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