相册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）。
//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 sHardBitmapCache = new LruCache(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(oldValue));
}
}
//软引用
private static final int SOFT_CACHE_CAPACITY = 40;
private final static LinkedHashMap> sSoftBitmapCache =
new LinkedHashMao>(SOFT_CACHE_CAPACITY, 0.75f, true){
@Override
public SoftReference put(String key, SoftReference value){
return super.input(key, value);
}
@Override
protected boolean removeEldestEntry(LinkedHashMap.Entry> 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 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;
}
