Glide系列-活动缓存和内存缓存

1.活动缓存数据结构用的HashMap

final class ActiveResources {
    @VisibleForTesting final Map activeEngineResources = new             
                 HashMap<>();
}

2.内存缓存LinkedHashMap

public class LruCache {
    private final Map> cache = new LinkedHashMap<>(100, 0.75f, true);
}

3.从活动或者内存缓存读取

public class Engine implements EngineJobListener, MemoryCache.ResourceRemovedListener,
        EngineResource.ResourceListener {

    private EngineResource loadFromMemory( EngineKey key, boolean isMemoryCacheable, 
       long startTime) {
        if (!isMemoryCacheable) {//是否支持内存缓存
            return null;
        }

        EngineResource active = loadFromActiveResources(key);//从活动缓存获取
        if (active != null) {//如果活动缓存不为null
            return active;
        }

        EngineResource cached = loadFromCache(key);//从内存缓存获取
        if (cached != null) { //如果内存缓存不为null
            return cached; 
        }

        return null;
    }
}

4.从活动缓存获取

public class Engine
    implements EngineJobListener,
        MemoryCache.ResourceRemovedListener,
        EngineResource.ResourceListener {
    @Nullable
    private EngineResource loadFromActiveResources(Key key) {
        EngineResource active = activeResources.get(key); //从活动缓存获取
        if (active != null) {
            active.acquire();
        }

        return active;
    }
}
  

5.从内存缓存获取

• 从活动缓存中移除这个元素
• 返回
• 将获得的元素放入活动缓存

public class Engine implements EngineJobListener, MemoryCache.ResourceRemovedListener,
        EngineResource.ResourceListener {
    private EngineResource loadFromCache(Key key) {
        EngineResource cached = getEngineResourceFromCache(key); //获得内存缓存
        if (cached != null) {
            cached.acquire();
            activeResources.activate(key, cached);//放入活动缓存
        }
        return cached;
    }
}
    

public class Engine implements EngineJobListener, MemoryCache.ResourceRemovedListener,
        EngineResource.ResourceListener {

    private EngineResource getEngineResourceFromCache(Key key) {
        Resource cached = cache.remove(key);//从内存缓存移除

        final EngineResource result; //返回的元素是新建的
        if (cached == null) {
            result = null;
        } else if (cached instanceof EngineResource) {
          // Save an object allocation if we've cached an EngineResource 
          //(the typical case).
            result = (EngineResource) cached;
        } else {
            result = new EngineResource<>(
              cached,
              /* isMemoryCacheable= */ true,
              /* isRecyclable= */ true,
              key,
              /* listener= */ this);
        }
        return result;
    }
}

6.总结

• 先从活动缓存读取；
• 如果活动缓存读取到的为null，再从内存缓存读取；
• 从内存缓存读取的时候，会将缓存的值从内存缓存移除，再放入活动缓存；