private int mCacheRefCount = 0;
private int mDisplayRefCount = 0;
...
// Notify the drawable that the displayed state has changed.
// Keep a count to determine when the drawable is no longer displayed.
public void setIsDisplayed(boolean isDisplayed) {
    synchronized (this) {
        if (isDisplayed) {
            mDisplayRefCount++;
            mHasBeenDisplayed = true;
        } else {
            mDisplayRefCount--;
        }
    }
    // Check to see if recycle() can be called.
    checkState();
}

// Notify the drawable that the cache state has changed.
// Keep a count to determine when the drawable is no longer being cached.
public void setIsCached(boolean isCached) {
    synchronized (this) {
        if (isCached) {
            mCacheRefCount++;
        } else {
            mCacheRefCount--;
        }
    }
    // Check to see if recycle() can be called.
    checkState();
}

private synchronized void checkState() {
    // If the drawable cache and display ref counts = 0, and this drawable
    // has been displayed, then recycle.
    if (mCacheRefCount <= 0 && mDisplayRefCount <= 0 && mHasBeenDisplayed
            && hasValidBitmap()) {
        getBitmap().recycle();
    }
}

private synchronized boolean hasValidBitmap() {
    Bitmap bitmap = getBitmap();
    return bitmap != null && !bitmap.isRecycled();
}

Set<SoftReference<Bitmap>> mReusableBitmaps;
private LruCache<String, BitmapDrawable> mMemoryCache;

// If you‘re running on Honeycomb or newer, create a
// synchronized HashSet of references to reusable bitmaps.
if (Utils.hasHoneycomb()) {
    mReusableBitmaps =
            Collections.synchronizedSet(new HashSet<SoftReference<Bitmap>>());
}

mMemoryCache = new LruCache<String, BitmapDrawable>(mCacheParams.memCacheSize) {

    // Notify the removed entry that is no longer being cached.
    @Override
    protected void entryRemoved(boolean evicted, String key,
            BitmapDrawable oldValue, BitmapDrawable newValue) {
        if (RecyclingBitmapDrawable.class.isInstance(oldValue)) {
            // The removed entry is a recycling drawable, so notify it
            // that it has been removed from the memory cache.
            ((RecyclingBitmapDrawable) oldValue).setIsCached(false);
        } else {
            // The removed entry is a standard BitmapDrawable.
            if (Utils.hasHoneycomb()) {
                // We‘re running on Honeycomb or later, so add the bitmap
                // to a SoftReference set for possible use with inBitmap later.
                mReusableBitmaps.add
                        (new SoftReference<Bitmap>(oldValue.getBitmap()));
            }
        }
    }
....
}

public static Bitmap decodeSampledBitmapFromFile(String filename,
        int reqWidth, int reqHeight, ImageCache cache) {

    final BitmapFactory.Options options = new BitmapFactory.Options();
    ...
    BitmapFactory.decodeFile(filename, options);
    ...

    // If we‘re running on Honeycomb or newer, try to use inBitmap.
    if (Utils.hasHoneycomb()) {
        addInBitmapOptions(options, cache);
    }
    ...
    return BitmapFactory.decodeFile(filename, options);
}

private static void addInBitmapOptions(BitmapFactory.Options options,
        ImageCache cache) {
    // inBitmap only works with mutable bitmaps, so force the decoder to
    // return mutable bitmaps.
    options.inMutable = true;

    if (cache != null) {
        // Try to find a bitmap to use for inBitmap.
        Bitmap inBitmap = cache.getBitmapFromReusableSet(options);

        if (inBitmap != null) {
            // If a suitable bitmap has been found, set it as the value of
            // inBitmap.
            options.inBitmap = inBitmap;
        }
    }
}

// This method iterates through the reusable bitmaps, looking for one 
// to use for inBitmap:
protected Bitmap getBitmapFromReusableSet(BitmapFactory.Options options) {
        Bitmap bitmap = null;

    if (mReusableBitmaps != null && !mReusableBitmaps.isEmpty()) {
        synchronized (mReusableBitmaps) {
            final Iterator<SoftReference<Bitmap>> iterator
                    = mReusableBitmaps.iterator();
            Bitmap item;

            while (iterator.hasNext()) {
                item = iterator.next().get();

                if (null != item && item.isMutable()) {
                    // Check to see it the item can be used for inBitmap.
                    if (canUseForInBitmap(item, options)) {
                        bitmap = item;

                        // Remove from reusable set so it can‘t be used again.
                        iterator.remove();
                        break;
                    }
                } else {
                    // Remove from the set if the reference has been cleared.
                    iterator.remove();
                }
            }
        }
    }
    return bitmap;
}

static boolean canUseForInBitmap(
        Bitmap candidate, BitmapFactory.Options targetOptions) {

    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
        // From Android 4.4 (KitKat) onward we can re-use if the byte size of
        // the new bitmap is smaller than the reusable bitmap candidate
        // allocation byte count.
        int width = targetOptions.outWidth / targetOptions.inSampleSize;
        int height = targetOptions.outHeight / targetOptions.inSampleSize;
        int byteCount = width * height * getBytesPerPixel(candidate.getConfig());
        return byteCount <= candidate.getAllocationByteCount();
    }

    // On earlier versions, the dimensions must match exactly and the inSampleSize must be 1
    return candidate.getWidth() == targetOptions.outWidth
            && candidate.getHeight() == targetOptions.outHeight
            && targetOptions.inSampleSize == 1;
}

/**
 * A helper function to return the byte usage per pixel of a bitmap based on its configuration.
 */
static int getBytesPerPixel(Config config) {
    if (config == Config.ARGB_8888) {
        return 4;
    } else if (config == Config.RGB_565) {
        return 2;
    } else if (config == Config.ARGB_4444) {
        return 2;
    } else if (config == Config.ALPHA_8) {
        return 1;
    }
    return 1;
}