管理Bitmap的内存 · Android Training官方培训课程中文版

> 编写:[kesenhoo](https://github.com/kesenhoo) - 原文:[http://developer.android.com/training/displaying-bitmaps/manage-memory.html](http://developer.android.com/training/displaying-bitmaps/manage-memory.html) 这节课将作为缓存Bitmaps课程的进一步延伸。为了优化垃圾回收机制与Bitmap的重用，我们还有一些特定的事情可以做。同时根据Android的不同版本，推荐的策略会有所差异。[DisplayingBitmaps](http://developer.android.com/downloads/samples/DisplayingBitmaps.zip)的示例程序会演示如何设计我们的程序，使得它能够在不同的Android平台上高效地运行. 为了给这节课奠定基础，我们首先要知道Android管理Bitmap内存使用的演变进程: - 在Android 2.2 (API level 8)以及之前，当垃圾回收发生时，应用的线程是会被暂停的，这会导致一个延迟滞后，并降低系统效率。 **从Android 2.3开始，添加了并发垃圾回收的机制，这意味着在一个Bitmap不再被引用之后，它所占用的内存会被立即回收。** - 在Android 2.3.3 (API level 10)以及之前, 一个Bitmap的像素级数据（pixel data）是存放在Native内存空间中的。这些数据与Bitmap本身是隔离的，Bitmap本身被存放在Dalvik堆中。我们无法预测在Native内存中的像素级数据何时会被释放，这意味着程序容易超过它的内存限制并且崩溃。 **自Android 3.0 (API Level 11)开始，像素级数据则是与Bitmap本身一起存放在Dalvik堆中。** 下面会介绍如何在不同的Android版本上优化Bitmap内存使用。 ### 管理Android 2.3.3及以下版本的内存使用在Android 2.3.3 (API level 10) 以及更低版本上，推荐使用[recycle()](http://developer.android.com/reference/android/graphics/Bitmap.html#recycle())方法。如果在应用中显示了大量的Bitmap数据，我们很可能会遇到[OutOfMemoryError](http://developer.android.com/reference/java/lang/OutOfMemoryError.html)的错误。 [recycle()](http://developer.android.com/reference/android/graphics/Bitmap.html#recycle())方法可以使得程序更快的释放内存。 > **Caution：**只有当我们确定这个Bitmap不再需要用到的时候才应该使用recycle()。在执行recycle()方法之后，如果尝试绘制这个Bitmap，我们将得到`"Canvas: trying to use a recycled bitmap"`的错误提示。下面的代码片段演示了使用`recycle()`的例子。它使用了引用计数的方法（`mDisplayRefCount` 与 `mCacheRefCount`）来追踪一个Bitmap目前是否有被显示或者是在缓存中。并且在下面列举的条件满足时，回收Bitmap： - `mDisplayRefCount` 与 `mCacheRefCount` 的引用计数均为 0； - bitmap不为`null`, 并且它还没有被回收。 ~~~ 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(); } ~~~ ### 管理Android 3.0及其以上版本的内存从Android 3.0 (API Level 11)开始，引进了[BitmapFactory.Options.inBitmap](http://developer.android.com/reference/android/graphics/BitmapFactory.Options.html#inBitmap)字段。如果使用了这个设置字段，decode方法会在加载Bitmap数据的时候去重用已经存在的Bitmap。这意味着Bitmap的内存是被重新利用的，这样可以提升性能，并且减少了内存的分配与回收。然而，使用[inBitmap](http://developer.android.com/reference/android/graphics/BitmapFactory.Options.html#inBitmap)有一些限制，特别是在Android 4.4 (API level 19)之前，只有同等大小的位图才可以被重用。详情请查看[inBitmap文档](http://developer.android.com/reference/android/graphics/BitmapFactory.Options.html#inBitmap)。 ### 保存Bitmap供以后使用下面演示了如何将一个已经存在的Bitmap存放起来以便后续使用。当一个应用运行在Android 3.0或者更高的平台上并且Bitmap从LruCache中移除时，Bitmap的一个软引用会被存放在[Hashset](http://developer.android.com/reference/java/util/HashSet.html)中，这样便于之后可能被[inBitmap](http://developer.android.com/reference/android/graphics/BitmapFactory.Options.html#inBitmap)重用： ~~~ 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())); } } } .... } ~~~ ### 使用已经存在的Bitmap 在运行的程序中，decode方法会检查看是否存在可重用的Bitmap。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); } ~~~ 下面的代码是上述代码片段中，`addInBitmapOptions()`方法的具体实现。它会为inBitmap查找一个已经存在的Bitmap，并将它设置为inBitmap的值。注意这个方法只有在找到合适且可重用的Bitmap时才会赋值给inBitmap（我们需要在赋值之前进行检查）： ~~~ 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，下面这个方法判断候选Bitmap是否满足inBitmap的大小条件: ~~~ 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; } ~~~