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
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
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;
}