相冊(cè)圖片預(yù)取緩存策略是內(nèi)存緩存（硬引用LruCache、軟引用SoftReference）、外部文件緩存（context.getCachedDir()），緩存中取不到的情況下再向服務(wù)端請(qǐng)求下載圖片。同時(shí)緩存三張圖片（當(dāng)前預(yù)覽的這張，前一張以及后一張）。
//需要導(dǎo)入外部jar文件 android-support-v4.jar
import android.support.v4.util.LruCache;
//開(kāi)辟8M硬緩存空間
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");
//硬引用緩存區(qū)滿，將一個(gè)最不經(jīng)常使用的oldvalue推入到軟引用緩存區(qū)
sSoftBitmapCahe.put(key, new SoftReference(oldValue));
}
}
//軟引用
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;
}
}
//緩存bitmap
public boolean putBitmap(String key, Bitmap bitmap){
if(bitmap != null){
synchronized(sHardBitmapCache){
sHardBitmapCache.put(key, bitmap);
}
return true;
}
return false;
}
//從緩存中獲取bitmap
public Bitmap getBitmap(String key){
synchronized(sHardBitmapCache){
final Bitmap bitmap = sHardBitmapCache.get(key);
if(bitmap != null)
return bitmap;
}
//硬引用緩存區(qū)間中讀取失敗，從軟引用緩存區(qū)間讀取
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 已經(jīng)被回收");
sSoftBitmapCache.remove(key);
}
}
}
return null;
}
