// Fresco.java
private static void initializeDrawee(Context context, @Nullable DraweeConfig draweeConfig) {
    sDraweeControllerBuilderSupplier = new PipelineDraweeControllerBuilderSupplier(context, draweeConfig);
    SimpleDraweeView.initialize(sDraweeControllerBuilderSupplier);
}

  private void init(Context context, @Nullable AttributeSet attrs) {
    try {
      if (FrescoSystrace.isTracing()) {
        FrescoSystrace.beginSection("SimpleDraweeView#init");
      }
      if (isInEditMode()) {
        getTopLevelDrawable().setVisible(true, false);
        getTopLevelDrawable().invalidateSelf();
      } else {
        Preconditions.checkNotNull(
            sDraweecontrollerbuildersupplier, "SimpleDraweeView was not initialized!");
        mControllerBuilder = sDraweecontrollerbuildersupplier.get(); // 调用一次就会创建一个新的实例
    }
    // ...... 省略其他代码  
}

SimpleDraweeView.java

public void setImageURI(Uri uri, @Nullable Object callerContext) {
    DraweeController controller =
        mControllerBuilder
            .setCallerContext(callerContext)
            .setUri(uri) //设置新的图片加载路径
            .setOldController(getController())  //复用 controller
            .build(); 
    setController(controller);
}

// AbstractDraweeController.java
protected void submitRequest() {
    mDataSource = getDataSource(); 
    final DataSubscriber<T> dataSubscriber = new BaseDataSubscriber<T>() { //可以简单的把它理解为一个监听者
        @Override
        public void onNewResultImpl(DataSource<T> dataSource) { //图片加载成功
            ...
        }
        ...
    };
    ...
    mDataSource.subscribe(dataSubscriber, mUiThreadImmediateExecutor); //mUiThreadImmediateExecutor是指 dataSubscriber 回调方法运行的线程，这里是主线程
}

// PipelineDraweeControllerBuilder 
 protected DataSource<CloseableReference<CloseableImage>> getDataSourceForRequest(
      DraweeController controller,
      String controllerId,
      ImageRequest imageRequest,
      Object callerContext,
      AbstractDraweeControllerBuilder.CacheLevel cacheLevel) {
    return mImagePipeline.fetchDecodedImage(
        imageRequest,
        callerContext,
        convertCacheLevelToRequestLevel(cacheLevel),
        getRequestListener(controller),
        controllerId);
  }

// CloseableProducerToDataSourceAdapter<T>

public static <T> DataSource<CloseableReference<T>> create(
      Producer<CloseableReference<T>> producer,
      SettableProducerContext settableProducerContext,
      RequestListener2 listener) {
   
    CloseableProducerToDataSourceAdapter<T> result =
        new CloseableProducerToDataSourceAdapter<T>(producer, settableProducerContext, listener);return result;
  }

// SimpleDraweeView.java
public void setImageURI(Uri uri, @Nullable Object callerContext) {
    DraweeController controller = mControllerBuilder
            .setCallerContext(callerContext)
            .setUri(uri)
            .setOldController(getController())
            .build();    //这里会复用 controller
    setController(controller);
}

public void setController(@Nullable DraweeController draweeController) {
    mDraweeHolder.setController(draweeController);
    super.setImageDrawable(mDraweeHolder.getTopLevelDrawable());  
}

// DraweeHolder.java
public @Nullable Drawable getTopLevelDrawable() {
    return mHierarchy == null ? null : mHierarchy.getTopLevelDrawable();
}

/** Sets a new controller. */
public void setController(@Nullable DraweeController draweeController) {
  boolean wasAttached = mIsControllerAttached;
  if (wasAttached) {
    detachController();
  }

  // Clear the old controller
  if (isControllerValid()) {
    mEventTracker.recordEvent(Event.ON_CLEAR_OLD_CONTROLLER);
    mController.setHierarchy(null);
  }
  mController = draweeController;
　// 注意这里是只有确定已经 attached 才会调用，也就是才回去加载图片
  if (wasAttached) {
    attachController();
  }
}

// AbstractDraweeController.java
public void onAttach() {
    ...
    mIsAttached = true;
    if (!mIsRequestSubmitted) {
      submitRequest();
    }
}

protected void submitRequest() {
    mDataSource = getDataSource(); 
    final DataSubscriber<T> dataSubscriber = new BaseDataSubscriber<T>() { //可以简单的把它理解为一个监听者
        @Override
        public void onNewResultImpl(DataSource<T> dataSource) { //图片加载成功
            ...
        }
        ...
    };
    ...
    mDataSource.subscribe(dataSubscriber, mUiThreadImmediateExecutor); //mUiThreadImmediateExecutor是指 dataSubscriber 回调方法运行的线程，这里是主线程
}

/**
 * <p> Execution of image request consists of multiple different tasks such as network fetch,
 * disk caching, memory caching, decoding, applying transformations etc. Producer<T> represents
 * single task whose result is an instance of T. Breaking entire request into sequence of
 * Producers allows us to construct different requests while reusing the same blocks.
 */
public interface Producer<T> {

  /**
   * Start producing results for given context. Provided consumer is notified whenever progress is made (new value is ready or error occurs).
   */
  void produceResults(Consumer<T> consumer, ProducerContext context);
}

public interface Consumer<T> {
/**
   * Called by a producer whenever new data is produced. This method should not throw an exception.
   *
   * <p>In case when result is closeable resource producer will close it after onNewResult returns.
   * Consumer needs to make copy of it if the resource must be accessed after that. Fortunately,
   * with CloseableReferences, that should not impose too much overhead.
   *
   * @param newResult
   * @param status bitwise values describing the returned result
   * @see Status for status flags
   */
  void onNewResult(T newResult, @Status int status);

  /**
   * Called by a producer whenever it terminates further work due to Throwable being thrown. This
   * method should not throw an exception.
   *
   * @param t
   */
  void onFailure(Throwable t);

  /** Called by a producer whenever it is cancelled and won't produce any more results */
  void onCancellation();

  /**
   * Called when the progress updates.
   *
   * @param progress in range [0, 1]
   */
  void onProgressUpdate(float progress);
}

// BitmapMemoryCacheProducer.java

public class BitmapMemoryCacheProducer implements Producer<CloseableReference<CloseableImage>> {

    private final Producer<CloseableReference<CloseableImage>> mInputProducer;

    // 我们假设 inputProducer 在这里为NetworkFetchProducer
    public BitmapMemoryCacheProducer(...,Producer<CloseableReference<CloseableImage>> inputProducer) { 
        ...
        mInputProducer = inputProducer;
    }

    @Override
    public void produceResults(Consumer<CloseableReference<CloseableImage>> consumer,...) {
        CloseableReference<CloseableImage> cachedReference = mMemoryCache.get(cacheKey);

        if (cachedReference != null) { //从缓存中获取成功，直接通知外界
            consumer.onNewResult(cachedReference, BaseConsumer.simpleStatusForIsLast(isFinal));
            return; //结束处理流程
        }

        Consumer<CloseableReference<CloseableImage>> wrappedConsumer = wrapConsumer(consumer..); //包了一层Consumer，即mInputProducer产生结果时，它自己可以观察到
        mInputProducer.produceResults(wrappedConsumer, producerContext); //网络加载
    }
}

// NetworkFetchProducer.java

public class NetworkFetchProducer implements Producer<EncodedImage> {

   // 它并没有 inputProducer, 对于 Fresco 的图片加载来说如果网络都获取失败，那么就是图片加载失败了

    @Override
    public void produceResults(final Consumer<CloseableReference<CloseableImage>> consumer,..) {

       // 网路获取
       //   ...
        if(获取到网络图片){
            notifyConsumer(...); //把结果通知给consumer，即观察者
        }
        ...
    }
}

// ProducerSequenceFactory.java
private Producer<CloseableReference<CloseableImage>> getBasicDecodedImageSequence(ImageRequest imageRequest) {
    switch (imageRequest.getSourceUriType()) {
        case SOURCE_TYPE_NETWORK: return getNetworkFetchSequence();
        ...
} 

/**
* swallow result if prefetch -> bitmap cache get -> background thread hand-off -> multiplex ->
* bitmap cache -> decode -> multiplex -> encoded cache -> disk cache -> (webp transcode) ->
* network fetch.
*/
private synchronized Producer<CloseableReference<CloseableImage>> getNetworkFetchSequence() {
    ...
    mNetworkFetchSequence = new BitmapCacheGetToDecodeSequence(getCommonNetworkFetchToEncodedMemorySequence());
    ...
    return mNetworkFetchSequence;
}