Android Bitmaps缓存
Android 开发中,bitmap是引起内存泄漏的罪魁祸首,关于bitmap的加载,缓存策略,官方已经给了很详细的方法:
缓存之Memory Cache:
缓存的策略,是利用应用程序的分配的内存拿出适当的一部分利用LruCache算法进行缓存。关于用多少内存来缓存图片,这个要根据不同的图片,机型和 屏幕的分辨率来进行综合考量,比如对于同一个图片,Galaxy Nexus 就比Nexus S需要的内存多。
以LruCache缓存示例代码:
private LruCache<String, Bitmap> mMemoryCache; @Override protected void onCreate(Bundle savedInstanceState) { ... // Get max available VM memory, exceeding this amount will throw an // OutOfMemory exception. Stored in kilobytes as LruCache takes an // int in its constructor. final int maxMemory = (int) (Runtime.getRuntime().maxMemory() / 1024); // Use 1/8th of the available memory for this memory cache. final int cacheSize = maxMemory / 8; mMemoryCache = new LruCache<String, Bitmap>(cacheSize) { @Override protected int sizeOf(String key, Bitmap bitmap) { // The cache size will be measured in kilobytes rather than // number of items. return bitmap.getByteCount() / 1024; } }; ... } public void addBitmapToMemoryCache(String key, Bitmap bitmap) { if (getBitmapFromMemCache(key) == null) { mMemoryCache.put(key, bitmap); } } public Bitmap getBitmapFromMemCache(String key) { return mMemoryCache.get(key); }
当加载图片的时候,如果缓存里面有图片,就直接从缓存加载,否者就在一个background thread执行加载图片的操作:
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); } }
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; } ... }
缓存之Disk Cache:
对于图片比较小,图片个数比较少的,已经对加载效率比较快的要求,用Memory Cache无疑是最好的选择,但是当对于,图片比较多的场景,比如,Gridview ,ListView加载大量的图片,内存就吃不消了,这时候,就需要用到磁盘等外部存储设备来进行缓存。
实际上,如果对图片切换比较频繁的话,那么 ContentProvider
绝壁是个好的选择,比如 image gallery application.
关于使用Disk Cache的官方示例代码如下:
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); }
memory Cache 在UI线程中执行,Disk Cache则是在Backound thread 中执行。
当屏幕旋转,或者其他配置改变的时候,比如电话进来,弹出对话框,对Activity进行销毁,重置的时候,需要对之前缓存进行保存;
比如在一个Fragment中保存缓存对象的代码示例:
private LruCache<String, Bitmap> mMemoryCache; @Override protected void onCreate(Bundle savedInstanceState) { ... RetainFragment retainFragment = RetainFragment.findOrCreateRetainFragment(getFragmentManager()); mMemoryCache = retainFragment.mRetainedCache; if (mMemoryCache == null) { mMemoryCache = new LruCache<String, Bitmap>(cacheSize) { ... // Initialize cache here as usual } retainFragment.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(); fm.beginTransaction().add(fragment, TAG).commit(); } return fragment; } @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setRetainInstance(true); } }
From :http://developer.android.com/training/displaying-bitmaps/cache-bitmap.html#config-changes