何时应该使用 LRUCache 回收位图？

Question

我正在使用LRUCache 来缓存存储在文件系统上的位图。我根据此处的示例构建了缓存：http://developer.android.com/training/displaying-bitmaps/cache-bitmap.html

问题是我在使用该应用程序时经常看到 OutOfMemory 崩溃。我相信当 LRUCache 驱逐一个图像为另一个图像腾出空间时，内存并没有被释放。

当图像被驱逐时，我添加了对 Bitmap.recycle() 的调用：

  // use 1/8 of the available memory for this memory cache
    final int cacheSize = 1024 * 1024 * memClass / 8;
                mImageCache = new LruCache<String, Bitmap>(cacheSize) {
                @Override
                protected int sizeOf(String key, Bitmap bitmap) {
                    return bitmap.getByteCount();
                }

                @Override
                protected void entryRemoved(boolean evicted, String key, Bitmap oldBitmap, Bitmap newBitmap) {
                    oldBitmap.recycle();
                    oldBitmap = null;
                }
            };

这修复了崩溃，但它也导致图像有时不会出现在应用程序中（只是图像应该存在的黑色空间）。任何时候发生这种情况，我都会在我的 Logcat 中看到这条消息：Cannot generate texture from bitmap。

快速谷歌搜索显示，这是因为正在显示的图像已被回收。

那么这里发生了什么？如果我只是在它们被删除后回收它们，为什么回收的图像仍然在 LRUCache 中？ 实现缓存的替代方法是什么？ Android 文档明确指出 LRUCache 是要走的路，但他们没有提到回收位图的必要性或如何这样做。

已解决： 如果它对其他人有用，则接受的答案所建议的解决此问题的方法是 NO 执行我在上面的代码示例中所做的操作（不要回收entryRemoved() 中的位图打电话）。

相反，当您使用完 ImageView（例如活动中的 onPause()，或在适配器中回收视图时）检查位图是否仍在缓存中（我添加了 isImageInCache() 方法到我的缓存类），如果不是，则回收位图。否则，别管它。这修复了我的 OutOfMemory 异常并阻止了回收仍在使用的位图。

Answer 1

我相信当 LRUCache 驱逐一个图像为另一个图像腾出空间时，内存并没有被释放。

在Bitmap 被回收或垃圾收集之前不会。

快速谷歌搜索显示，这是因为正在显示的图像已被回收。

这就是为什么你不应该在那里回收。

如果我只是在它们被删除后回收它们，为什么回收的图像仍然在 LRUCache 中？

据推测，它们不在LRUCache 中。它们位于ImageView 或其他仍在使用Bitmap 的地方。

实现缓存的替代方法是什么？

为了论证，假设您在ImageView 小部件中使用Bitmap 对象，例如在ListView 的行中。

当您处理完 Bitmap（例如，ListView 中的行被回收）时，您检查它是否仍在缓存中。如果是，你别管它。如果不是，你recycle()它。

缓存只是让您知道哪些Bitmap 对象值得持有。缓存无法知道 Bitmap 是否仍在某处使用。

顺便说一句，如果您使用的是 API 级别 11+，请考虑使用 inBitmap。 OutOMemoryErrors 在分配无法完成时触发。最后我检查了一下，Android 没有压缩垃圾收集器，所以你可以得到一个 OutOfMemoryError 由于碎片（想要分配比最大的单个可用块更大的东西）。

Answer 2

遇到同样的问题，感谢@CommonsWare 的讨论。 在此处发布完整的解决方案，以帮助更多人来这里解决相同的问题。 欢迎编辑和评论。干杯

 When should I recycle a bitmap using LRUCache?

• 正是当您的位图既不在缓存中，也没有被任何 ImageView 引用时。

• 为了维护位图的引用计数，我们必须扩展 BitmapDrawable 类并添加 引用它们的属性。

• 这个 android 示例有准确的答案。 DisplayingBitmaps.zip

我们将在下面了解详细信息和代码。

(don't recycle the bitmaps in the entryRemoved() call).

不完全是。

• 在 entryRemoved 委托中检查 Bitmap 是否仍被任何 ImageView 引用。 如果不。自己在那里回收。

• 反之亦然，在接受的答案中提到，当视图即将被重用或被转储时，检查它的位图（如果视图被重用，则为前一个位图）是否在缓存中。如果它在那里，别管它，否则回收它。

• 这里的关键是我们要检查这两个地方是否可以回收位图。

我将解释我使用 LruCache 为我保存位图的具体情况。 并在 ListView 中显示它们。并在不再使用的位图上调用回收。

上述示例的

RecyclingBitmapDrawable.java和RecyclingImageView.java是我们这里需要的核心部分。他们处理事情很漂亮。 他们的 setIsCached 和 setIsDisplayed 方法正在做我们需要的事情。

代码可以在上面提到的示例链接中找到。但也要在答案底部发布文件的完整代码，以防将来链接断开或更改。对覆盖 setImageResource 进行了小修改，以检查先前位图的状态。

--- 这里是你的代码---

所以你的 LruCache 管理器应该看起来像这样。

LruCacheManager.java

package com.example.cache;

import android.os.Build;
import android.support.v4.util.LruCache;

public class LruCacheManager {

    private LruCache<String, RecyclingBitmapDrawable> mMemoryCache;

    private static LruCacheManager instance;

    public static LruCacheManager getInstance() {
        if(instance == null) {
            instance = new LruCacheManager();
            instance.init();
        } 

        return instance;
    }

    private void init() {

        // We are declaring a cache of 6Mb for our use.
        // You need to calculate this on the basis of your need 
        mMemoryCache = new LruCache<String, RecyclingBitmapDrawable>(6 * 1024 * 1024) {
            @Override
            protected int sizeOf(String key, RecyclingBitmapDrawable bitmapDrawable) {
                // The cache size will be measured in kilobytes rather than
                // number of items.
                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.HONEYCOMB_MR1) {
                    return bitmapDrawable.getBitmap().getByteCount() ;
                } else {
                    return bitmapDrawable.getBitmap().getRowBytes() * bitmapDrawable.getBitmap().getHeight();
                }
            }

            @Override
            protected void entryRemoved(boolean evicted, String key, RecyclingBitmapDrawable oldValue, RecyclingBitmapDrawable newValue) {
                super.entryRemoved(evicted, key, oldValue, newValue);
                oldValue.setIsCached(false);
            }
        };

    }

    public void addBitmapToMemoryCache(String key, RecyclingBitmapDrawable bitmapDrawable) {
        if (getBitmapFromMemCache(key) == null) {
            // The removed entry is a recycling drawable, so notify it
            // that it has been added into the memory cache
            bitmapDrawable.setIsCached(true);
            mMemoryCache.put(key, bitmapDrawable);
        }
    }

    public RecyclingBitmapDrawable getBitmapFromMemCache(String key) {
        return mMemoryCache.get(key);
    }

    public void clear() {
        mMemoryCache.evictAll();
    }
}

您的 ListView/GridView 适配器的 getView() 应该看起来像往常一样正常。 就像您使用 setImageDrawable 方法在 ImageView 上设置新图像一样。 它在内部检查前一个位图的引用计数，如果不在 lrucache 中，它将在内部调用它的回收。

@Override
    public View getView(int position, View convertView, ViewGroup parent) {
        RecyclingImageView imageView;
        if (convertView == null) { // if it's not recycled, initialize some attributes
            imageView = new RecyclingImageView(getActivity());
            imageView.setLayoutParams(new GridView.LayoutParams(
                    GridView.LayoutParams.WRAP_CONTENT,
                    GridView.LayoutParams.WRAP_CONTENT));
            imageView.setScaleType(ImageView.ScaleType.FIT_CENTER);
            imageView.setPadding(5, 5, 5, 5);

        } else {
            imageView = (RecyclingImageView) convertView;
        }

        MyDataObject dataItem = (MyDataObject) getItem(position);
        RecyclingBitmapDrawable  image = lruCacheManager.getBitmapFromMemCache(dataItem.getId());

        if(image != null) {
            // This internally is checking reference count on previous bitmap it used.
            imageView.setImageDrawable(image);
        } else {
            // You have to implement this method as per your code structure.
            // But it basically doing is preparing bitmap in the background
            // and adding that to LruCache.
            // Also it is setting the empty view till bitmap gets loaded.
            // once loaded it just need to call notifyDataSetChanged of adapter. 
            loadImage(dataItem.getId(), R.drawable.empty_view);
        }

        return imageView;

    }

这是你的 RecyclingImageView.java

/*
 * Copyright (C) 2013 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.
 */

package com.example.cache;

import android.content.Context;
import android.graphics.drawable.Drawable;
import android.graphics.drawable.LayerDrawable;
import android.util.AttributeSet;
import android.widget.ImageView;


/**
 * Sub-class of ImageView which automatically notifies the drawable when it is
 * being displayed.
 */
public class RecyclingImageView extends ImageView {

    public RecyclingImageView(Context context) {
        super(context);
    }

    public RecyclingImageView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    /**
     * @see android.widget.ImageView#onDetachedFromWindow()
     */
    @Override
    protected void onDetachedFromWindow() {
        // This has been detached from Window, so clear the drawable
        setImageDrawable(null);

        super.onDetachedFromWindow();
    }

    /**
     * @see android.widget.ImageView#setImageDrawable(android.graphics.drawable.Drawable)
     */
    @Override
    public void setImageDrawable(Drawable drawable) {
        // Keep hold of previous Drawable
        final Drawable previousDrawable = getDrawable();

        // Call super to set new Drawable
        super.setImageDrawable(drawable);

        // Notify new Drawable that it is being displayed
        notifyDrawable(drawable, true);

        // Notify old Drawable so it is no longer being displayed
        notifyDrawable(previousDrawable, false);
    }

    /**
     * @see android.widget.ImageView#setImageResource(android.graphics.drawable.Drawable)
     */
    @Override
    public void setImageResource(int resId) {
        // Keep hold of previous Drawable
        final Drawable previousDrawable = getDrawable();

        // Call super to set new Drawable
        super.setImageResource(resId);

        // Notify old Drawable so it is no longer being displayed
        notifyDrawable(previousDrawable, false);
    }


    /**
     * Notifies the drawable that it's displayed state has changed.
     *
     * @param drawable
     * @param isDisplayed
     */
    private static void notifyDrawable(Drawable drawable, final boolean isDisplayed) {
        if (drawable instanceof RecyclingBitmapDrawable) {
            // The drawable is a CountingBitmapDrawable, so notify it
            ((RecyclingBitmapDrawable) drawable).setIsDisplayed(isDisplayed);
        } else if (drawable instanceof LayerDrawable) {
            // The drawable is a LayerDrawable, so recurse on each layer
            LayerDrawable layerDrawable = (LayerDrawable) drawable;
            for (int i = 0, z = layerDrawable.getNumberOfLayers(); i < z; i++) {
                notifyDrawable(layerDrawable.getDrawable(i), isDisplayed);
            }
        }
    }

}

这是你的 RecyclingBitmapDrawable.java

/*
 * Copyright (C) 2013 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.
 */

package com.example.cache;

import android.content.res.Resources;
import android.graphics.Bitmap;
import android.graphics.drawable.BitmapDrawable;

import android.util.Log;

/**
 * A BitmapDrawable that keeps track of whether it is being displayed or cached.
 * When the drawable is no longer being displayed or cached,
 * {@link android.graphics.Bitmap#recycle() recycle()} will be called on this drawable's bitmap.
 */
public class RecyclingBitmapDrawable extends BitmapDrawable {

    static final String TAG = "CountingBitmapDrawable";

    private int mCacheRefCount = 0;
    private int mDisplayRefCount = 0;

    private boolean mHasBeenDisplayed;

    public RecyclingBitmapDrawable(Resources res, Bitmap bitmap) {
        super(res, bitmap);
    }

    /**
     * Notify the drawable that the displayed state has changed. Internally a
     * count is kept so that the drawable knows when it is no longer being
     * displayed.
     *
     * @param isDisplayed - Whether the drawable is being displayed or not
     */
    public void setIsDisplayed(boolean isDisplayed) {
        //BEGIN_INCLUDE(set_is_displayed)
        synchronized (this) {
            if (isDisplayed) {
                mDisplayRefCount++;
                mHasBeenDisplayed = true;
            } else {
                mDisplayRefCount--;
            }
        }

        // Check to see if recycle() can be called
        checkState();
        //END_INCLUDE(set_is_displayed)
    }

    /**
     * Notify the drawable that the cache state has changed. Internally a count
     * is kept so that the drawable knows when it is no longer being cached.
     *
     * @param isCached - Whether the drawable is being cached or not
     */
    public void setIsCached(boolean isCached) {
        //BEGIN_INCLUDE(set_is_cached)
        synchronized (this) {
            if (isCached) {
                mCacheRefCount++;
            } else {
                mCacheRefCount--;
            }
        }

        // Check to see if recycle() can be called
        checkState();
        //END_INCLUDE(set_is_cached)
    }

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

            Log.d(TAG, "No longer being used or cached so recycling. "
                        + toString());

        getBitmap().recycle();
    }
        //END_INCLUDE(check_state)
    }

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

}