简单聊聊Glide的内存缓存

Posted 2021-12-24 BennuCTech

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);
    

    ...

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);
      
    
  

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);
  

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");
  

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);
  

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;
  

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);

这个attachStateListener的源码：