注：为了突出重点，文中的源码有大量删节，有的用//..进行了标识，有的没有。

引子

图片加载

Engine#load()

public 
    
      LoadStatus load(//..) {    //..    // 一级内存缓存    EngineResource
      active = loadFromActiveResources(key, isMemoryCacheable);    if (active != null) {      //..      return null;    }    // 二级内存内存    EngineResource
      cached = loadFromCache(key, isMemoryCacheable);    if (cached != null) {      //..      return null;    }    //..  }复制代码
    

ActiveResources解析

获取 ActiveResource 缓存

Engine#loadFromActiveResources()

private final ActiveResources activeResources;    @Nullable  private EngineResource
     loadFromActiveResources(Key key, boolean isMemoryCacheable) {    //..    EngineResource
     active = activeResources.get(key);    //..    return active;  }复制代码

ActiveResource

本质是一个HashMap，用WeekReference保存EngineResource，用作内存缓存。

final class ActiveResources {  final Map
    
      activeEngineResources = new HashMap<>();      static final class ResourceWeakReference extends WeakReference
     
      > {    //..  }    void activate(Key key, EngineResource
       resource) {    ResourceWeakReference toPut = new ResourceWeakReference(//..)    ResourceWeakReference removed = activeEngineResources.put(key, toPut);    //..  }      void deactivate(Key key) {    ResourceWeakReference removed = activeEngineResources.remove(key);    if (removed != null) {      removed.reset();    }  }        EngineResource
       get(Key key) {    ResourceWeakReference activeRef = activeEngineResources.get(key);    //..    return active;  }}复制代码
     
    

ActiveResources 缓存的添加和移除

从调用关系可以看到activate是在EngineJobComplete或loadFromcache时调用的，deactivate是在onResourceRelease时调用的。调用都是在Engine中，可见ActiveResources的维护都是有Engine维护的。

MemoryCache 解析

获取 MemoryCache 缓存

Engine#loadFromCache

private final MemoryCache cache;  private EngineResource
     loadFromCache(Key key, boolean isMemoryCacheable) {    //..    EngineResource
     cached = getEngineResourceFromCache(key);    //..    return cached;  }    private EngineResource
     getEngineResourceFromCache(Key key) {    Resource
     cached = cache.remove(key);    //..    return result;  }复制代码

MemoryCache 的具体实现

MemoryCache 是一个接口， 默认的实现是LruResourceCache。 初始化逻辑： GlideBuilder#build

@NonNull  Glide build(@NonNull Context context) {    //..    if (memoryCache == null) {      memoryCache = new LruResourceCache(memorySizeCalculator.getMemoryCacheSize());    }        if (engine == null) {      engine =          new Engine(              memoryCache,            //..);    }    //..  }复制代码

LruResourceCache 的继承结构

LruResourceCache extends LruCache
    
     > implements MemoryCache复制代码
    

MemoryCache 缓存的添加和移除

加入是在onResourceReleased，联系前面的源码只可以看到，在onResourceReleased中cache从ActiveResource中移出来，放到LruResourceCache中了。

再来回顾一下Glide内存缓存的逻辑 onEngineJobComplete 图片被缓存到ActiveResources onResourceReleased 图片从ActiveResources移到LruResourceCache loadFromCache 图片从LruResourceCache移到ActiveResources

@Override  public void onEngineJobComplete(EngineJob
     engineJob, Key key, EngineResource
     resource) {    //..     if (resource.isCacheable()) {        activeResources.activate(key, resource);    }  }    @Override  public void onResourceReleased(Key cacheKey, EngineResource
     resource) {    activeResources.deactivate(cacheKey);    //..    cache.put(cacheKey, resource);    //..  }    private EngineResource
     loadFromCache(Key key, boolean isMemoryCacheable) {    //..    EngineResource
     cached = getEngineResourceFromCache(key);    activeResources.activate(key, cached);    return cached;  }    private EngineResource
     getEngineResourceFromCache(Key key) {    Resource
     cached = cache.remove(key);    //..  }复制代码

回顾Engine、ActivityResources、LruCacheResourceCache

从上面的分析可以看到，cache的加入和移除都是通过Engine的回调方法进行处理的，也就是说Engine监听图片加载过程中的各种事件，在事件发生的时候采取相应的行动维护缓存。Engine实现了3个interface，一个这对图片加载任务，一个针对ActiveResources，一个针对LruCacheResourceCache，结构非常清晰。

public class Engine implements EngineJobListener,    MemoryCache.ResourceRemovedListener,    EngineResource.ResourceListener{}    interface EngineJobListener {  void onEngineJobComplete(EngineJob
     engineJob, Key key, EngineResource
     resource);  void onEngineJobCancelled(EngineJob
     engineJob, Key key);}MemoryCacheinterface ResourceRemovedListener {  void onResourceRemoved(@NonNull Resource
     removed);}EngineResourceinterface ResourceListener {  void onResourceReleased(Key key, EngineResource
     resource);}复制代码