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