前言

glide的內存緩存有兩級：LruCache、ActiveResources

其中LruCache老生常談了，這里就不細說了。

ActiveResources實際上內含一個HashMap，Map中value則是資源的弱引用。

那么這兩級是如何工作的？

取出

先從LruCache取，沒有再從ActiveResources取

如果LruCache中有，則取出存入ActiveResources，并從LruCache移除

代碼如下：

public class Engine implements EngineJobListener,MemoryCache.ResourceRemovedListener,EngineResource.ResourceListener {... public <T, Z, R> LoadStatus load(Key signature, int width, int height, DataFetcher<T> fetcher,DataLoadProvider<T, Z> loadProvider, Transformation<Z> transformation, ResourceTranscoder<Z, R> transcoder,Priority priority, boolean isMemoryCacheable, DiskCacheStrategy diskCacheStrategy, ResourceCallback cb) {Util.assertMainThread();long startTime = LogTime.getLogTime();final String id = fetcher.getId();//生成緩存的keyEngineKey key = keyFactory.buildKey(id, signature, width, height, loadProvider.getCacheDecoder(),loadProvider.getSourceDecoder(), transformation, loadProvider.getEncoder(),transcoder, loadProvider.getSourceEncoder());//從LruCache獲取緩存圖片EngineResource<?> cached = loadFromCache(key, isMemoryCacheable);if (cached != null) {cb.onResourceReady(cached);if (Log.isLoggable(TAG, Log.VERBOSE)) {logWithTimeAndKey("Loaded resource from cache", startTime, key);}return null;}//從弱引用獲取圖片EngineResource<?> active = loadFromActiveResources(key, isMemoryCacheable);if (active != null) {cb.onResourceReady(active);if (Log.isLoggable(TAG, Log.VERBOSE)) {logWithTimeAndKey("Loaded resource from active resources", startTime, key);}return null;}EngineJob current = jobs.get(key);if (current != null) {current.addCallback(cb);if (Log.isLoggable(TAG, Log.VERBOSE)) {logWithTimeAndKey("Added to existing load", startTime, key);}return new LoadStatus(cb, current);}EngineJob engineJob = engineJobFactory.build(key, isMemoryCacheable);DecodeJob<T, Z, R> decodeJob = new DecodeJob<T, Z, R>(key, width, height, fetcher, loadProvider, transformation,transcoder, diskCacheProvider, diskCacheStrategy, priority);EngineRunnable runnable = new EngineRunnable(engineJob, decodeJob, priority);jobs.put(key, engineJob);engineJob.addCallback(cb);engineJob.start(runnable);if (Log.isLoggable(TAG, Log.VERBOSE)) {logWithTimeAndKey("Started new load", startTime, key);}return new LoadStatus(cb, engineJob);}... }

存儲

如果內存本地都沒有，則從網絡獲取，獲取后先存入ActiveResources，ActiveResources中存儲的是EngineResource對象的弱引用。

EngineResource是將資源進行封裝的一個類，它有一個計數acquired，記錄資源被引用的次數，當資源被取出使用時+1（acquired函數），當資源被釋放時-1（release函數）。當acquired為0時，會將它從ActiveResources中移除，存入LruCache。

代碼如下：

void release() {synchronized (listener) {synchronized (this) {if (acquired <= 0) {throw new IllegalStateException("Cannot release a recycled or not yet acquired resource");}if (--acquired == 0) {listener.onResourceReleased(key, this);}}} }

listener是Engine對象

@Override public synchronized void onResourceReleased(Key cacheKey, EngineResource<?> resource) {activeResources.deactivate(cacheKey);if (resource.isCacheable()) {cache.put(cacheKey, resource);} else {resourceRecycler.recycle(resource);} }

可以看到如果開啟內存緩存，則存入LruCache，否則直接釋放。

兩級緩存

這樣我們就比較明白glide內存的兩級緩存是怎么回事了，實際上是對緩存的資源進行了劃分：使用中的和使用過的。

使用中的放入ActiveResources，這樣可以防止被LruCache算法回收掉；而使用過的放在LruCache中，通過算法控制內存總量。

release何時執行

上面我們知道當資源被使用時會調用EngineResource的acquired函數，釋放的時候會調用EngineResource的release函數。

使用的時候我們比較好理解，取出的時候其實就是使用的時候，這是一個主動的動作。

但是何時釋放？glide中是怎么監控資源釋放的？

通過查找EngineResource的release函數的調用，找到在Engine中

public void release(Resource<?> resource) {if (resource instanceof EngineResource) {((EngineResource<?>) resource).release();} else {throw new IllegalArgumentException("Cannot release anything but an EngineResource");} }

繼續查找這個函數在哪里調用，發現在SingleRequest中

@Override public synchronized void clear() {assertNotCallingCallbacks();stateVerifier.throwIfRecycled();if (status == Status.CLEARED) {return;}cancel();// Resource must be released before canNotifyStatusChanged is called.if (resource != null) {releaseResource(resource);}if (canNotifyCleared()) {target.onLoadCleared(getPlaceholderDrawable());}status = Status.CLEARED;if (toRelease != null) {engine.release(toRelease);} }

那么這個clear函數又在哪里調用？在ViewTarget中

@Synthetic void pauseMyRequest() {Request request = getRequest();// If the Request were cleared by the developer, it would be null here. The only way it's// present is if the developer hasn't previously cleared this Target.if (request != null) {isClearedByUs = true;request.clear();isClearedByUs = false;} }

ViewTarget是對要加載圖片的ImageView進行封裝，而資源的釋放也必然與View有關系。

ViewTarget有一個字段protected final T view;這就是要加載圖片的ImageView，另外在ViewTarget中可以看到對這個view添加了attach監聽：

view.addOnAttachStateChangeListener(attachStateListener);

這個attachStateListener的源碼：

總結

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