This lesson teaches you to
You should also read
- Memory Analysis for Android Applications blog post
- Memory management for Android Apps Google I/O presentation
- Android Design: Swipe Views
- Android Design: Grid Lists
Try it out
DisplayingBitmaps.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 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:
- On Android Android 2.2 (API level 8) and lower, when garbage collection occurs, your app's threads get stopped. This causes a lag that can degrade performance. Android 2.3 adds concurrent garbage collection, which means that the memory is reclaimed soon after a bitmap is no longer referenced.
- On Android 2.3.3 (API level 10) and lower, the backing pixel data for a bitmap is stored in native memory. It is separate from the bitmap itself, which is stored in the Dalvik heap. The pixel data in native memory is not released in a predictable manner, potentially causing an application to briefly exceed its memory limits and crash. As of Android 3.0 (API level 11), the pixel data is stored on the Dalvik heap along with the associated bitmap.
The following sections describe how to optimize bitmap memory management for different Android versions.
Manage Memory on Android 2.3.3 and Lower
On Android 2.3.3 (API level 10) and lower, using
recycle()
is recommended. If you're displaying large amounts of bitmap data in your app,
you're likely to run into
OutOfMemoryError
errors. The
recycle()
method allows an app
to reclaim memory as soon as possible.
Caution: You should use
recycle()
only when you are sure that the
bitmap is no longer being used. If you call recycle()
and later attempt to draw the bitmap, you will get the error:
"Canvas: trying to use a recycled bitmap"
.
The following code snippet gives an example of calling
recycle()
. It uses reference counting
(in the variables mDisplayRefCount
and mCacheRefCount
) to track
whether a bitmap is currently being displayed or in the cache. The
code recycles the bitmap when these conditions are met:
- The reference count for both
mDisplayRefCount
andmCacheRefCount
is 0. - The bitmap is not
null
, and it hasn't been recycled yet.
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(); }
Manage Memory on Android 3.0 and Higher
Android 3.0 (API level 11) introduces the
BitmapFactory.Options.inBitmap
field. If this option is set, decode methods that take the
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. However, there are certain restrictions with how
inBitmap
can be used. In particular, before Android
4.4 (API level 19), only equal sized bitmaps are supported. For details, please see the
inBitmap
documentation.
Save a bitmap for later use
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 LruCache
,
a soft reference to the bitmap is placed
in a HashSet
, for possible reuse later with
inBitmap
:
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())); } } } .... }
Use an existing bitmap
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
addInBitmapOptions()
method that is called in the
above snippet. It looks for an existing bitmap to set as the value for
inBitmap
. Note that this
method only sets a value for 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()) { 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; }
Finally, this method determines whether a candidate bitmap
satisfies the size criteria to be used for
inBitmap
:
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; }