該類作為memorycache的實作類，它實作了 baseMemoryCache的抽想方法。
該實作類，僅實作了弱引用的緩存。
源碼：
/**
* Memory cache with {@linkplain WeakReference weak references} to {@linkplain android.graphics.Bitmap bitmaps}<br />
* <br />
* <b>NOTE:</b> This cache uses only weak references for stored Bitmaps.
*
* @author Sergey Tarasevich (nostra13[at]gmail[dot]com)
* @since 1.5.3
*/
public class WeakMemoryCache extends BaseMemoryCache {
@Override
protected Reference<Bitmap> createReference(Bitmap value) {
return new WeakReference<Bitmap>(value);
}
}