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简单聊聊Glide的内存缓存

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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:

attachStateListener = new OnAttachStateChangeListener() {
  @Override
  public void onViewAttachedToWindow(View v) {
    resumeMyRequest();
  }

  @Override
  public void onViewDetachedFromWindow(View v) {
    pauseMyRequest();
  }
};

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。

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