| page.title=Caching Bitmaps |
| parent.title=Displaying Bitmaps Efficiently |
| parent.link=index.html |
| |
| trainingnavtop=true |
| next.title=Displaying Bitmaps in Your UI |
| next.link=display-bitmap.html |
| previous.title=Processing Bitmaps Off the UI Thread |
| previous.link=process-bitmap.html |
| |
| @jd:body |
| |
| <div id="tb-wrapper"> |
| <div id="tb"> |
| |
| <h2>This lesson teaches you to</h2> |
| <ol> |
| <li><a href="#memory-cache">Use a Memory Cache</a></li> |
| <li><a href="#disk-cache">Use a Disk Cache</a></li> |
| <li><a href="#config-changes">Handle Configuration Changes</a></li> |
| </ol> |
| |
| <h2>You should also read</h2> |
| <ul> |
| <li><a href="{@docRoot}guide/topics/resources/runtime-changes.html">Handling Runtime Changes</a></li> |
| </ul> |
| |
| <h2>Try it out</h2> |
| |
| <div class="download-box"> |
| <a href="{@docRoot}shareables/training/BitmapFun.zip" class="button">Download the sample</a> |
| <p class="filename">BitmapFun.zip</p> |
| </div> |
| |
| </div> |
| </div> |
| |
| <p>Loading a single bitmap into your user interface (UI) is straightforward, however things get more |
| complicated if you need to load a larger set of images at once. In many cases (such as with |
| components like {@link android.widget.ListView}, {@link android.widget.GridView} or {@link |
| android.support.v4.view.ViewPager }), the total number of images on-screen combined with images that |
| might soon scroll onto the screen are essentially unlimited.</p> |
| |
| <p>Memory usage is kept down with components like this by recycling the child views as they move |
| off-screen. The garbage collector also frees up your loaded bitmaps, assuming you don't keep any |
| long lived references. This is all good and well, but in order to keep a fluid and fast-loading UI |
| you want to avoid continually processing these images each time they come back on-screen. A memory |
| and disk cache can often help here, allowing components to quickly reload processed images.</p> |
| |
| <p>This lesson walks you through using a memory and disk bitmap cache to improve the responsiveness |
| and fluidity of your UI when loading multiple bitmaps.</p> |
| |
| <h2 id="memory-cache">Use a Memory Cache</h2> |
| |
| <p>A memory cache offers fast access to bitmaps at the cost of taking up valuable application |
| memory. The {@link android.util.LruCache} class (also available in the <a |
| href="{@docRoot}reference/android/support/v4/util/LruCache.html">Support Library</a> for use back |
| to API Level 4) is particularly well suited to the task of caching bitmaps, keeping recently |
| referenced objects in a strong referenced {@link java.util.LinkedHashMap} and evicting the least |
| recently used member before the cache exceeds its designated size.</p> |
| |
| <p class="note"><strong>Note:</strong> In the past, a popular memory cache implementation was a |
| {@link java.lang.ref.SoftReference} or {@link java.lang.ref.WeakReference} bitmap cache, however |
| this is not recommended. Starting from Android 2.3 (API Level 9) the garbage collector is more |
| aggressive with collecting soft/weak references which makes them fairly ineffective. In addition, |
| prior to Android 3.0 (API Level 11), the backing data of a bitmap was stored in native memory which |
| is not released in a predictable manner, potentially causing an application to briefly exceed its |
| memory limits and crash.</p> |
| |
| <p>In order to choose a suitable size for a {@link android.util.LruCache}, a number of factors |
| should be taken into consideration, for example:</p> |
| |
| <ul> |
| <li>How memory intensive is the rest of your activity and/or application?</li> |
| <li>How many images will be on-screen at once? How many need to be available ready to come |
| on-screen?</li> |
| <li>What is the screen size and density of the device? An extra high density screen (xhdpi) device |
| like <a href="http://www.android.com/devices/detail/galaxy-nexus">Galaxy Nexus</a> will need a |
| larger cache to hold the same number of images in memory compared to a device like <a |
| href="http://www.android.com/devices/detail/nexus-s">Nexus S</a> (hdpi).</li> |
| <li>What dimensions and configuration are the bitmaps and therefore how much memory will each take |
| up?</li> |
| <li>How frequently will the images be accessed? Will some be accessed more frequently than others? |
| If so, perhaps you may want to keep certain items always in memory or even have multiple {@link |
| android.util.LruCache} objects for different groups of bitmaps.</li> |
| <li>Can you balance quality against quantity? Sometimes it can be more useful to store a larger |
| number of lower quality bitmaps, potentially loading a higher quality version in another |
| background task.</li> |
| </ul> |
| |
| <p>There is no specific size or formula that suits all applications, it's up to you to analyze your |
| usage and come up with a suitable solution. A cache that is too small causes additional overhead with |
| no benefit, a cache that is too large can once again cause {@code java.lang.OutOfMemory} exceptions |
| and leave the rest of your app little memory to work with.</p> |
| |
| <p>Here’s an example of setting up a {@link android.util.LruCache} for bitmaps:</p> |
| |
| <pre> |
| private LruCache<String, Bitmap> mMemoryCache; |
| |
| @Override |
| protected void onCreate(Bundle savedInstanceState) { |
| ... |
| // Get memory class of this device, exceeding this amount will throw an |
| // OutOfMemory exception. |
| final int memClass = ((ActivityManager) context.getSystemService( |
| Context.ACTIVITY_SERVICE)).getMemoryClass(); |
| |
| // Use 1/8th of the available memory for this memory cache. |
| final int cacheSize = 1024 * 1024 * memClass / 8; |
| |
| mMemoryCache = new LruCache<String, Bitmap>(cacheSize) { |
| @Override |
| protected int sizeOf(String key, Bitmap bitmap) { |
| // The cache size will be measured in bytes rather than number of items. |
| return bitmap.getByteCount(); |
| } |
| }; |
| ... |
| } |
| |
| public void addBitmapToMemoryCache(String key, Bitmap bitmap) { |
| if (getBitmapFromMemCache(key) == null) { |
| mMemoryCache.put(key, bitmap); |
| } |
| } |
| |
| public Bitmap getBitmapFromMemCache(String key) { |
| return mMemoryCache.get(key); |
| } |
| </pre> |
| |
| <p class="note"><strong>Note:</strong> In this example, one eighth of the application memory is |
| allocated for our cache. On a normal/hdpi device this is a minimum of around 4MB (32/8). A full |
| screen {@link android.widget.GridView} filled with images on a device with 800x480 resolution would |
| use around 1.5MB (800*480*4 bytes), so this would cache a minimum of around 2.5 pages of images in |
| memory.</p> |
| |
| <p>When loading a bitmap into an {@link android.widget.ImageView}, the {@link android.util.LruCache} |
| is checked first. If an entry is found, it is used immediately to update the {@link |
| android.widget.ImageView}, otherwise a background thread is spawned to process the image:</p> |
| |
| <pre> |
| public void loadBitmap(int resId, ImageView imageView) { |
| final String imageKey = String.valueOf(resId); |
| |
| final Bitmap bitmap = getBitmapFromMemCache(imageKey); |
| if (bitmap != null) { |
| mImageView.setImageBitmap(bitmap); |
| } else { |
| mImageView.setImageResource(R.drawable.image_placeholder); |
| BitmapWorkerTask task = new BitmapWorkerTask(mImageView); |
| task.execute(resId); |
| } |
| } |
| </pre> |
| |
| <p>The <a href="process-bitmap.html#BitmapWorkerTask">{@code BitmapWorkerTask}</a> also needs to be |
| updated to add entries to the memory cache:</p> |
| |
| <pre> |
| class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> { |
| ... |
| // Decode image in background. |
| @Override |
| protected Bitmap doInBackground(Integer... params) { |
| final Bitmap bitmap = decodeSampledBitmapFromResource( |
| getResources(), params[0], 100, 100)); |
| addBitmapToMemoryCache(String.valueOf(params[0]), bitmap); |
| return bitmap; |
| } |
| ... |
| } |
| </pre> |
| |
| <h2 id="disk-cache">Use a Disk Cache</h2> |
| |
| <p>A memory cache is useful in speeding up access to recently viewed bitmaps, however you cannot |
| rely on images being available in this cache. Components like {@link android.widget.GridView} with |
| larger datasets can easily fill up a memory cache. Your application could be interrupted by another |
| task like a phone call, and while in the background it might be killed and the memory cache |
| destroyed. Once the user resumes, your application has to process each image again.</p> |
| |
| <p>A disk cache can be used in these cases to persist processed bitmaps and help decrease loading |
| times where images are no longer available in a memory cache. Of course, fetching images from disk |
| is slower than loading from memory and should be done in a background thread, as disk read times can |
| be unpredictable.</p> |
| |
| <p class="note"><strong>Note:</strong> A {@link android.content.ContentProvider} might be a more |
| appropriate place to store cached images if they are accessed more frequently, for example in an |
| image gallery application.</p> |
| |
| <p>The sample code of this class uses a {@code DiskLruCache} implementation that is pulled from the |
| <a href="https://android.googlesource.com/platform/libcore/+/master/luni/src/main/java/libcore/io/DiskLruCache.java">Android source</a>. Here’s updated example code that adds a disk cache in addition |
| to the existing memory cache:</p> |
| |
| <pre> |
| private DiskLruCache mDiskLruCache; |
| private final Object mDiskCacheLock = new Object(); |
| private boolean mDiskCacheStarting = true; |
| private static final int DISK_CACHE_SIZE = 1024 * 1024 * 10; // 10MB |
| private static final String DISK_CACHE_SUBDIR = "thumbnails"; |
| |
| @Override |
| protected void onCreate(Bundle savedInstanceState) { |
| ... |
| // Initialize memory cache |
| ... |
| // Initialize disk cache on background thread |
| File cacheDir = getDiskCacheDir(this, DISK_CACHE_SUBDIR); |
| new InitDiskCacheTask().execute(cacheDir); |
| ... |
| } |
| |
| class InitDiskCacheTask extends AsyncTask<File, Void, Void> { |
| @Override |
| protected Void doInBackground(File... params) { |
| synchronized (mDiskCacheLock) { |
| File cacheDir = params[0]; |
| mDiskLruCache = DiskLruCache.open(cacheDir, DISK_CACHE_SIZE); |
| mDiskCacheStarting = false; // Finished initialization |
| mDiskCacheLock.notifyAll(); // Wake any waiting threads |
| } |
| return null; |
| } |
| } |
| |
| class BitmapWorkerTask extends AsyncTask<Integer, Void, Bitmap> { |
| ... |
| // Decode image in background. |
| @Override |
| protected Bitmap doInBackground(Integer... params) { |
| final String imageKey = String.valueOf(params[0]); |
| |
| // Check disk cache in background thread |
| Bitmap bitmap = getBitmapFromDiskCache(imageKey); |
| |
| if (bitmap == null) { // Not found in disk cache |
| // Process as normal |
| final Bitmap bitmap = decodeSampledBitmapFromResource( |
| getResources(), params[0], 100, 100)); |
| } |
| |
| // Add final bitmap to caches |
| addBitmapToCache(imageKey, bitmap); |
| |
| return bitmap; |
| } |
| ... |
| } |
| |
| public void addBitmapToCache(String key, Bitmap bitmap) { |
| // Add to memory cache as before |
| if (getBitmapFromMemCache(key) == null) { |
| mMemoryCache.put(key, bitmap); |
| } |
| |
| // Also add to disk cache |
| synchronized (mDiskCacheLock) { |
| if (mDiskLruCache != null && mDiskLruCache.get(key) == null) { |
| mDiskLruCache.put(key, bitmap); |
| } |
| } |
| } |
| |
| public Bitmap getBitmapFromDiskCache(String key) { |
| synchronized (mDiskCacheLock) { |
| // Wait while disk cache is started from background thread |
| while (mDiskCacheStarting) { |
| try { |
| mDiskCacheLock.wait(); |
| } catch (InterruptedException e) {} |
| } |
| if (mDiskLruCache != null) { |
| return mDiskLruCache.get(key); |
| } |
| } |
| return null; |
| } |
| |
| // Creates a unique subdirectory of the designated app cache directory. Tries to use external |
| // but if not mounted, falls back on internal storage. |
| public static File getDiskCacheDir(Context context, String uniqueName) { |
| // Check if media is mounted or storage is built-in, if so, try and use external cache dir |
| // otherwise use internal cache dir |
| final String cachePath = |
| Environment.MEDIA_MOUNTED.equals(Environment.getExternalStorageState()) || |
| !isExternalStorageRemovable() ? getExternalCacheDir(context).getPath() : |
| context.getCacheDir().getPath(); |
| |
| return new File(cachePath + File.separator + uniqueName); |
| } |
| </pre> |
| |
| <p class="note"><strong>Note:</strong> Even initializing the disk cache requires disk operations |
| and therefore should not take place on the main thread. However, this does mean there's a chance |
| the cache is accessed before initialization. To address this, in the above implementation, a lock |
| object ensures that the app does not read from the disk cache until the cache has been |
| initialized.</p> |
| |
| <p>While the memory cache is checked in the UI thread, the disk cache is checked in the background |
| thread. Disk operations should never take place on the UI thread. When image processing is |
| complete, the final bitmap is added to both the memory and disk cache for future use.</p> |
| |
| <h2 id="config-changes">Handle Configuration Changes</h2> |
| |
| <p>Runtime configuration changes, such as a screen orientation change, cause Android to destroy and |
| restart the running activity with the new configuration (For more information about this behavior, |
| see <a href="{@docRoot}guide/topics/resources/runtime-changes.html">Handling Runtime Changes</a>). |
| You want to avoid having to process all your images again so the user has a smooth and fast |
| experience when a configuration change occurs.</p> |
| |
| <p>Luckily, you have a nice memory cache of bitmaps that you built in the <a |
| href="#memory-cache">Use a Memory Cache</a> section. This cache can be passed through to the new |
| activity instance using a {@link android.app.Fragment} which is preserved by calling {@link |
| android.app.Fragment#setRetainInstance setRetainInstance(true)}). After the activity has been |
| recreated, this retained {@link android.app.Fragment} is reattached and you gain access to the |
| existing cache object, allowing images to be quickly fetched and re-populated into the {@link |
| android.widget.ImageView} objects.</p> |
| |
| <p>Here’s an example of retaining a {@link android.util.LruCache} object across configuration |
| changes using a {@link android.app.Fragment}:</p> |
| |
| <pre> |
| private LruCache<String, Bitmap> mMemoryCache; |
| |
| @Override |
| protected void onCreate(Bundle savedInstanceState) { |
| ... |
| RetainFragment mRetainFragment = |
| RetainFragment.findOrCreateRetainFragment(getFragmentManager()); |
| mMemoryCache = RetainFragment.mRetainedCache; |
| if (mMemoryCache == null) { |
| mMemoryCache = new LruCache<String, Bitmap>(cacheSize) { |
| ... // Initialize cache here as usual |
| } |
| mRetainFragment.mRetainedCache = mMemoryCache; |
| } |
| ... |
| } |
| |
| class RetainFragment extends Fragment { |
| private static final String TAG = "RetainFragment"; |
| public LruCache<String, Bitmap> mRetainedCache; |
| |
| public RetainFragment() {} |
| |
| public static RetainFragment findOrCreateRetainFragment(FragmentManager fm) { |
| RetainFragment fragment = (RetainFragment) fm.findFragmentByTag(TAG); |
| if (fragment == null) { |
| fragment = new RetainFragment(); |
| } |
| return fragment; |
| } |
| |
| @Override |
| public void onCreate(Bundle savedInstanceState) { |
| super.onCreate(savedInstanceState); |
| <strong>setRetainInstance(true);</strong> |
| } |
| } |
| </pre> |
| |
| <p>To test this out, try rotating a device both with and without retaining the {@link |
| android.app.Fragment}. You should notice little to no lag as the images populate the activity almost |
| instantly from memory when you retain the cache. Any images not found in the memory cache are |
| hopefully available in the disk cache, if not, they are processed as usual.</p> |