Glide 源码解析

Picasso.with(this).load(“http://nuuneoi.com/uploads/source/playstore/cover.jpg“).into(ivImgPicasso);

Picasso.with(this).load(“http://nuuneoi.com/uploads/source/playstore/cover.jpg“).resize(768, 432).into(ivImgPicasso);

Picasso.with(this).load(“http://nuuneoi.com/uploads/source/playstore/cover.jpg“).fit().centerCrop().into(ivImgPicasso);

//单例模式获取Picasso 对象
public static Picasso with(Context context) {
    if (singleton == null) {
      synchronized (Picasso.class) {
        if (singleton == null) {
          singleton = new Builder(context).build();
        }
      }
    }
    return singleton;
  }

// 真正new 的地方在build()方法里
 public Picasso build() {
      Context context = this.context;

      if (downloader == null) {
        //配置默认的下载器，首先通过反射获取OkhttpClient,如果获取到了，就使用OkHttpDwownloader作为默认下载器
        //如果获取不到就使用UrlConnectionDownloader作为默认下载器
        downloader = Utils.createDefaultDownloader(context);
      }
      if (cache == null) {
       // 配置内存缓存，默认大小为手机内存的15%
        cache = new LruCache(context);
      }
      if (service == null) {
       // 配置Picaso 线程池，默认核心池大小为3
        service = new PicassoExecutorService();
      }
      if (transformer == null) {
       // 配置请求转换器，默认的请求转换器没有做任何事，直接返回原请求
        transformer = RequestTransformer.IDENTITY;
      }

      Stats stats = new Stats(cache);
      //分发器 
      Dispatcher dispatcher = new Dispatcher(context, service, HANDLER, downloader, cache, stats);

      return new Picasso(context, dispatcher, cache, listener, transformer, requestHandlers, stats,
          defaultBitmapConfig, indicatorsEnabled, loggingEnabled);
    }

//load有几个重载方法，参数为string和File 的重载最重都会包装成一个Uri 调用这个方法
 public RequestCreator load(Uri uri) {
    return new RequestCreator(this, uri, 0);
  }
// 如果是加载资源id 的图片会调用这个方法
 public RequestCreator load(int resourceId) {
    if (resourceId == 0) {
      throw new IllegalArgumentException("Resource ID must not be zero.");
    }
    return new RequestCreator(this, null, resourceId);
  }

// 配置占位图，在加载图片的时候显示
public RequestCreator placeholder(int placeholderResId) {
    if (!setPlaceholder) {
      throw new IllegalStateException("Already explicitly declared as no placeholder.");
    }
    if (placeholderResId == 0) {
      throw new IllegalArgumentException("Placeholder image resource invalid.");
    }
    if (placeholderDrawable != null) {
      throw new IllegalStateException("Placeholder image already set.");
    }
    this.placeholderResId = placeholderResId;
    return this;
  }

// 配置真正显示的大小
 public RequestCreator resize(int targetWidth, int targetHeight) {
    data.resize(targetWidth, targetHeight);
    return this;
  }

//将action 保存到了一个Map 中，目标View作为key
 void enqueueAndSubmit(Action action) {
    Object target = action.getTarget();
    if (target != null && targetToAction.get(target) != action) {
      // This will also check we are on the main thread.
      cancelExistingRequest(target);
      targetToAction.put(target, action);
    }
    submit(action);
  }

// 交给分发器分发提交请求
void submit(Action action) {
    dispatcher.dispatchSubmit(action);
  }

 
//执行请求提交
//1, 先查看保存暂停tag表里面没有包含Action的tag,如果包含，则将Action 存到暂停Action表里
//2，从BitmapHunter表里查找有没有对应action的hunter,如果有直接attach
//3, 为这个请求生成一个BitmapHunter，提交给线程池执行
 void performSubmit(Action action, boolean dismissFailed) {
    // 先查看保存暂停tag表里面没有包含Action的tag,如果包含，则将Action 存到暂停Action表里
    if (pausedTags.contains(action.getTag())) {
      pausedActions.put(action.getTarget(), action);
      if (action.getPicasso().loggingEnabled) {
        log(OWNER_DISPATCHER, VERB_PAUSED, action.request.logId(),
            "because tag '" + action.getTag() + "' is paused");
      }
      return;
    }

    BitmapHunter hunter = hunterMap.get(action.getKey());
    if (hunter != null) {
      hunter.attach(action);
      return;
    }
   // 如果线程池被shutDown,直接return
    if (service.isShutdown()) {
      if (action.getPicasso().loggingEnabled) {
        log(OWNER_DISPATCHER, VERB_IGNORED, action.request.logId(), "because shut down");
      }
      return;
    }
    // 为请求生成一个BitmapHunter
    hunter = forRequest(action.getPicasso(), this, cache, stats, action);
    hunter.future = service.submit(hunter);//提交执行
    hunterMap.put(action.getKey(), hunter);
    if (dismissFailed) {
      failedActions.remove(action.getTarget());
    }

    if (action.getPicasso().loggingEnabled) {
      log(OWNER_DISPATCHER, VERB_ENQUEUED, action.request.logId());
    }
  }

static BitmapHunter forRequest(Picasso picasso, Dispatcher dispatcher, Cache cache, Stats stats,
      Action action) {
    Request request = action.getRequest();
    List<RequestHandler> requestHandlers = picasso.getRequestHandlers();

    // Index-based loop to avoid allocating an iterator.
    //noinspection ForLoopReplaceableByForEach
    for (int i = 0, count = requestHandlers.size(); i < count; i++) {
      RequestHandler requestHandler = requestHandlers.get(i);
     // 循环请求处理器列表，如果找到有能处理这个请求的请求处理器
     // 则生成BitmapHunter
      if (requestHandler.canHandleRequest(request)) {
        return new BitmapHunter(picasso, dispatcher, cache, stats, action, requestHandler);
      }
    }

    return new BitmapHunter(picasso, dispatcher, cache, stats, action, ERRORING_HANDLER);
  }

// 1,首先调用了getRequestHandlers
 List<RequestHandler> getRequestHandlers() {
    return requestHandlers;
  }

// 2 requestHandlers 列表是在Picasso 构造函数里出实话的
 Picasso(Context context, Dispatcher dispatcher, Cache cache, Listener listener,
      RequestTransformer requestTransformer, List<RequestHandler> extraRequestHandlers, Stats stats,
      Bitmap.Config defaultBitmapConfig, boolean indicatorsEnabled, boolean loggingEnabled) {
 
    ....
   //前面代码省略
 
   // 添加了7个内置的请求处理器
  // 如果你自己通过Builder添了额外的处理器，也会添加在这个列表里面

    int builtInHandlers = 7; // Adjust this as internal handlers are added or removed.
    int extraCount = (extraRequestHandlers != null ? extraRequestHandlers.size() : 0);
    List<RequestHandler> allRequestHandlers =
        new ArrayList<RequestHandler>(builtInHandlers + extraCount);

    // ResourceRequestHandler needs to be the first in the list to avoid
    // forcing other RequestHandlers to perform null checks on request.uri
    // to cover the (request.resourceId != 0) case.
    allRequestHandlers.add(new ResourceRequestHandler(context));
    if (extraRequestHandlers != null) {
      allRequestHandlers.addAll(extraRequestHandlers);
    }
    allRequestHandlers.add(new ContactsPhotoRequestHandler(context));
    allRequestHandlers.add(new MediaStoreRequestHandler(context));
    allRequestHandlers.add(new ContentStreamRequestHandler(context));
    allRequestHandlers.add(new AssetRequestHandler(context));
    allRequestHandlers.add(new FileRequestHandler(context));
    allRequestHandlers.add(new NetworkRequestHandler(dispatcher.downloader, stats));
    requestHandlers = Collections.unmodifiableList(allRequestHandlers);
   
  //后面代码省略 
    ...

  }

@Override public Result load(Request request, int networkPolicy) throws IOException {
    //最终调用downloader的load方法获取结果
    Response response = downloader.load(request.uri, request.networkPolicy);
    if (response == null) {
      return null;
    }

    Picasso.LoadedFrom loadedFrom = response.cached ? DISK : NETWORK;

    Bitmap bitmap = response.getBitmap();
    if (bitmap != null) {
      return new Result(bitmap, loadedFrom);
    }

    InputStream is = response.getInputStream();
    if (is == null) {
      return null;
    }
    // Sometimes response content length is zero when requests are being replayed. Haven't found
    // root cause to this but retrying the request seems safe to do so.
    if (loadedFrom == DISK && response.getContentLength() == 0) {
      Utils.closeQuietly(is);
      throw new ContentLengthException("Received response with 0 content-length header.");
    }
    if (loadedFrom == NETWORK && response.getContentLength() > 0) {
      stats.dispatchDownloadFinished(response.getContentLength());
    }
    return new Result(is, loadedFrom);
  }

@Override public Response load(Uri uri, int networkPolicy) throws IOException {
   // 如果SDK 版本大于等于14,安装磁盘缓存，用的是HttpResponseCache（缓存http或者https的response到文件系统）
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
      installCacheIfNeeded(context);
    }

    HttpURLConnection connection = openConnection(uri);
   //设置使用缓存
    connection.setUseCaches(true);

    if (networkPolicy != 0) {
      String headerValue;
     // 下面一段代码是设置缓存策略
      if (NetworkPolicy.isOfflineOnly(networkPolicy)) {
        headerValue = FORCE_CACHE;
      } else {
        StringBuilder builder = CACHE_HEADER_BUILDER.get();
        builder.setLength(0);

        if (!NetworkPolicy.shouldReadFromDiskCache(networkPolicy)) {
          builder.append("no-cache");
        }
        if (!NetworkPolicy.shouldWriteToDiskCache(networkPolicy)) {
          if (builder.length() > 0) {
            builder.append(',');
          }
          builder.append("no-store");
        }

        headerValue = builder.toString();
      }

      connection.setRequestProperty("Cache-Control", headerValue);
    }

    int responseCode = connection.getResponseCode();
    if (responseCode >= 300) {
      connection.disconnect();
      throw new ResponseException(responseCode + " " + connection.getResponseMessage(),
          networkPolicy, responseCode);
    }

    long contentLength = connection.getHeaderFieldInt("Content-Length", -1);
    boolean fromCache = parseResponseSourceHeader(connection.getHeaderField(RESPONSE_SOURCE));
    // 最后获取InputStream流包装成Response返回
    return new Response(connection.getInputStream(), fromCache, contentLength);
  }

//1,
void performComplete(BitmapHunter hunter) {
    // 这里将结果缓存到内存
    if (shouldWriteToMemoryCache(hunter.getMemoryPolicy())) {
      cache.set(hunter.getKey(), hunter.getResult());
    }
    hunterMap.remove(hunter.getKey());// 请求完毕，将hunter从表中移除
    batch(hunter);
    if (hunter.getPicasso().loggingEnabled) {
      log(OWNER_DISPATCHER, VERB_BATCHED, getLogIdsForHunter(hunter), "for completion");
    }
  }

// 2,然后将BitmapHunter添加到一个批处理列表，通过Hander发送一个批处理消息
private void batch(BitmapHunter hunter) {
    if (hunter.isCancelled()) {
      return;
    }
    batch.add(hunter);
    if (!handler.hasMessages(HUNTER_DELAY_NEXT_BATCH)) {
      handler.sendEmptyMessageDelayed(HUNTER_DELAY_NEXT_BATCH, BATCH_DELAY);
    }
  }
// 3，最后执行performBatchComplete 方法，通过主线程的Handler送处理完成的消息
void performBatchComplete() {
    List<BitmapHunter> copy = new ArrayList<BitmapHunter>(batch);
    batch.clear();
    mainThreadHandler.sendMessage(mainThreadHandler.obtainMessage(HUNTER_BATCH_COMPLETE, copy));
    logBatch(copy);
  }

// 4,最后在Picasso 中handleMessage,显示图片
 static final Handler HANDLER = new Handler(Looper.getMainLooper()) {
    @Override public void handleMessage(Message msg) {
      switch (msg.what) {
        case HUNTER_BATCH_COMPLETE: {
          @SuppressWarnings("unchecked") List<BitmapHunter> batch = (List<BitmapHunter>) msg.obj;
          //noinspection ForLoopReplaceableByForEach
          for (int i = 0, n = batch.size(); i < n; i++) {
            BitmapHunter hunter = batch.get(i);
            hunter.picasso.complete(hunter);
          }
          break;
        }
      //后面代码省略
      ...
  };
// 5,最后回调到ImageViewAction 的complete方法显示图片
@Override public void complete(Bitmap result, Picasso.LoadedFrom from) {
    if (result == null) {
      throw new AssertionError(
          String.format("Attempted to complete action with no result!\n%s", this));
    }

    ImageView target = this.target.get();
    if (target == null) {
      return;
    }

    Context context = picasso.context;
    boolean indicatorsEnabled = picasso.indicatorsEnabled;
   //将结果包装成一个PicassoDrawable 并显示
    PicassoDrawable.setBitmap(target, context, result, from, noFade, indicatorsEnabled);

    if (callback != null) {
      callback.onSuccess(); 回调callback
    }
  }

public OkHttpDownloader(final File cacheDir, final long maxSize) {
    this(defaultOkHttpClient());
    try {
      client.setCache(new com.squareup.okhttp.Cache(cacheDir, maxSize));
    } catch (IOException ignored) {
    }
  }

if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH) {
      installCacheIfNeeded(context);
    }

private static void installCacheIfNeeded(Context context) {
    // DCL + volatile should be safe after Java 5.
    if (cache == null) {
      try {
        synchronized (lock) {
          if (cache == null) {
            cache = ResponseCacheIcs.install(context);
          }
        }
      } catch (IOException ignored) {
      }
    }
  }

  private static class ResponseCacheIcs {
    static Object install(Context context) throws IOException {
      File cacheDir = Utils.createDefaultCacheDir(context);
      HttpResponseCache cache = HttpResponseCache.getInstalled();
      if (cache == null) {
        long maxSize = Utils.calculateDiskCacheSize(cacheDir);
        cache = HttpResponseCache.install(cacheDir, maxSize);
      }
      return cache;
    }

    static void close(Object cache) {
      try {
        ((HttpResponseCache) cache).close();
      } catch (IOException ignored) {
      }
    }
  }

Glide.with(fragment)
     .load(myUrl)
     .into(imageView);

public static RequestManager with(Context context) {
        RequestManagerRetriever retriever = RequestManagerRetriever.get();
        return retriever.get(context);
    }

    public static RequestManager with(Activity activity) {
        RequestManagerRetriever retriever = RequestManagerRetriever.get();
        return retriever.get(activity);
    }
    
    public static RequestManager with(FragmentActivity activity) {
        RequestManagerRetriever retriever = RequestManagerRetriever.get();
        return retriever.get(activity);
    }
    
    public static RequestManager with(android.app.Fragment fragment) {
        RequestManagerRetriever retriever = RequestManagerRetriever.get();
        return retriever.get(fragment);
    }
    
    public static RequestManager with(Fragment fragment) {
        RequestManagerRetriever retriever = RequestManagerRetriever.get();
        return retriever.get(fragment);
    }

public class RequestManagerRetriever implements Handler.Callback {
    //饿汉式创建单例
    private static final RequestManagerRetriever INSTANCE = new RequestManagerRetriever();
    
    //返回单例对象
    public static RequestManagerRetriever get() {
        return INSTANCE;
    }

    //根据传入的参数，获取不同的RequestManager
    public RequestManager get(Context context) {
        if (context == null) {
            throw new IllegalArgumentException("You cannot start a load on a null Context");
        } else if (Util.isOnMainThread() && !(context instanceof Application)) {
            if (context instanceof FragmentActivity) {
                return get((FragmentActivity) context);
            } else if (context instanceof Activity) {
                return get((Activity) context);
            } else if (context instanceof ContextWrapper) {
                return get(((ContextWrapper) context).getBaseContext());
            }
        }

        return getApplicationManager(context);
    }
    
    //省略无关代码......
}

private RequestManager getApplicationManager(Context context) {
        // 返回一个单例
        if (applicationManager == null) {
            synchronized (this) {
                if (applicationManager == null) {
                    applicationManager = new RequestManager(context.getApplicationContext(),
                            new ApplicationLifecycle(), new EmptyRequestManagerTreeNode());
                }
            }
        }

        return applicationManager;
    }

public RequestManager get(Activity activity) {
        //如果不在主线程或者Android SDK的版本低于HONEYCOMB，传入的还是Application类型的context
        if (Util.isOnBackgroundThread() || Build.VERSION.SDK_INT < Build.VERSION_CODES.HONEYCOMB) {
            return get(activity.getApplicationContext());
        } else {
            //判断当前activity是否被销毁
            assertNotDestroyed(activity);
            android.app.FragmentManager fm = activity.getFragmentManager();
           //通过fragmentGet(activity, fm)获取RequestManager
            return fragmentGet(activity, fm);
        }
    }

RequestManager fragmentGet(Context context, android.app.FragmentManager fm) {
        //在当前activity中创建一个没有界面的的fragment并add到当前activity中
        RequestManagerFragment current = getRequestManagerFragment(fm);
        RequestManager requestManager = current.getRequestManager();
        if (requestManager == null) {
            //创建一个requestManager
            requestManager = new RequestManager(context, current.getLifecycle(), current.getRequestManagerTreeNode());
            //将requestManager与fragment绑定        
            current.setRequestManager(requestManager);
        }
        return requestManager;
    }

public DrawableTypeRequest<String> load(String string) {
        return (DrawableTypeRequest<String>) fromString().load(string);
    }

public DrawableTypeRequest<String> fromString() {
        return loadGeneric(String.class);
    }

private <T> DrawableTypeRequest<T> loadGeneric(Class<T> modelClass) {
        ModelLoader<T, InputStream> streamModelLoader = Glide.buildStreamModelLoader(modelClass, context);
        ModelLoader<T, ParcelFileDescriptor> fileDescriptorModelLoader =
                Glide.buildFileDescriptorModelLoader(modelClass, context);
        if (modelClass != null && streamModelLoader == null && fileDescriptorModelLoader == null) {
            throw new IllegalArgumentException("Unknown type " + modelClass + ". You must provide a Model of a type for"
                    + " which there is a registered ModelLoader, if you are using a custom model, you must first call"
                    + " Glide#register with a ModelLoaderFactory for your custom model class");
        }

        //这句是核心，本质是创建并返回了一个DrawableTypeRequest
        return optionsApplier.apply(
                new DrawableTypeRequest<T>(modelClass, streamModelLoader, fileDescriptorModelLoader, context,
                        glide, requestTracker, lifecycle, optionsApplier));
    }

@Override
    public DrawableRequestBuilder<ModelType> load(ModelType model) {
        super.load(model);
        return this;
    }

public GenericRequestBuilder<ModelType, DataType, ResourceType, TranscodeType> load(ModelType model) {
        this.model = model;
        isModelSet = true;
        return this;
    }

public Target<GlideDrawable> into(ImageView view) {
        return super.into(view);
    }

public Target<TranscodeType> into(ImageView view) {
        //确保在主线程
        Util.assertMainThread();
        //确保view不为空
        if (view == null) {
            throw new IllegalArgumentException("You must pass in a non null View");
        }
        //对ScaleType进行配置
        if (!isTransformationSet && view.getScaleType() != null) {
            switch (view.getScaleType()) {
                case CENTER_CROP:
                    applyCenterCrop();
                    break;
                case FIT_CENTER:
                case FIT_START:
                case FIT_END:
                    applyFitCenter();
                    break;
                //$CASES-OMITTED$
                default:
                    // Do nothing.
            }
        }

        //核心
        return into(glide.buildImageViewTarget(view, transcodeClass));
    }

 <R> Target<R> buildImageViewTarget(ImageView imageView, Class<R> transcodedClass) {
        return imageViewTargetFactory.buildTarget(imageView, transcodedClass);
    }

public <Z> Target<Z> buildTarget(ImageView view, Class<Z> clazz) {
        //图片来源是GlideDrawable
        if (GlideDrawable.class.isAssignableFrom(clazz)) {
            //创建GlideDrawable对应的target
            return (Target<Z>) new GlideDrawableImageViewTarget(view);
        } else if (Bitmap.class.equals(clazz)) {
            //如果图片来源是Bitmap，创建Bitmap对应的target
            return (Target<Z>) new BitmapImageViewTarget(view);
        } else if (Drawable.class.isAssignableFrom(clazz)) {
            //如果图片来源是Drawable，创建Drawable对应的target
            return (Target<Z>) new DrawableImageViewTarget(view);
        } else {
            throw new IllegalArgumentException("Unhandled class: " + clazz
                    + ", try .as*(Class).transcode(ResourceTranscoder)");
        }
    }

public <Y extends Target<TranscodeType>> Y into(Y target) {
        //确保在主线程
        Util.assertMainThread();
        //确保target不为空
        if (target == null) {
            throw new IllegalArgumentException("You must pass in a non null Target");
        }
        //确保数据来源已经确定，即已经调用了load(url)方法
        if (!isModelSet) {
            throw new IllegalArgumentException("You must first set a model (try #load())");
        }

        //获取当前target已经绑定的Request对象
        Request previous = target.getRequest();

        //如果当前target已经绑定了Request对象，则清空这个Request对象
        if (previous != null) {
            previous.clear();
            //停止绑定到当前target的上一个Request的图片请求处理
            requestTracker.removeRequest(previous);
            previous.recycle();
        }
        
        //创建Request对象
        Request request = buildRequest(target);
        //与target绑定
        target.setRequest(request);
        lifecycle.addListener(target);
        //执行request
        requestTracker.runRequest(request);

        return target;
    }

public void runRequest(Request request) {
        //将请求加入请求集合
        requests.add(request);
        
        if (!isPaused) {
            如果处于非暂停状态，开始执行请求
            request.begin();
        } else {
            //如果处于暂停状态，将请求添加到等待集合
            pendingRequests.add(request);
        }
    }

 @Override
    public void begin() {
        startTime = LogTime.getLogTime();
        if (model == null) {
            onException(null);
            return;
        }

        status = Status.WAITING_FOR_SIZE;
        if (Util.isValidDimensions(overrideWidth, overrideHeight)) {
            //如果长宽尺寸已经确定
            onSizeReady(overrideWidth, overrideHeight);
        } else {
            //获取长宽尺寸，获取完之后会调用onSizeReady(overrideWidth, overrideHeight)
            target.getSize(this);
        }

        if (!isComplete() && !isFailed() && canNotifyStatusChanged()) {
            //开始加载图片，先显示占位图
            target.onLoadStarted(getPlaceholderDrawable());
        }
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("finished run method in " + LogTime.getElapsedMillis(startTime));
        }
    }

@Override
    public void onSizeReady(int width, int height) {
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("Got onSizeReady in " + LogTime.getElapsedMillis(startTime));
        }
        if (status != Status.WAITING_FOR_SIZE) {
            return;
        }
        status = Status.RUNNING;

        width = Math.round(sizeMultiplier * width);
        height = Math.round(sizeMultiplier * height);

        ModelLoader<A, T> modelLoader = loadProvider.getModelLoader();
        final DataFetcher<T> dataFetcher = modelLoader.getResourceFetcher(model, width, height);

        if (dataFetcher == null) {
            onException(new Exception("Failed to load model: \'" + model + "\'"));
            return;
        }
        ResourceTranscoder<Z, R> transcoder = loadProvider.getTranscoder();
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("finished setup for calling load in " + LogTime.getElapsedMillis(startTime));
        }
        loadedFromMemoryCache = true;
        
        //核心代码，加载图片
        loadStatus = engine.load(signature, width, height, dataFetcher, loadProvider, transformation, transcoder,
                priority, isMemoryCacheable, diskCacheStrategy, this);
                
        loadedFromMemoryCache = resource != null;
        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("finished onSizeReady in " + LogTime.getElapsedMillis(startTime));
        }
    }

    engine.load(signature, width, height, dataFetcher, loadProvider, transformation, transcoder, priority, isMemoryCacheable, diskCacheStrategy, this)，

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();
        EngineKey key = keyFactory.buildKey(id, signature, width, height, loadProvider.getCacheDecoder(),
                loadProvider.getSourceDecoder(), transformation, loadProvider.getEncoder(),
                transcoder, loadProvider.getSourceEncoder());

        //使用LruCache获取缓存
        EngineResource<?> cached = loadFromCache(key, isMemoryCacheable);
        if (cached != null) {
            //从缓存中获取资源成功
            cb.onResourceReady(cached);
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                logWithTimeAndKey("Loaded resource from cache", startTime, key);
            }
            return null;
        }

        //从弱引用中获取缓存
        EngineResource<?> active = loadFromActiveResources(key, isMemoryCacheable);
        if (active != null) {
            //从缓存中获取资源成功
            cb.onResourceReady(active);
            if (Log.isLoggable(TAG, Log.VERBOSE)) {
                logWithTimeAndKey("Loaded resource from active resources", startTime, key);
            }
            return null;
        }

        //开启线程从网络中加载图片......
        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);
    }

@Override
    public void onResourceReady(Resource<?> resource) {
        if (resource == null) {
            onException(new Exception("Expected to receive a Resource<R> with an object of " + transcodeClass
                    + " inside, but instead got null."));
            return;
        }

        Object received = resource.get();
        if (received == null || !transcodeClass.isAssignableFrom(received.getClass())) {
            releaseResource(resource);
            onException(new Exception("Expected to receive an object of " + transcodeClass
                    + " but instead got " + (received != null ? received.getClass() : "") + "{" + received + "}"
                    + " inside Resource{" + resource + "}."
                    + (received != null ? "" : " "
                        + "To indicate failure return a null Resource object, "
                        + "rather than a Resource object containing null data.")
            ));
            return;
        }

        if (!canSetResource()) {
            releaseResource(resource);
            // We can't set the status to complete before asking canSetResource().
            status = Status.COMPLETE;
            return;
        }
        
        //核心是这一句
        onResourceReady(resource, (R) received);
    }

 private void onResourceReady(Resource<?> resource, R result) {
        // We must call isFirstReadyResource before setting status.
        boolean isFirstResource = isFirstReadyResource();
        status = Status.COMPLETE;
        this.resource = resource;

        if (requestListener == null || !requestListener.onResourceReady(result, model, target, loadedFromMemoryCache,
                isFirstResource)) {
            GlideAnimation<R> animation = animationFactory.build(loadedFromMemoryCache, isFirstResource);
            
            //核心，通过调用target的onResourceReady方法加载图片
            target.onResourceReady(result, animation);
        }

        notifyLoadSuccess();

        if (Log.isLoggable(TAG, Log.VERBOSE)) {
            logV("Resource ready in " + LogTime.getElapsedMillis(startTime) + " size: "
                    + (resource.getSize() * TO_MEGABYTE) + " fromCache: " + loadedFromMemoryCache);
        }
    }

public class DrawableImageViewTarget extends ImageViewTarget<Drawable> {
    public DrawableImageViewTarget(ImageView view) {
        super(view);
    }

    @Override
    protected void setResource(Drawable resource) {
       view.setImageDrawable(resource);
    }
}

public class LruCache<K, V> {
    //存储缓存
    private final LinkedHashMap<K, V> map;
    //当前缓存的总大小
    private int size;
    //最大缓存大小
    private int maxSize;
    //添加到缓存的个数
    private int putCount;
    //创建的个数
    private int createCount;
    //移除的个数
    private int evictionCount;
    //命中个数
    private int hitCount;
    //未命中个数
    private int missCount;

    public LruCache(int maxSize) {
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }
        this.maxSize = maxSize;
        this.map = new LinkedHashMap<K, V>(0, 0.75f, true);
    }

    //重新设置最大缓存
    public void resize(int maxSize) {
        //确保最大缓存大于0
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }

        synchronized (this) {
            this.maxSize = maxSize;
        }
        //对当前的缓存做一些操作以适应新的最大缓存大小
        trimToSize(maxSize);
    }

    //获取缓存
    public final V get(K key) {
        //确保key不为null
        if (key == null) {
            throw new NullPointerException("key == null");
        }

        V mapValue;
        synchronized (this) {
            mapValue = map.get(key);
            //如果可以获取key对应的value
            if (mapValue != null) {
                //命中数加一
                hitCount++;
                return mapValue;
            }
            //如果根据key获取的value为null，未命中数加一
            missCount++;
        }
        
        //省略无关代码......
    }

    
    public final V put(K key, V value) {
        if (key == null || value == null) {
            throw new NullPointerException("key == null || value == null");
        }

        V previous;
        synchronized (this) {
            //添加到缓存的个数加一
            putCount++;
            //更新当前缓存大小
            size += safeSizeOf(key, value);
            previous = map.put(key, value);
            if (previous != null) {
                //如果之前map中对应key存在value不为null，由于重复的key新添加的value会覆盖上一个value，所以当前缓存大小应该再减去之前value的大小
                size -= safeSizeOf(key, previous);
            }
        }
        //根据缓存最大值调整缓存
        trimToSize(maxSize);
        return previous;
    }

    //根据最大缓存大小对map中的缓存做调整
    public void trimToSize(int maxSize) {
        while (true) {
            K key;
            V value;
            synchronized (this) {
                if (size < 0 || (map.isEmpty() && size != 0)) {
                    throw new IllegalStateException(getClass().getName()
                            + ".sizeOf() is reporting inconsistent results!");
                }
                //当前缓存大小小于最大缓存，或LinkedHashMap为空时跳出循环
                if (size <= maxSize || map.isEmpty()) {
                    break;
                }
                
                //遍历LinkedHashMap,删除顶部的（也就是最先添加的）元素，直到当前缓存大小小于最大缓存，或LinkedHashMap为空
                Map.Entry<K, V> toEvict = map.entrySet().iterator().next();
                key = toEvict.getKey();
                value = toEvict.getValue();
                map.remove(key);
                size -= safeSizeOf(key, value);
                evictionCount++;
            }

          //省略无关代码......
        }
    }

    public final V remove(K key) {
        if (key == null) {
            throw new NullPointerException("key == null");
        }

        V previous;
        synchronized (this) {
            previous = map.remove(key);
            if (previous != null) {
                size -= safeSizeOf(key, previous);
            }
        }

        return previous;
    }

    private int safeSizeOf(K key, V value) {
        int result = sizeOf(key, value);
        if (result < 0) {
            throw new IllegalStateException("Negative size: " + key + "=" + value);
        }
        return result;
    }

     protected int sizeOf(K key, V value) {
        return 1;
    }


    public final void evictAll() {
        trimToSize(-1); // -1 will evict 0-sized elements
    }

    @Override public synchronized final String toString() {
        int accesses = hitCount + missCount;
        int hitPercent = accesses != 0 ? (100 * hitCount / accesses) : 0;
        return String.format(Locale.US, "LruCache[maxSize=%d,hits=%d,misses=%d,hitRate=%d%%]",
                maxSize, hitCount, missCount, hitPercent);
    }
}

//根据最大缓存大小对map中的缓存做调整
    public void trimToSize(int maxSize) {
        while (true) {
            K key;
            V value;
            synchronized (this) {
                if (size < 0 || (map.isEmpty() && size != 0)) {
                    throw new IllegalStateException(getClass().getName()
                            + ".sizeOf() is reporting inconsistent results!");
                }
                //当前缓存大小小于最大缓存，或LinkedHashMap为空时跳出循环
                if (size <= maxSize || map.isEmpty()) {
                    break;
                }
                
                //遍历LinkedHashMap,删除顶部的（也就是最先添加的）元素，直到当前缓存大小小于最大缓存，或LinkedHashMap为空
                Map.Entry<K, V> toEvict = map.entrySet().iterator().next();
                key = toEvict.getKey();
                value = toEvict.getValue();
                map.remove(key);
                size -= safeSizeOf(key, value);
                evictionCount++;
            }

          //省略无关代码......
        }
    }