Bitmap缓存机制
Bitmap缓存机制
使用内存缓存
Li brary for use back to API Level 4),很适合缓存bitmaps。他使用LinkedHashMap,它会在超过缓存大小的时候回收近期最少使用的指向。
Note:过去。我们常使用 SoftReference or WeakReference 来缓存。可是如今不推荐了。从Android2.3(API
Level 9),垃圾回收器抵制使用它们。Android3.0(11)之后。bitmap被存储在有效的缓存里面。在可预測的情况下并不能被释放。这样导致超过内存限制而且导致崩溃。
为了为LrcCache选择合适的内存空间,以下几个因素要被大家重视的:
- 你应用的空余内存是多大?
- 一次将要载入多少张图片显示?如今已经显示了多少张图片?
- 屏幕的尺寸大小和密度是多少?高密度的是被比方Galaxy Nexus要比低密度的设备须要更大的缓存。
- bitmap的尺寸和配置是什么。没一张图片所占资源的大小是多少?
- 你须要什么样的用户体验?还是有一部分须要流畅的体验?假设是这种话,你能够把他们长久的放到内存里面或者使用LrcCache缓存。
- 你须要在质量(内存大小)和“质量”(图片的质量)上做出选择?有时候,我们能够选择存储缩略图,后台载入更高质量的图片。
没有一个特定大小的内存能够使用全部的应用。他取决于你去分析内存的使用情况寻找一个合适的解决方法。缓存太小的话没有什么意义,缓存太大easy引起java.lang.OutOfMemory exceptions而且仅仅留给你的应用非常少的一部分内存。
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);
}
Note: 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 fullscreen 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 2.5pages of images in memory.
假设一旦找到接口。我们能够非常高速的更新这个ImageView,否则,我们启动一个线程去运行载入这张图片。
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);
}
}
BitmapWorkerTask须要更新资源和资源的缓存接口:
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;
}
...
}
使用本地缓存
be a more appropriate place to store cached images if they are accessed more frequently, for example in an image gallery application.
以下的代码DiskLrcCache。是从本地载入的一个样例:
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) {<pre name="code" class="java">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);
}
}
Note: 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.
处理配置的变化
Runtime Changes),你想避免再次process你的图片,为了更快的体验。
setRetainInstance(true),
当Activity被又一次创建的时候,该保留的Fragment相同会被又一次附着到你的应用上面。
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);
}
}
为了測试它,我们能够旋转屏幕在保留和不保留Fragment的情况下。你应该会注意到。当你保留Fragment的时候。你会注意到图片会毫无滞留的从内存缓存载入。内存中没有找到的会被缓存到本地。假设不这种话,和常规一样。
缓存的图片的优化请看上一篇:多线程处理Bitmaps
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