Bitmap緩存機制
阿新 • • 發佈:2017-06-19
mean set wake instance ads images obj hat 內存大小
當我們載入一張圖片到ImageView,首先檢查Lrucache。
Bitmap緩存機制
載入一個bitmap到UI裏面比較簡單直接。可是,假設我們一次載入大量的bitmap數據的時候就變得復雜了。很多情況下(比方這些組件:ListVIew,GridView或者ViewPager),顯示的圖片和將要滾動顯示的圖片通常是沒有限制的。 內存的使用量能夠被降下來。通過回收那些移出屏幕之外的組件。Android的垃圾回收機制相同會釋放你的資源,假設它們沒有被引用。這樣的機制是好的,可是為了獲得流暢高速的UI體驗,我們想避免反復下載圖片。一種本地內存緩存的方法提供了非常大的幫助。能夠高速的又一次載入本地緩存的資源。 本章將帶你使用內存緩存的機制來提高UI的響應速度和流暢的體驗,當載入多張圖片的時候。原文:http://wear.techbrood.com/training/displaying-bitmaps/cache-bitmap.html#config-changes
使用內存緩存
內存緩存使用有限的應用內存去緩存bitmap。LruCache這個類(also available in the Support Li brary for use back to API Level 4),很適合緩存bitmaps。他使用LinkedHashMap,它會在超過緩存大小的時候回收近期最少使用的指向。Note:過去。我們常使用 SoftReference or WeakReference 來緩存。可是如今不推薦了。從Android2.3(API Level 9),垃圾回收器抵制使用它們。為了為LrcCache選擇合適的內存空間,以下幾個因素要被大家重視的:Android3.0(11)之後。bitmap被存儲在有效的緩存裏面。在可預測的情況下並不能被釋放。這樣導致超過內存限制而且導致崩潰。
- 你應用的空余內存是多大?
- 一次將要載入多少張圖片顯示?如今已經顯示了多少張圖片?
- 屏幕的尺寸大小和密度是多少?高密度的是被比方Galaxy Nexus要比低密度的設備須要更大的緩存。
- bitmap的尺寸和配置是什麽。沒一張圖片所占資源的大小是多少?
- 你須要什麽樣的用戶體驗?還是有一部分須要流暢的體驗?假設是這種話,你能夠把他們長久的放到內存裏面或者使用LrcCache緩存。
- 你須要在質量(內存大小)和“質量”(圖片的質量)上做出選擇?有時候,我們能夠選擇存儲縮略圖,後臺載入更高質量的圖片。
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.
當我們載入一張圖片到ImageView,首先檢查Lrucache。
假設一旦找到接口。我們能夠非常高速的更新這個ImageView,否則,我們啟動一個線程去運行載入這張圖片。
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須要更新資源和資源的緩存接口:
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; } ... }
使用本地緩存
內存緩存是一種非常好的提快速度的方法,可是你不能全然依靠它。像GridView組件非常快的占用掉大量內存。你的應用可能會被其它任務打斷。比方來電,而且後臺進程可能會被終止,內存緩存也可能會被釋放。當你的應用再次啟動時候,你不得不又一次載入。 本地緩存能夠解決問題,幫助你存儲那些緩存不須要的資源來降低反復載入的次數,當然,本地緩存的使用要比內存緩存的速度要慢,須要在後臺操作。擔任果然讀取的事件是不可預知的 Note: A ContentProvider might be a more appropriate place to store cached images if they are accessed more frequently, for example in an image gallery application. 以下的代碼DiskLrcCache。是從本地載入的一個樣例: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. 內存緩存在UI線程裏面檢測。本地緩存須要在後臺使用,本地緩存不能代替內存緩存在UI線程裏面的地位,終於,為了以後的使用bitmap同事被放到內存和本地
處理配置的變化
執行時的配置變化。比方屏幕方向的變化會引起Android摧毀和重新啟動Activity(For more information about this behavior, see Handling Runtime Changes),你想避免再次process你的圖片,為了更快的體驗。 幸運的是,你有一個非常好的內存緩存機制,你能夠使用Fragment來忽略這些通過使用setRetainInstance(true),
當Activity被又一次創建的時候,該保留的Fragment相同會被又一次附著到你的應用上面。
以下是一個應對配置改變時的樣例:
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); } }為了測試它,我們能夠旋轉屏幕在保留和不保留Fragment的情況下。你應該會註意到。當你保留Fragment的時候。你會註意到圖片會毫無滯留的從內存緩存載入。內存中沒有找到的會被緩存到本地。假設不這種話,和常規一樣。
緩存的圖片的優化請看上一篇:多線程處理Bitmaps
Bitmap緩存機制