前言
glide的内存缓存有两级:LruCache、ActiveResources
其中LruCache老生常谈了,这里就不细说了。
ActiveResources实际上内含一个HashMap,Map中value则是资源的弱引用。
那么这两级是如何工作的?
取出
先从LruCache取,没有再从ActiveResources取
如果LruCache中有,则取出存入ActiveResources,并从LruCache移除
代码如下:
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(); //生成缓存的key EngineKey key = keyFactory.buildKey(id, signature, width, height, loadProvider.getCacheDecoder(), loadProvider.getSourceDecoder(), transformation, loadProvider.getEncoder(), transcoder, loadProvider.getSourceEncoder()); //从LruCache获取缓存图片 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; } //从弱引用获取图片 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); } ... } 复制代码
存储
如果内存本地都没有,则从网络获取,获取后先存入ActiveResources,ActiveResources中存储的是EngineResource对象的弱引用。
EngineResource是将资源进行封装的一个类,它有一个计数acquired,记录资源被引用的次数,当资源被取出使用时+1(acquired函数),当资源被释放时-1(release函数)。当acquired为0时,会将它从ActiveResources中移除,存入LruCache。
代码如下:
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); } } } } 复制代码
listener是Engine对象
@Override public synchronized void onResourceReleased(Key cacheKey, EngineResource<?> resource) { activeResources.deactivate(cacheKey); if (resource.isCacheable()) { cache.put(cacheKey, resource); } else { resourceRecycler.recycle(resource); } } 复制代码
可以看到如果开启内存缓存,则存入LruCache,否则直接释放。
两级缓存
这样我们就比较明白glide内存的两级缓存是怎么回事了,实际上是对缓存的资源进行了划分:使用中的和使用过的。
使用中的放入ActiveResources,这样可以防止被LruCache算法回收掉;而使用过的放在LruCache中,通过算法控制内存总量。
release何时执行
上面我们知道当资源被使用时会调用EngineResource的acquired函数,释放的时候会调用EngineResource的release函数。
使用的时候我们比较好理解,取出的时候其实就是使用的时候,这是一个主动的动作。
但是何时释放?glide中是怎么监控资源释放的?
通过查找EngineResource的release函数的调用,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"); } } 复制代码
继续查找这个函数在哪里调用,发现在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); } } 复制代码
那么这个clear函数又在哪里调用?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; } } 复制代码
ViewTarget是对要加载图片的ImageView进行封装,而资源的释放也必然与View有关系。
ViewTarget有一个字段protected final T view;
这就是要加载图片的ImageView,另外在ViewTarget中可以看到对这个view添加了attach监听:
view.addOnAttachStateChangeListener(attachStateListener); 复制代码
这个attachStateListener的源码:
attachStateListener = new OnAttachStateChangeListener() { @Override public void onViewAttachedToWindow(View v) { resumeMyRequest(); } @Override public void onViewDetachedFromWindow(View v) { pauseMyRequest(); } }; 复制代码
这样就很明显了,每个加载图片的view都会注册一个OnAttachStateChangeListener,当这个view从界面移除的时候,也就是资源不再被引用的时候,就会调用pauseMyRequest,最终会将EngineResource的引用计数-1。
这样就保证了当ActiveResources中的资源不再被引用时,将这个资源转移到LruCache中。