在使用Fresco时，最常用的莫过于这个方法:void setImageURI(@Nullable String uriString)。那么就以这个方法为起点，追踪fresco的图片加载流程。
setImageURI()
SimpleDraweeView.java
public void setImageURIroller = mSimpleDraweeControllerBuilder
.setCallerContext(callerContext)
.setUri(uri)
.setOldController(getController())
.build();
setController(controller);
}
可以看到这个方法就设置了一个DraweeController。其实就是一个PipelineDraweeController对象。这个对象持有要加载图片的Uri，那么看来对于图片的加载PipelineDraweeController是关键。
继续追踪
setController->DraweeView.java
public void setController(@Nullable DraweeController draweeController) {
mDraweeHolder.setController(draweeController);
super.setImageDrawable(mDraweeHolder.getTopLevelDrawable());
}
即实际上是把Controller设置给了DraweeHolder。并立刻显示DraweeHolder中的最顶层的图片。
第一章已经知道Fresco在加载图片时，对于不同的阶段会显示不同的图片，具体使用一个
DraweeHierarchy的概念来组织。这里其实可以猜出mDraweeHolder.getTopLevelDrawable()就是拿出DraweeHierarchy中我们设置的 placeholder 图片。
这段代码把要做的事情都委托给了DraweeHolder。在看具体下面流程之前，先大致看一下DraweeHolder是干什么的。
DraweeHolder
/**
* A holder class for Drawee controller and hierarchy.
*
* <p>Drawee users, should, as a rule, use {@link DraweeView} or its subclasses. There are
* situations where custom views are required, however, and this class is for those circumstances.
*
* <p>Each {@link DraweeHierarchy} object should be contained in a single instance of this
* class.
...
*/
public class DraweeHolder<DH extends DraweeHierarchy> implements VisibilityCallback
即这个类看样是维护 Drawee controller 和 hierarchy关系的。DraweeHierarchy和DraweeHolder是一对一的关系。
接下来继续追踪
setController->DraweeHolder
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;
if (mController != null) {
mEventTracker.recordEvent(Event.ON_SET_CONTROLLER);
mController.setHierarchy(mHierarchy);
} else {
mEventTracker.recordEvent(Event.ON_CLEAR_CONTROLLER);
}
if (wasAttached) {
attachController();
}
}
上面代码维护了Hierarchy与Controller的关系，一个Hierarchy被一个Controller引用。把Controller操作相关的一些事件记录到DraweeEventTracker。然后调用attachController()。
DraweeEventTracker是一个
This class keeps a record of internal events that take place in the Drawee。他主要用来调试。
attachController()最终会调用 mController.onAttach()，在这个方法中发起了图片加载事件。那么来看一下onAttach() -> AbstractDraweeController.java
public void onAttach() {
mEventTracker.recordEvent(Event.ON_ATTACH_CONTROLLER);
Preconditions.checkNotNull(mSettableDraweeHierarchy);
mDeferredReleaser.cancelDeferredRelease(this);
mIsAttached = true;
if (!mIsRequestSubmitted) {
submitRequest();
}
}
可以看到这个方法主要记录了一些状态，并且要mSettableDraweeHierarchy不为null,这里其实就有点问题了，我们其实并没有在加载图片时设置过DraweeHierarchy那这个是哪里来的呢？其实我们在布局文件，或者手动new SimpleDraweeView时都会触发GenericDraweeView的inflateHierarchy。触发时机当然是在设置DraweeHierarchy时。
protected void inflateHierarchy(Context context, @Nullable AttributeSet attrs) {
GenericDraweeHierarchyBuilder builder =
GenericDraweeHierarchyInflater.inflateBuilder(context, attrs);
setAspectRatio(builder.getDesiredAspectRatio());
setHierarchy(builder.build());
}
即mSettableDraweeHierarchy其实就是一个GenericDraweeHierarchy对象。在判断这个对象不为null后，Controller调用了submitRequest(),这个方法看起来好像开始加载图片了。
protected void submitRequest() {
final T closeableImage = getCachedImage();
if (closeableImage != null) {
mDataSource = null;
mIsRequestSubmitted = true;
mHasFetchFailed = false;
mEventTracker.recordEvent(Event.ON_SUBMIT_CACHE_HIT);
getControllerListener().onSubmit(mId, mCallerContext);
onImageLoadedFromCacheImmediately(mId, closeableImage);
onNewResultInternal(mId, mDataSource, closeableImage, 1.0f, true, true);
return;
}
......
final String id = mId;
final boolean wasImmediate = mDataSource.hasResult();
final DataSubscriber<T> dataSubscriber =
new BaseDataSubscriber<T>() {
@Override
public void onNewResultImpl(DataSource<T> dataSource) {
boolean isFinished = dataSource.isFinished();
float progress = dataSource.getProgress();
T image = dataSource.getResult();
if (image != null) {
onNewResultInternal(id, dataSource, image, progress, isFinished, wasImmediate);
} else if (isFinished) {
onFailureInternal(id, dataSource, new NullPointerException(), /* isFinished */ true);
}
}
@Override
public void onFailureImpl(DataSource<T> dataSource) {
onFailureInternal(id, dataSource, dataSource.getFailureCause(), /* isFinished */ true);
}
@Override
public void onProgressUpdate(DataSource<T> dataSource) {
boolean isFinished = dataSource.isFinished();
float progress = dataSource.getProgress();
onProgressUpdateInternal(id, dataSource, progress, isFinished);
}
};
mDataSource.subscribe(dataSubscriber, mUiThreadImmediateExecutor);
}
大体上逻辑是这样的:在有缓存的情况下直接使用缓存中的图片，在没有缓存时，使用DataSource加载图片。
看一下是如何从内存中获取缓存图片的：PipelineDraweeController -> getCachedImage():
protected CloseableReference<CloseableImage> getCachedImage() {
if (mMemoryCache == null || mCacheKey == null) {
return null;
}
// We get the CacheKey
CloseableReference<CloseableImage> closeableImage = mMemoryCache.get(mCacheKey);
if (closeableImage != null && !closeableImage.get().getQualityInfo().isOfFullQuality()) {
closeableImage.close();
return null;
}
return closeableImage;
}
CloseableReference实际上是一个持有引用计数的指针，当他所引用的对象引用数量为0，调用close()。
private @Nullable MemoryCache<CacheKey, CloseableImage> mMemoryCache
那么如何使用DataSource加载图片的呢?
final boolean wasImmediate = mDataSource.hasResult();
DataSource
跟着mDataSource.hasResult(),追踪一下mDataSource是哪个类的实例,如何加载数据。
dataSoure通过 AbstractDraweeController -> obtainDataSourceSupplier()方法获得。
/** Gets the top-level data source supplier to be used by a controller. */
protected Supplier<DataSource<IMAGE>> obtainDataSourceSupplier() {
//....
// final image supplier;
if (mImageRequest != null) {
supplier = getDataSourceSupplierForRequest(mImageRequest);
....
return supplier;
}
可以看到应根据不同的image request获得不同的datasource。追踪源码，最终的data soure是在ImagePipeline中fetchDecodedImage()方法获得。
public DataSource<CloseableReference<CloseableImage>> fetchDecodedImage(
ImageRequest imageRequest,
Object callerContext,
ImageRequest.RequestLevel lowestPermittedRequestLevelOnSubmit) {
try {
Producer<CloseableReference<CloseableImage>> producerSequence =
mProducerSequenceFactory.getDecodedImageProducerSequence(imageRequest);
return submitFetchRequest(
producerSequence,
imageRequest,
lowestPermittedRequestLevelOnSubmit,
callerContext);
} catch (Exception exception) {
return DataSources.immediateFailedDataSource(exception);
}
}
private <T> DataSource<CloseableReference<T>> submitFetchRequest(
Producer<CloseableReference<T>> producerSequence,
ImageRequest imageRequest,
ImageRequest.RequestLevel lowestPermittedRequestLevelOnSubmit,
Object callerContext) {
......
return CloseableProducerToDataSourceAdapter.create(
producerSequence,
settableProducerContext,
requestListener);
} catch (Exception exception) {
return DataSources.immediateFailedDataSource(exception);
}
}
...
可以看到这里DataSource返回的是CloseableProducerToDataSourceAdapter的实例。那么数据源到底如何发起图片加载请求的呢？看一下CloseableProducerToDataSourceAdapter构造函数：
protected AbstractProducerToDataSourceAdapter(
Producer<T> producer,
SettableProducerContext settableProducerContext,
RequestListener requestListener) {
//....
producer.produceResults(createConsumer(), settableProducerContext);
}
这里其实就是调用producer的produceResults()来产生请求结果，并通过回调来设置返回结果。而上面的mDataSource.hasResult()就是判断当前有没有返回结果。
通过上面的源码分析，这里已经知道，DataSource并不会去加载图片，它会把图片加载的细节委托给Producer,它只是负责发起Producer的图片加载请求，并监听返回结果给外部。
使用下面这张图总结一下流程：
progress.png
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