);
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if(file != null)
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file.delete();
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}
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}
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private File getFile(String fileName) throws FileNotFoundException {
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File file = new File(mCacheDir, fileName);
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if(!file.exists() || !file.isFile())
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throw new FileNotFoundException("文件不存在或有同名文件夹");
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return file;
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}
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//缓存bitmap到外部存储
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public boolean putBitmap(String key, Bitmap bitmap){
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File file = getFile(key);
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if(file != null){
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Log.v("tag", "文件已经存在");
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return true;
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}
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FileOutputStream fos = getOutputStream(key);
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boolean saved = bitmap.compress(CompressFormat.JPEG, 100, fos);
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fos.flush();
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fos.close();
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if(saved){
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synchronized(sFileCache){
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sFileCache.put(key, getFile(key).length());
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}
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return true;
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}
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return false;
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}
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//根据key获取OutputStream
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private FileOutputStream getOutputStream(String key){
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if(mCacheDir == null)
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return null;
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FileOutputStream fos = new FileOutputStream(mCacheDir.getAbsolutePath() + File.separator + key);
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return fos;
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}
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//获取bitmap
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private static BitmapFactory.Options sBitmapOptions;
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static {
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sBitmapOptions = new BitmapFactory.Options();
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sBitmapOptions.inPurgeable=true; //bitmap can be purged to disk
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}
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public Bitmap getBitmap(String key){
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File bitmapFile = getFile(key);
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if(bitmapFile != null){
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Bitmap bitmap = BitmapFactory.decodeStream(new FileInputStream(bitmapFile), null, sBitmapOptions);
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if(bitmap != null){
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//重新将其缓存至硬引用中
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...
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}
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}
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}
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
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private static String generateKey(String fileId, int width, int height) {
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String ret = fileId + "_" + Integer.toString(width) + "x" + Integer.toString(height);
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return ret;
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}
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String key = generateKey(...)即可生成唯一的key值
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