管理Bitmap的內存 · Android Training官方培訓課程中文版

> 編寫:[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) 這節課將作為緩存Bitmaps課程的進一步延伸。為了優化垃圾回收機制與Bitmap的重用，我們還有一些特定的事情可以做。同時根據Android的不同版本，推薦的策略會有所差異。[DisplayingBitmaps](http://developer.android.com/downloads/samples/DisplayingBitmaps.zip)的示例程序會演示如何設計我們的程序，使得它能夠在不同的Android平臺上高效地運行. 為了給這節課奠定基礎，我們首先要知道Android管理Bitmap內存使用的演變進程: - 在Android 2.2 (API level 8)以及之前，當垃圾回收發生時，應用的線程是會被暫停的，這會導致一個延遲滯后，并降低系統效率。 **從Android 2.3開始，添加了并發垃圾回收的機制，這意味著在一個Bitmap不再被引用之后，它所占用的內存會被立即回收。** - 在Android 2.3.3 (API level 10)以及之前, 一個Bitmap的像素級數據（pixel data）是存放在Native內存空間中的。這些數據與Bitmap本身是隔離的，Bitmap本身被存放在Dalvik堆中。我們無法預測在Native內存中的像素級數據何時會被釋放，這意味著程序容易超過它的內存限制并且崩潰。 **自Android 3.0 (API Level 11)開始，像素級數據則是與Bitmap本身一起存放在Dalvik堆中。** 下面會介紹如何在不同的Android版本上優化Bitmap內存使用。 ### 管理Android 2.3.3及以下版本的內存使用在Android 2.3.3 (API level 10) 以及更低版本上，推薦使用[recycle()](http://developer.android.com/reference/android/graphics/Bitmap.html#recycle())方法。如果在應用中顯示了大量的Bitmap數據，我們很可能會遇到[OutOfMemoryError](http://developer.android.com/reference/java/lang/OutOfMemoryError.html)的錯誤。 [recycle()](http://developer.android.com/reference/android/graphics/Bitmap.html#recycle())方法可以使得程序更快的釋放內存。 > **Caution：**只有當我們確定這個Bitmap不再需要用到的時候才應該使用recycle()。在執行recycle()方法之后，如果嘗試繪制這個Bitmap，我們將得到`"Canvas: trying to use a recycled bitmap"`的錯誤提示。下面的代碼片段演示了使用`recycle()`的例子。它使用了引用計數的方法（`mDisplayRefCount` 與 `mCacheRefCount`）來追蹤一個Bitmap目前是否有被顯示或者是在緩存中。并且在下面列舉的條件滿足時，回收Bitmap： - `mDisplayRefCount` 與 `mCacheRefCount` 的引用計數均為 0； - bitmap不為`null`, 并且它還沒有被回收。 ~~~ 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及其以上版本的內存從Android 3.0 (API Level 11)開始，引進了[BitmapFactory.Options.inBitmap](http://developer.android.com/reference/android/graphics/BitmapFactory.Options.html#inBitmap)字段。如果使用了這個設置字段，decode方法會在加載Bitmap數據的時候去重用已經存在的Bitmap。這意味著Bitmap的內存是被重新利用的，這樣可以提升性能，并且減少了內存的分配與回收。然而，使用[inBitmap](http://developer.android.com/reference/android/graphics/BitmapFactory.Options.html#inBitmap)有一些限制，特別是在Android 4.4 (API level 19)之前，只有同等大小的位圖才可以被重用。詳情請查看[inBitmap文檔](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將一個已經存在的Bitmap存放起來以便后續使用。當一個應用運行在Android 3.0或者更高的平臺上并且Bitmap從LruCache中移除時，Bitmap的一個軟引用會被存放在[Hashset](http://developer.android.com/reference/java/util/HashSet.html)中，這樣便于之后可能被[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())); } } } .... } ~~~ ### 使用已經存在的Bitmap 在運行的程序中，decode方法會檢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); } ~~~ 下面的代碼是上述代碼片段中，`addInBitmapOptions()`方法的具體實現。它會為inBitmap查找一個已經存在的Bitmap，并將它設置為inBitmap的值。注意這個方法只有在找到合適且可重用的Bitmap時才會賦值給inBitmap（我們需要在賦值之前進行檢查）： ~~~ 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，下面這個方法判斷候選Bitmap是否滿足inBitmap的大小條件: ~~~ 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; } ~~~