管理bitmap内存

在上个章节《如何缓存你的Bitmap》中，说明了Bitmap对象的缓存与回收。本章来说Bitmap对象内存的管理，由于Adroid版本的差异，所以bitmap的内存管理方式有所不同。

先来简单说下Android系统各个版本管理Bitmap内存的差异，看看管理Bitmap的进化史。

• Android2.2及以下：当垃圾回收器回收时线程停止，导致渲染延迟。Android2.3以后增加了concurrent垃圾回收器，意味着不再引用的Bitmap对象能够立即回收。
• Android2.3.3及以下：Bitmap的像素数据时保持在native内存中的，而Bitmap对象本身保存在Dalvik head 中。在native内存中的像素信息的回收并不是按照常规回收，这使得对内存的把控变难，容易引起OOM。Android3.0开始Bitmap的像素数据也存储在Dalvik head中了。

接下来讲述在不同Android版本中如何优化管理Bitmap内存。

在Android2.3.3中管理Bitmap内存

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在Android2.3.3及以下中，推荐使用recycle（）方法释放内存。但是要十分谨慎：你要确保使用recycle（）方法的Bitmap对象不再使用，如果你使用recycle（）释放了Bitmap，然后又尝试绘制这个Bitmap，你就会得到一个“Canvas：trying to use a recycled bitmap”的异常。
下面给一个例子：什么时候调用recycle（）。使用相关计数（使用两个变量：mDisplayRefCount和mCacheRefCount）来跟踪bitmap对象当前是否在显示或在缓存。bitmap回收满足两个条件：

•  mDisplayRefCount和mCacheRefCount同时为0.
•  bitmap不为null，切没有回收。

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();}

在Android3.0及以上管理Bitmap内存

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 在Android3.0中引入BitmapFactory.Options.inBitmap这个变量，如果这个变量被设置，便可以尝试使用已经存在的Bitmap加载内容，这就意味着bitmap的内存能够重用，从而提高显示速度，减少每次分配和回收吞吐过程。然而必须清楚使用inBitmap 的条件和限制，在Android4.4之前，只有相同大小的bitmap可以重用，在Android4.4以后只要小于inBitmap的都可以使用。
 

 保存一份bitmap内存

下面例子的代码片段演示如何使用inBitmap来重复利用bitmap的内存。使用LruCache来缓存bitmap，当bitmap从缓存LruCache中移除时我们再将bitmap的内存设置到inBitmap使用软引用保存，以便后续可能的重复利用。

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()));            }        }    }....}

 使用存在的bitmap内存decode

 先判断版本环境，例如：

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);}

 addInBitmapOptions()，查询有无存在的bitmap内存可以设置到inBitmap。代码实现片段：

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;}

最后，下面这个方法用来判断候选bitmap内存大小是否满足inBitmap的要求：

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;}

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