Bitmaps加载之内存管理

除了前面说的Bitmap缓存之外,还有一些事情我们可以做来使用好GC和Bitmap的重用. 对于不同的Android版本要做不同的处理,这样才能达到高效使用Bitmap的效果,这是推荐的策略.

这里先介绍一些关于Android中Bitmap内存管理的基础知识铺垫一下：

在Android2.2(API 8)及以前,当GC开始回收时,app的所有线程都将停止, 这就导致了延迟的产生,进而影响体验. Android2.3及之后就不会用这个问题了,因此增加了GC的并发处理,也就意味着Bitmap被清理之后app的可用空间会很快回收回来.
Android2.3.3(API 10)及以前,Bitmap的图片数据是保存在native memory上, 而Bitmap对象是保存在Dalvik的heap上,这样这两个就分离开了,就会导致内存释放不及时从而带来潜在的OOM. 在Android 3.0(API 11)之后这个问题就解决了,因为这两个都被放在Dalvik的heap上.

下面介绍如何根据不同的Android版本来管理Bitmap的内存.

Android2.3.3及以下

在这个版本范围内,推荐使用recycle()方法, 该方法会尽快把Bitmap的内存回收回来.

注意: 你只有在确定这个Bitmap不再使用的情况下才去调用recycle()方法. 不然如果你调用recycle()之后又要想去使用之前的Bitmap,会抛出一个异常:"Canvas: trying to use a recycled bitmap"

下面的代码是Demo中RecyclingBitmapDrawable的一部分,其中mDisplayRefCount和mCacheRefCount这两个变量用来记录该Bitmap显示和缓存情况,具体回收条件如下:

mDisplayRefCount和mCacheRefCount的值都为0.
Bitmap不为空.

完整代码请参考官方demo,下面Reference带下载地址.

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();}

Android 3.0及以上

Android 3.0(API 11)引入了 BitmapFactory.Options.inBitmap属性.如果设置了该属性, BitmapFactory带有Options参数的decode相关方法会尝试去重用已存在的Bitmap, 这就意味这Bitmap的内存空间得到了重用, 就可以改善性能,减少内存分配和回收.
但是使用inBitmap这个属性有一些限制, 有一点比较特别的是在Android4.4以前(API 19),只有相同大小的Bitmap才可以重用,具体可以看inBitmap文档.

下面看具体实例:

1. 保存Bitmap

下面是用一个HashSet来保存从LruCache中移除的Bitmap的软引用.

Set> mReusableBitmaps;private LruCache 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>());}mMemoryCache = new LruCache(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(oldValue.getBitmap()));            }        }    }....}

2. 重用Bitmap

查找是否有可重用的Bitmap

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);}

如果找到可用的就设置inBitmap

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> 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;}

查找时具体的匹配条件如下

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;}

Reference

Managing Bitmap Memory
官方DisplayingBitmaps Demo

Android2.3.3及以下

Android 3.0及以上

Reference

更多相关文章

随机推荐