BitmapFun.zip
In addition to the steps described in Caching Bitmaps, there are specific things you can do to facilitate garbage collection and bitmap reuse. The recommended strategy depends on which version(s) of Android you are targeting. The {@code BitmapFun} sample app included with this class shows you how to design your app to work efficiently across different versions of Android.
To set the stage for this lesson, here is how Android's management of bitmap memory has evolved:
The following sections describe how to optimize bitmap memory management for different Android versions.
On Android 2.3.3 (API level 10) and lower, using {@link android.graphics.Bitmap#recycle recycle()} is recommended. If you're displaying large amounts of bitmap data in your app, you're likely to run into {@link java.lang.OutOfMemoryError} errors. The {@link android.graphics.Bitmap#recycle recycle()} method allows an app to reclaim memory as soon as possible.
Caution: You should use {@link android.graphics.Bitmap#recycle recycle()} only when you are sure that the bitmap is no longer being used. If you call {@link android.graphics.Bitmap#recycle recycle()} and later attempt to draw the bitmap, you will get the error: {@code "Canvas: trying to use a recycled bitmap"}.
The following code snippet gives an example of calling {@link android.graphics.Bitmap#recycle recycle()}. It uses reference counting (in the variables {@code mDisplayRefCount} and {@code mCacheRefCount}) to track whether a bitmap is currently being displayed or in the cache. The code recycles the bitmap when these conditions are met:
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 (API Level 11) introduces the {@link android.graphics.BitmapFactory.Options#inBitmap BitmapFactory.Options.inBitmap} field. If this option is set, decode methods that take the {@link android.graphics.BitmapFactory.Options Options} object will attempt to reuse an existing bitmap when loading content. This means that the bitmap's memory is reused, resulting in improved performance, and removing both memory allocation and de-allocation. There are some caveats in using {@link android.graphics.BitmapFactory.Options#inBitmap}:
The following snippet demonstrates how an existing bitmap is stored for possible later use in the sample app. When an app is running on Android 3.0 or higher and a bitmap is evicted from the {@link android.util.LruCache}, a soft reference to the bitmap is placed in a {@link java.util.HashSet}, for possible reuse later with {@link android.graphics.BitmapFactory.Options#inBitmap}:
HashSet<SoftReference<Bitmap>> mReusableBitmaps;
private LruCache<String, BitmapDrawable> mMemoryCache;
// If you're running on Honeycomb or newer, create
// a HashSet of references to reusable bitmaps.
if (Utils.hasHoneycomb()) {
mReusableBitmaps = 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()));
}
}
}
....
}
In the running app, decoder methods check to see if there is an existing bitmap they can use. For example:
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);
}The next snippet shows the {@code addInBitmapOptions()} method that is called in the
above snippet. It looks for an existing bitmap to set as the value for
{@link android.graphics.BitmapFactory.Options#inBitmap}. Note that this
method only sets a value for {@link android.graphics.BitmapFactory.Options#inBitmap}
if it finds a suitable match (your code should never assume that a match will be found):
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()) {
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;
}
Finally, this method determines whether a candidate bitmap satisfies the size criteria to be used for {@link android.graphics.BitmapFactory.Options#inBitmap}:
private static boolean canUseForInBitmap(
Bitmap candidate, BitmapFactory.Options targetOptions) {
int width = targetOptions.outWidth / targetOptions.inSampleSize;
int height = targetOptions.outHeight / targetOptions.inSampleSize;
// Returns true if "candidate" can be used for inBitmap re-use with
// "targetOptions".
return candidate.getWidth() == width && candidate.getHeight() == height;
}