Android资源优化

大多数情况下，安卓应用的运行内存，res目录下图片资源占了一大部分，虽然系统对Resource增加了缓存机制，但是仍然还是有优化空间的。

1.Resources#loadDrawable存在优化空间。

2.在Activity或者Fragment销毁的时候，部分Bitmap是可以及时回收的。

先贴出代码，后续慢慢完善

BitmapHelper.java

import android.app.Activity;
import android.content.Context;
import android.content.res.Resources;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.graphics.drawable.BitmapDrawable;
import android.graphics.drawable.Drawable;
import android.os.Build;
import android.support.annotation.NonNull;
import android.support.v4.app.Fragment;
import android.support.v4.util.ArrayMap;
import android.support.v4.util.LongSparseArray;
import android.text.TextUtils;
import android.util.TypedValue;
import android.view.View;
import android.widget.ImageView;

import com.<span style="font-family:FangSong_GB2312;">lorss</span>.util.IBLog;

import java.lang.ref.WeakReference;
import java.util.Map;


/**
 * Created by <span style="font-family:FangSong_GB2312;">lorss</span> on 16-4-20.
 */
public class BitmapHelper {
    public static final String TAG = "Mem#BitmapHelper";
    private static final boolean CACHE_RESOURCES = true;
    private static int sReqWidth;
    private static int sReqHeight;
    private static LongSparseArray<ResourceCache> sResourceCaches;

    /**
     * return the {@link cn.qihoo.reader.util.BitmapHelper.ResourceCache} related to target, every target holds single one
     */
    public static ResourceCache getResourceCache(Object target) {
        if (target == null) {
            return ResourceCache.getSingleInstance();
        } else {
            if (sResourceCaches == null) sResourceCaches = new LongSparseArray<>();
            ResourceCache cache;
            int key = target.hashCode();
            if ((cache = sResourceCaches.get(key)) == null) {
                cache = ResourceCache.newInstance();
                sResourceCaches.put(key, cache);
                IBLog.v(ResourceCache.TAG, "# : sResourceCaches put " + target.getClass().getSimpleName());
            }
            return cache;
        }
    }

    /**
     * recycle all the Resource Cache.<br/>
     * Please ensure that all the Bitmap cached not used again after recycled, otherwise it will throw Exception.
     * <br/>See : {@link ResourceCache#recycleAll()}
     */
    public static void recycleAllResourceCache() {
        IBLog.v(ResourceCache.TAG, "#recycleAllResourceCache : ");
        if (sResourceCaches != null) {
            for (int i = 0; i < sResourceCaches.size(); i++) {
                ResourceCache cache = sResourceCaches.valueAt(i);
                if (cache != null) {
                    cache.recycleAll();
                }
            }
            sResourceCaches.clear();
        }
    }

    public static void loadBackground(Fragment fragment, int viewId, int resId) {
        if (fragment == null || viewId < 0 || resId < 0) return;
        Activity activity = fragment.getActivity();
        if (activity != null) {
            loadBackground(fragment, activity.findViewById(viewId), resId);
        }
    }

    public static void loadBackground(Activity activity, int viewId, int resId) {
        if (activity == null || viewId < 0 || resId < 0) return;
        View view = activity.findViewById(viewId);
        loadBackground(activity, view, resId);
    }

    public static void loadBackground(Context context, View view, int resId) {
        if (context == null || view == null || resId < 0) return;
        loadBackground(view, context.getResources(), resId, CACHE_RESOURCES ? getResourceCache(context) : null, false);
    }

    public static void loadBackground(Fragment fragment, View view, int resId) {
        if (fragment == null || view == null || resId < 0) return;
        loadBackground(view, fragment.getResources(), resId, CACHE_RESOURCES ? getResourceCache(fragment) : null, false);
    }

    /**
     * @param exactSize see {@link #decodeSampledBitmapFromResource(Resources, int, int, int, boolean, ResourceCache)}
     */
    private static void loadBackground(View view, Resources res, int resId, ResourceCache cache, boolean exactSize) {
        IBLog.v(TAG, "#loadBackground : resId = " + resId);
        if (view == null || res == null || resId < 0) return;
        final Resources r = res;
        loadDrawable(view, res, resId, cache, exactSize, new Callback<View, Drawable>() {
            @Override
            public void callback(View view, Drawable drawable) {
                if (view != null && drawable != null) {
                    setBackground(view, drawable);
                }
            }
        });
    }

    public static void loadImage(Fragment fragment, int viewId, int resId) {
        if (fragment == null || viewId < 0 || resId < 0) return;
        Activity activity = fragment.getActivity();
        View view;
        if (activity != null && (view = activity.findViewById(viewId)) instanceof ImageView) {
            loadImage(fragment, (ImageView) view, resId);
        }
    }

    public static void loadImage(Activity activity, int viewId, int resId) {
        if (activity == null || viewId < 0 || resId < 0) return;
        View view = activity.findViewById(viewId);
        if (view instanceof ImageView)
            loadImage(activity, (ImageView) view, resId);
    }

    public static void loadImage(Context context, ImageView view, int resId) {
        if (context == null || view == null || resId < 0) return;
        loadImage(view, context.getResources(), resId, CACHE_RESOURCES ? getResourceCache(context) : null, false);
    }

    public static void loadImage(Fragment fragment, ImageView view, int resId) {
        if (fragment == null || view == null || resId < 0) return;
        loadImage(view, fragment.getResources(), resId, CACHE_RESOURCES ? getResourceCache(fragment) : null, false);
    }

    /**
     * @param exactSize see {@link #decodeSampledBitmapFromResource(Resources, int, int, int, boolean, ResourceCache)}
     */
    private static void loadImage(ImageView view, Resources res, int resId, ResourceCache cache, boolean exactSize) {
        IBLog.v(TAG, "#loadImage : resId = " + resId);
        if (view == null || res == null || resId < 0) return;
        final Resources r = res;
        loadDrawable(view, res, resId, cache, exactSize, new Callback<View, Drawable>() {
            @Override
            public void callback(View view, Drawable drawable) {
                if (view != null && drawable != null) {
                    ((ImageView) view).setImageDrawable(drawable);
                }
            }
        });
    }

    /**
     * @param exactSize see {@link #decodeSampledBitmapFromResource(Resources, int, int, int, boolean, ResourceCache)}
     */
    private static void loadDrawable(View view, Resources res, int resId, ResourceCache cache, boolean exactSize, Callback<View, Drawable> callback) {
        IBLog.v(TAG, "#loadDrawable : resId = " + resId);
        if (view == null || res == null || resId < 0) return;
        Drawable drawable = null;
        if (isResourcePicture(res, resId)) {
            int reqWidth;
            int reqHeight;

            if (view.getWidth() != 0 && view.getHeight() != 0) {
                reqWidth = view.getWidth();
                reqHeight = view.getHeight();
                IBLog.v(TAG, "#loadDrawable : reqWidth = " + reqWidth + ", reqHeight = " + reqHeight);
            } else {
                initScreenSize();
                reqWidth = sReqWidth;
                reqHeight = sReqHeight;
            }

            IBLog.v(TAG, "#loadDrawable : [isResourcePicture]");
            Bitmap bitmap = decodeSampledBitmapFromResource(res, resId, reqWidth, reqHeight, exactSize, cache);
            drawable = new BitmapDrawable(res, bitmap);
        } else {
            IBLog.v(TAG, "#loadDrawable : [is NOT ResourcePicture]");
            try {
                drawable = res.getDrawable(resId);
            } catch (Exception ignored) {
            } catch (Error ignored) {
            }
        }
        if (callback != null) callback.callback(view, drawable);
    }

    public static void setBackground(View view, Drawable drawable) {
        if (view == null) return;
        if (Build.VERSION.SDK_INT >= 16) {
            view.setBackground(drawable);
        } else {
            view.setBackgroundDrawable(drawable);
        }
    }


    private static void initScreenSize() {
        sReqWidth = ResolutionUtil.getScreenWidth();
        sReqHeight = ResolutionUtil.getScreenHeight();
        IBLog.v(TAG, "#initScreenSize : sReqWidth = " + sReqWidth);
        IBLog.v(TAG, "#initScreenSize : sReqHeight = " + sReqHeight);
    }

    /**
     * calculate the InSampleSize which indicates the rate to scale the loading Bitmap.
     */
    public static int calculateInSampleSize(BitmapFactory.Options options, int reqWidth, int reqHeight) {
        //default inSampleSize
        int inSampleSize = 1;
        if (options == null) return inSampleSize;
        //size of original picture
        final int height = options.outHeight;
        final int width = options.outWidth;

        if (height > reqHeight || width > reqWidth) {
            // calculate the ratio for scaling
            final int heightRatio = Math.round((float) height / (float) reqHeight);
            final int widthRatio = Math.round((float) width / (float) reqWidth);
            inSampleSize = heightRatio < widthRatio ? heightRatio : widthRatio;
        }
        return inSampleSize;
    }

    /**
     * @param exactSize if true, to decode Bitmap by exact reqWidth and reqHeight; otherwise to scale by {@link #calculateInSampleSize(BitmapFactory.Options, int, int)}
     */
    public static Bitmap decodeSampledBitmapFromResource(Resources res, int resId, int reqWidth, int reqHeight, boolean exactSize, ResourceCache cache) {
        IBLog.v(TAG, "#decodeSampledBitmapFromResource : resId = " + resId);
        Bitmap bitmap = null;
        if (cache != null && (bitmap = cache.get(resId)) != null) {
            IBLog.v(TAG, "#decodeSampledBitmapFromResource : ResourceCache -- Hit !!!");
            if (!bitmap.isRecycled()) {
                return bitmap;
            } else {
                IBLog.v(TAG, "#decodeSampledBitmapFromResource : ResourceCache -- Hit But Recycled !!!");
                cache.remove(resId);
                //TODO m:<span style="font-family:FangSong_GB2312;">lorss</span> how to deal with the views which has set background with this Bitmap
            }
        }
        if (!isResourcePicture(res, resId)) {
            IBLog.v(TAG, "#decodeSampledBitmapFromResource : [is not ResourcePicture]");
        } else if (exactSize) {
            IBLog.v(TAG, "#decodeSampledBitmapFromResource : [exactSize]");
            final BitmapFactory.Options op = new BitmapFactory.Options();
            op.outWidth = reqWidth;
            op.outHeight = reqHeight;
            op.inPreferredConfig = Bitmap.Config.RGB_565;
            try {
                bitmap = BitmapFactory.decodeResource(res, resId, op);
            } catch (Exception ignored) {
            } catch (Error ignored) {
            }
        } else {
            IBLog.v(TAG, "#decodeSampledBitmapFromResource : [InSample]");
            try {
                if (res == null || resId < 0 || reqWidth == 0 || reqHeight == 0) return null;
                // get the size of original picture
                final BitmapFactory.Options options = new BitmapFactory.Options();
                options.inJustDecodeBounds = true;
                BitmapFactory.decodeResource(res, resId, options);

                options.inSampleSize = calculateInSampleSize(options, reqWidth, reqHeight);
                options.inPreferredConfig = Bitmap.Config.RGB_565;
                IBLog.v(TAG, "#decodeSampledBitmapFromResource : resId = " + resId + " , inSampleSize = " + options.inSampleSize);
                options.inJustDecodeBounds = false;
                bitmap = BitmapFactory.decodeResource(res, resId, options);
            } catch (Exception ignored) {
            } catch (Error ignored) {
            }
        }
        if (bitmap != null && cache != null) {
            cache.addBitmapToMemoryCache(resId, bitmap);
        }
        return bitmap;
    }

    /**
     * whether the Resource represented by resId is picture(e.g. jpg or png).
     */
    public static boolean isResourcePicture(Resources res, int resId) {
        IBLog.v(TAG, "#isResourcePicture : resId = " + resId);
        if (res == null || resId < 0) return false;
        TypedValue value = new TypedValue();
        res.getValue(resId, value, true);
        IBLog.v(TAG, "#isResourcePicture : resId = " + resId + ", file = " + value.string);
        if (value.type >= TypedValue.TYPE_FIRST_COLOR_INT
                && value.type <= TypedValue.TYPE_LAST_COLOR_INT) {
            //isColorDrawable
            IBLog.v(TAG, "#isResourcePicture : resId = " + resId + " -- isColorDrawable!!!");
            return false;
        }

        if (!TextUtils.isEmpty(value.string)) {
            final String file = value.string.toString();
            if (file.endsWith(".xml")) {
                //isDrawable but xml
                return false;
            }
        }
        return true;
    }

    public static int bytesOf(Bitmap bitmap) {
        if (bitmap == null) return 0;
        if (Build.VERSION.SDK_INT >= 19) {
            return bitmap.getAllocationByteCount();
        } else if (Build.VERSION.SDK_INT >= 12) {
            return bitmap.getByteCount();
        } else {
            return bitmap.getRowBytes();
        }
    }

    public static interface Callback<T, R> {
        void callback(T t, R r);
    }

    public static class ResourceCache extends LruCache<Integer, Bitmap> {

        private static ResourceCache sResourceCache;
        private LongSparseArray<WeakReference<Bitmap>> mLeakBitmaps;
        private static final String TAG = "Mem#ResourceCache";

        /**
         * @param maxSize for caches that do not override {@link #sizeOf}, this is
         *                the maximum number of entries in the cache. For all other caches,
         *                this is the maximum sum of the sizes of the entries in this cache.
         */
        public ResourceCache(int maxSize) {
            super(maxSize);
        }

        /**
         * get the Single Instance.
         */
        public static ResourceCache getSingleInstance() {
            if (sResourceCache == null) {
                synchronized (BitmapHelper.class) {
                    if (sResourceCache == null) {
                        sResourceCache = ResourceCache.newInstance();
                    }
                }
            }
            return sResourceCache;
        }

        public static ResourceCache newInstance() {
            long maxMemory/*KB*/ = (Runtime.getRuntime().maxMemory()/*byte*/ / 1024);
            long cacheSize = maxMemory / 8;
            if (cacheSize > Integer.MAX_VALUE) {
                IBLog.v(TAG, "#ResourceCache : cacheSize larger than Integer.MAX_VALUE(" + Integer.MAX_VALUE + ")");
            }

            IBLog.v(TAG, "#ResourceCache : maxMemory = " + maxMemory + "KB, cacheSize = " + cacheSize + "KB");
            return new ResourceCache((int) cacheSize);
        }

        @NonNull
        @Override
        protected Map<Integer, Bitmap> onCreateMap() {
            IBLog.v(TAG, "#onCreateMap : ");
            return new ArrayMap<Integer, Bitmap>();
        }

        @Override
        protected int sizeOf(Integer key, Bitmap bitmap) {
            return bytesOf(bitmap) / 1024;
        }

        @Override
        protected void entryRemoved(boolean evicted, Integer key, Bitmap oldValue, Bitmap newValue) {
            IBLog.v(TAG, "#entryRemoved : ");
            if (evicted) {/*caused by evictAll (methods which called trimToSize)  */
                if (oldValue != null) {
                    //m:<span style="font-family:FangSong_GB2312;">lorss</span> just record the leaked Bitmap which has not been recycled
                    if (mLeakBitmaps == null) mLeakBitmaps = new LongSparseArray<>();
                    mLeakBitmaps.put(key, new WeakReference<Bitmap>(oldValue));
                    IBLog.v(TAG, "#entryRemoved : leakBitmaps add " + key);
                }
            } else /*caused by remove or put*/ {
                if (oldValue != null && !oldValue.isRecycled()) {
                    try {
                        oldValue.recycle();
                        IBLog.v(TAG, "#entryRemoved : @oldValue#recycle!!!");
                        System.gc();
                    } catch (Exception ignored) {
                    } catch (Error ignored) {
                    }
                }
                logStatus();
            }
        }

        public void addBitmapToMemoryCache(Integer key, Bitmap bitmap) {
            IBLog.v(TAG, "#addBitmapToMemoryCache : ");
            if (get(key) == null) {
                put(key, bitmap);
            }
            logStatus();
        }

        public LongSparseArray<WeakReference<Bitmap>> getLeakBitmaps() {
            return mLeakBitmaps;
        }

        /**
         * recycle all the Cached Resource, including the leaking Bitmap that has not been recycled which maybe is still being used.
         * <br/>Please ensure that all the Bitmap cached not used again after recycled, otherwise it will throw Exception.
         * <br/>See : {@link Bitmap#recycle()}
         */
        public void recycleAll() {
            removeAll();
            if (mLeakBitmaps != null) {
                for (int i = 0; i < mLeakBitmaps.size(); i++) {
                    WeakReference<Bitmap> ref = mLeakBitmaps.valueAt(i);
                    if (ref != null && ref.get() != null) {
                        ref.get().recycle();
                    }
                }
            }
        }

        private void logStatus() {
            IBLog.v(TAG, "#logStatus : Size : " + size() + "/" + maxSize() + ", hitCount : " + hitCount() + ", missCount : " + missCount());
            IBLog.v(TAG, toString());
        }

    }
}

LruCache.java

/*
 * Copyright (C) 2011 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import android.support.annotation.NonNull;

import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Set;

/**
 * BEGIN LAYOUTLIB CHANGE
 * This is a custom version that doesn't use the non standard LinkedHashMap#eldest.
 * END LAYOUTLIB CHANGE
 * <p/>
 * A cache that holds strong references to a limited number of values. Each time
 * a value is accessed, it is moved to the head of a queue. When a value is
 * added to a full cache, the value at the end of that queue is evicted and may
 * become eligible for garbage collection.
 * <p/>
 * <p>If your cached values hold resources that need to be explicitly released,
 * override {@link #entryRemoved}.
 * <p/>
 * <p>If a cache miss should be computed on demand for the corresponding keys,
 * override {@link #create}. This simplifies the calling code, allowing it to
 * assume a value will always be returned, even when there's a cache miss.
 * <p/>
 * <p>By default, the cache size is measured in the number of entries. Override
 * {@link #sizeOf} to size the cache in different units. For example, this cache
 * is limited to 4MiB of bitmaps:
 * <pre>   {@code
 *   int cacheSize = 4 * 1024 * 1024; // 4MiB
 *   LruCache<String, Bitmap> bitmapCache = new LruCache<String, Bitmap>(cacheSize) {
 *       protected int sizeOf(String key, Bitmap value) {
 *           return value.getByteCount();
 *       }
 *   }}</pre>
 * <p/>
 * <p>This class is thread-safe. Perform multiple cache operations atomically by
 * synchronizing on the cache: <pre>   {@code
 *   synchronized (cache) {
 *     if (cache.get(key) == null) {
 *         cache.put(key, value);
 *     }
 *   }}</pre>
 * <p/>
 * <p>This class does not allow null to be used as a key or value. A return
 * value of null from {@link #get}, {@link #put} or {@link #remove} is
 * unambiguous: the key was not in the cache.
 * <p/>
 * <p>This class appeared in Android 3.1 (Honeycomb MR1); it's available as part
 * of <a href="http://developer.android.com/sdk/compatibility-library.html">Android's
 * Support Package</a> for earlier releases.
 */
public class LruCache<K, V> {
    private final Map<K, V> map;

    /**
     * Size of this cache in units. Not necessarily the number of elements.
     */
    private int size;
    private int maxSize;

    private int putCount;
    private int createCount;
    private int evictionCount;
    private int hitCount;
    private int missCount;

    /**
     * @param maxSize for caches that do not override {@link #sizeOf}, this is
     *                the maximum number of entries in the cache. For all other caches,
     *                this is the maximum sum of the sizes of the entries in this cache.
     */
    public LruCache(int maxSize) {
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }
        this.maxSize = maxSize;
        this.map = onCreateMap();
    }

    @NonNull
    protected Map<K, V> onCreateMap() {
        return new LinkedHashMap<K, V>(0, 0.75f, true);
    }

    /**
     * Sets the size of the cache.
     *
     * @param maxSize The new maximum size.
     * @hide
     */
    public void resize(int maxSize) {
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }

        synchronized (this) {
            this.maxSize = maxSize;
        }
        trimToSize(maxSize);
    }

    /**
     * Returns the value for {@code key} if it exists in the cache or can be
     * created by {@code #create}. If a value was returned, it is moved to the
     * head of the queue. This returns null if a value is not cached and cannot
     * be created.
     */
    public final V get(K key) {
        if (key == null) {
            throw new NullPointerException("key == null");
        }

        V mapValue;
        synchronized (this) {
            mapValue = map.get(key);
            if (mapValue != null) {
                hitCount++;
                return mapValue;
            }
            missCount++;
        }

        /*
         * Attempt to create a value. This may take a long time, and the map
         * may be different when create() returns. If a conflicting value was
         * added to the map while create() was working, we leave that value in
         * the map and release the created value.
         */

        V createdValue = create(key);
        if (createdValue == null) {
            return null;
        }

        synchronized (this) {
            createCount++;
            mapValue = map.put(key, createdValue);

            if (mapValue != null) {
                // There was a conflict so undo that last put
                map.put(key, mapValue);
            } else {
                size += safeSizeOf(key, createdValue);
            }
        }

        if (mapValue != null) {
            entryRemoved(false, key, createdValue, mapValue);
            return mapValue;
        } else {
            trimToSize(maxSize);
            return createdValue;
        }
    }

    /**
     * Caches {@code value} for {@code key}. The value is moved to the head of
     * the queue.
     *
     * @return the previous value mapped by {@code key}.
     */
    public final V put(K key, V value) {
        if (key == null || value == null) {
            throw new NullPointerException("key == null || value == null");
        }

        V previous;
        synchronized (this) {
            putCount++;
            size += safeSizeOf(key, value);
            previous = map.put(key, value);
            if (previous != null) {
                size -= safeSizeOf(key, previous);
            }
        }

        if (previous != null) {
            entryRemoved(false, key, previous, value);
        }

        trimToSize(maxSize);
        return previous;
    }

    /**
     * @param maxSize the maximum size of the cache before returning. May be -1
     *                to evict even 0-sized elements.
     */
    private void trimToSize(int maxSize) {
        while (true) {
            K key;
            V value;
            synchronized (this) {
                if (size < 0 || (map.isEmpty() && size != 0)) {
                    throw new IllegalStateException(getClass().getName()
                            + ".sizeOf() is reporting inconsistent results!");
                }

                if (size <= maxSize) {
                    break;
                }

                // BEGIN LAYOUTLIB CHANGE
                // get the last item in the linked list.
                // This is not efficient, the goal here is to minimize the changes
                // compared to the platform version.
                Map.Entry<K, V> toEvict = null;
                for (Map.Entry<K, V> entry : map.entrySet()) {
                    toEvict = entry;
                }
                // END LAYOUTLIB CHANGE

                if (toEvict == null) {
                    break;
                }

                key = toEvict.getKey();
                value = toEvict.getValue();
                map.remove(key);
                size -= safeSizeOf(key, value);
                evictionCount++;
            }

            entryRemoved(true, key, value, null);
        }
    }

    /**
     * Removes the entry for {@code key} if it exists.
     *
     * @return the previous value mapped by {@code key}.
     */
    public final V remove(K key) {
        if (key == null) {
            throw new NullPointerException("key == null");
        }

        V previous;
        synchronized (this) {
            previous = map.remove(key);
            if (previous != null) {
                size -= safeSizeOf(key, previous);
            }
        }

        if (previous != null) {
            entryRemoved(false, key, previous, null);
        }

        return previous;
    }

    /**
     * Called for entries that have been evicted or removed. This method is
     * invoked when a value is evicted to make space, removed by a call to
     * {@link #remove}, or replaced by a call to {@link #put}. The default
     * implementation does nothing.
     * <p/>
     * <p>The method is called without synchronization: other threads may
     * access the cache while this method is executing.
     *
     * @param evicted  true if the entry is being removed to make space, false
     *                 if the removal was caused by a {@link #put} or {@link #remove}.
     * @param newValue the new value for {@code key}, if it exists. If non-null,
     *                 this removal was caused by a {@link #put}. Otherwise it was caused by
     *                 an eviction or a {@link #remove}.
     */
    protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) {
    }

    /**
     * Called after a cache miss to compute a value for the corresponding key.
     * Returns the computed value or null if no value can be computed. The
     * default implementation returns null.
     * <p/>
     * <p>The method is called without synchronization: other threads may
     * access the cache while this method is executing.
     * <p/>
     * <p>If a value for {@code key} exists in the cache when this method
     * returns, the created value will be released with {@link #entryRemoved}
     * and discarded. This can occur when multiple threads request the same key
     * at the same time (causing multiple values to be created), or when one
     * thread calls {@link #put} while another is creating a value for the same
     * key.
     */
    protected V create(K key) {
        return null;
    }

    private int safeSizeOf(K key, V value) {
        int result = sizeOf(key, value);
        if (result < 0) {
            throw new IllegalStateException("Negative size: " + key + "=" + value);
        }
        return result;
    }

    /**
     * Returns the size of the entry for {@code key} and {@code value} in
     * user-defined units.  The default implementation returns 1 so that size
     * is the number of entries and max size is the maximum number of entries.
     * <p/>
     * <p>An entry's size must not change while it is in the cache.
     */
    protected int sizeOf(K key, V value) {
        return 1;
    }

    /**
     * Clear the cache, calling {@link #entryRemoved} on each removed entry.
     */
    public final void evictAll() {
        trimToSize(-1); // -1 will evict 0-sized elements
    }

    /**
     * For caches that do not override {@link #sizeOf}, this returns the number
     * of entries in the cache. For all other caches, this returns the sum of
     * the sizes of the entries in this cache.
     */
    public synchronized final int size() {
        return size;
    }

    /**
     * For caches that do not override {@link #sizeOf}, this returns the maximum
     * number of entries in the cache. For all other caches, this returns the
     * maximum sum of the sizes of the entries in this cache.
     */
    public synchronized final int maxSize() {
        return maxSize;
    }

    /**
     * Returns the number of times {@link #get} returned a value that was
     * already present in the cache.
     */
    public synchronized final int hitCount() {
        return hitCount;
    }

    /**
     * Returns the number of times {@link #get} returned null or required a new
     * value to be created.
     */
    public synchronized final int missCount() {
        return missCount;
    }

    /**
     * Returns the number of times {@link #create(Object)} returned a value.
     */
    public synchronized final int createCount() {
        return createCount;
    }

    /**
     * Returns the number of times {@link #put} was called.
     */
    public synchronized final int putCount() {
        return putCount;
    }

    /**
     * Returns the number of values that have been evicted.
     */
    public synchronized final int evictionCount() {
        return evictionCount;
    }

    /**
     * Returns a copy of the current contents of the cache, ordered from least
     * recently accessed to most recently accessed.
     */
    public synchronized final Map<K, V> snapshot() {
        return new LinkedHashMap<K, V>(map);
    }

    @Override
    public synchronized final String toString() {
        int accesses = hitCount + missCount;
        int hitPercent = accesses != 0 ? (100 * hitCount / accesses) : 0;
        return String.format("LruCache[maxSize=%d,hits=%d,misses=%d,hitRate=%d%%]",
                maxSize, hitCount, missCount, hitPercent);
    }

    /**
     * Return whether the Cache contains value to the key.
     */
    public boolean contains(K key) {
        if (map != null) {
            Set<K> set;
            if ((set = map.keySet()) != null) {
                return set.contains(key);
            }
        }
        return false;
    }

    /**
     * Clear the cache, calling {@link #remove(Object)} on each removed entry.
     */
    public void removeAll() {
        if (map != null) {
            Set<K> set;
            if ((set = map.keySet()) != null) {
                for (K key : set) {
                    if (key != null) {
                        remove(key);
                    }
                }
            }
        }
    }
}

在Fragment和Activity销毁时候调用{@link BitmapHelper#getResourceCache(Object)}和{@link ResourceCache#recycleAll()}

    //Activity
    @Override
    protected void onDestroy() {
        super.onDestroy();
        BitmapHelper.getResourceCache(this).recycleAll();
    }

    //Fragment
    @Override
    public void onDestroyView() {
        super.onDestroyView();
        BitmapHelper.getResourceCache(this).recycleAll();
    }

在应用退出时，或者所有界面销毁时，调用{@link BitmapHelper#recycleAllResourceCache()}

    //Application exit or all foreground item exit.
    @Override
    public void onApplicationDestroy() {
        BitmapHelper.recycleAllResourceCache();
    }