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Fresco 是一个强大的图片加载组件。
Fresco 中设计有一个叫做 image pipeline 的模块。它负责从网络，从本地文件系统，本地资源加载图片。为了最大限度节省空间和CPU时间，它含有3级缓存设计（2级内存，1级文件）。
Fresco 中设计有一个叫做 Drawees 模块，方便地显示loading图，当图片不再显示在屏幕上时，及时地释放内存和空间占用。
Fresco 支持 Android2.3(API level 9) 及其以上系统。
这么强大而优秀的开源项目，学习学习人家的源码是非常不错的，看了一两天，记录一下自己的笔记与见解。源码比较复杂，我的看法不一定对，如有错误之处欢迎指出，欢迎交流学习。
fresco作为一个图片加载框架肯定离不开数据或者，数据缓存等基本流程。如下图描述那样。
实际上，通过查看源码，Fresco使用的时候是这样的：提交请求------》获取数据
提交请求很容易理解，难点就是获取数据的过程，它获取数据的过程基本是上图的逆过程进行的，就是提交了请求-----》判断有没有缓存内存--------》没有获取网络数据
下面通过源码去看看整个流程，为了不被细节干扰，我们主要看主干线就行了，这样可以把握整体的逻辑流程。
fresco的简单使用是这样的：
Uri uri = Uri.parse("https://raw.githubusercontent.com/facebook/fresco/gh-pages/static/fresco-logo.png");
SimpleDraweeView draweeView = (SimpleDraweeView) findViewById(R.id.my_image_view);
draweeView.setImageURI(uri);
/**
* Displays an image given by the uri.
*
* @param uri uri of the image
* @undeprecate
*/
@Override
public void setImageURI(Uri uri) {
setImageURI(uri, null);
}
/**
* Displays an image given by the uri.
*
* @param uri uri of the image
* @param callerContext caller context
*/
public void setImageURI(Uri uri, @Nullable Object callerContext) {
//controller是com.facebook.drawee.backends.pipeline.PipelineDraweeController
DraweeController controller = mSimpleDraweeControllerBuilder
.setCallerContext(callerContext)
.setUri(uri)
.setOldController(getController())
.build();
setController(controller);
}
private void init() {
Preconditions.checkNotNull(
sDraweeControllerBuilderSupplier,
"SimpleDraweeView was not initialized!");
//可以看到这个初始化的时候初始化了mSimpleDraweeControllerBuilder，是通过sDraweeControllerBuilderSupplier获取的
mSimpleDraweeControllerBuilder = sDraweeControllerBuilderSupplier.get();
}
然而在初始化的时候初始化了这个sDraweeControllerBuilderSupplier
/** Initializes {@link SimpleDraweeView} with supplier of Drawee controller builders. */
public static void initialize(
Supplier<? extends SimpleDraweeControllerBuilder> draweeControllerBuilderSupplier) {
sDraweeControllerBuilderSupplier = draweeControllerBuilderSupplier;
}
/** Initializes Fresco with the default config. */
public static void initialize(Context context) {
ImagePipelineFactory.initialize(context);
initializeDrawee(context);
}
/** Initializes Fresco with the specified config. */
public static void initialize(Context context, ImagePipelineConfig imagePipelineConfig) {
ImagePipelineFactory.initialize(imagePipelineConfig);
initializeDrawee(context);
}
private static void initializeDrawee(Context context) {
sDraweeControllerBuilderSupplier = new PipelineDraweeControllerBuilderSupplier(context);
SimpleDraweeView.initialize(sDraweeControllerBuilderSupplier);
}
同时 我们知道了mSimpleDraweeControllerBuilder 就是PipelineDraweeControllerBuilderSupplier的get()，返回的，看看这个get的方法：
@Override
public PipelineDraweeControllerBuilder get() {
return newPipelineDraweeControllerBuilder(
mContext,
mPipelineDraweeControllerFactory,
mImagePipeline,
mBoundControllerListeners);
}
看看这个builder的build的方法返回的是什么：
/** Builds the specified controller. */
@Override
public AbstractDraweeController build() {
validate();
// if only a low-res request is specified, treat it as a final request.
if (mImageRequest == null && mMultiImageRequests == null && mLowResImageRequest != null) {
mImageRequest = mLowResImageRequest;
mLowResImageRequest = null;
}
return buildController();
}
/** Builds a regular controller. */
protected AbstractDraweeController buildController() {
AbstractDraweeController controller = obtainController();
maybeBuildAndSetRetryManager(controller);
maybeAttachListeners(controller);
return controller;
}
@Override
protected PipelineDraweeController obtainController() {
DraweeController oldController = getOldController();
PipelineDraweeController controller;
if (oldController instanceof PipelineDraweeController) {
controller = (PipelineDraweeController) oldController;
controller.initialize(
obtainDataSourceSupplier(),
generateUniqueControllerId(),
getCallerContext());
} else {
mPipelineDraweeControllerFactory.newController(
obtainDataSourceSupplier(),
generateUniqueControllerId(),
getCallerContext());
}
return controller;
}
public PipelineDraweeController newController(
Supplier<DataSource<CloseableReference<CloseableImage>>> dataSourceSupplier,
String id,
Object callerContext) {
return new PipelineDraweeController</span>(
mResources,
mDeferredReleaser,
mAnimatedDrawableFactory,
mUiThreadExecutor,
dataSourceSupplier,
id,
callerContext);
}走了那么远，终于可以回去，回到原点看。
/**
* Displays an image given by the uri.
*
* @param uri uri of the image
* @param callerContext caller context
*/
public void setImageURI(Uri uri, @Nullable Object callerContext) {
//controller是com.facebook.drawee.backends.pipeline.PipelineDraweeController
DraweeController controller</span> = mSimpleDraweeControllerBuilder
.setCallerContext(callerContext)
.setUri(uri)
.setOldController(getController())
.build();
setController(controller);
}
现在问题是setController(controller);后发生了什么，为什么用了这个方法后所有处理都自动完成了呢。
我们看看这个setController(controller);方法：
/** Sets the controller. */
public void setController(@Nullable DraweeController draweeController) {
mDraweeHolder.setController(draweeController);
super.setImageDrawable(mDraweeHolder.getTopLevelDrawable());
}
这里将控制器传进了holder里面。这里简单说一下，fresco采用的是典型的mvc模式，DraweeHierarchy 负责的是跟显示相关的，DraweeController 负责的是后台相关的，DraweeHolder主要是统筹两者的，其他还有一下事件记录等。官方是这么说的：
A holder class for Drawee controller and hierarchy.
返回到上面看这个setController方法：
/*
* Sets a new controller.
*/
public void setController(@Nullable DraweeController draweeController) {
boolean wasAttached = mIsControllerAttached;
if (wasAttached) {
detatchController();
}
.....
if (wasAttached) {
<span style="color: rgb(255, 0, 0);">attachController();</span>
}
} private void attachController() {
if (mIsControllerAttached) {
return;
}
mEventTracker.recordEvent(Event.ON_ATTACH_CONTROLLER);
mIsControllerAttached = true;
if (mController != null &&
mController.getHierarchy() != null) {
mController.onAttach();
}
}
@Override
public void onAttach() {
if (FLog.isLoggable(FLog.VERBOSE)) {
FLog.v(
TAG,
"controller %x %s: onAttach: %s",
System.identityHashCode(this),
mId,
mIsRequestSubmitted ? "request already submitted" : "request needs submit");
}
mEventTracker.recordEvent(Event.ON_ATTACH_CONTROLLER);
Preconditions.checkNotNull(mSettableDraweeHierarchy);
mDeferredReleaser.cancelDeferredRelease(this);
mIsAttached = true;
if (!mIsRequestSubmitted) {
submitRequest();//看到没这里进行了提交请求
}
}
protected void submitRequest() {
mEventTracker.recordEvent(Event.ON_DATASOURCE_SUBMIT);
getControllerListener().onSubmit(mId, mCallerContext);
mSettableDraweeHierarchy.setProgress(0, true);
mIsRequestSubmitted = true;
mHasFetchFailed = false;
mDataSource = getDataSource();//获取数据
if (FLog.isLoggable(FLog.VERBOSE)) {
FLog.v(
TAG,
"controller %x %s: submitRequest: dataSource: %x",
System.identityHashCode(this),
mId,
System.identityHashCode(mDataSource));
}
final String id = mId;
final boolean wasImmediate = mDataSource.hasResult();
final DataSubscriber<T> dataSubscriber =
new BaseDataSubscriber<T>() {
@Override
public void onNewResultImpl(DataSource<T> dataSource) {
// isFinished must be obtained before image, otherwise we might set intermediate result
// as final image.
boolean isFinished = dataSource.isFinished();
T image = dataSource.getResult();
if (image != null) {
onNewResultInternal(id, dataSource, image, 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);
}
};
mDataSource.subscribe(dataSubscriber, mUiThreadImmediateExecutor);//这个将获取的数据回调到ui线程</span>
}
可以看到，这里大概可以知道了，获取到数据，然后将数据提交到ui线程去，那么我们这里关心的有两个，1.如何获取到数据。 2.提交数据后怎么处理。
我们分开去看比较好，这里将获取后数据处理标志为Q1,先不管这个，我们先重点关心它是怎么获取到数据的。
然后获取到数据，那么 我们看这个方法mDataSource = getDataSource();//获取数据，这里发现是一个抽象方法，找到其实现：
@Override
protected DataSource<CloseableReference<CloseableImage>> getDataSource() {
if (FLog.isLoggable(FLog.VERBOSE)) {
FLog.v(TAG, "controller %x: getDataSource", System.identityHashCode(this));
}
return mDataSourceSupplier.get();
}
这里出现了一个mDataSourceSupplier，发现这个mDataSourceSupplier只有在两个地方传递进去，一个是构造器，还有一个是initialize方法，而这个方法只有在PipelineDraweeControllerBuilder的obtainController()时调用到：
@Ove<span style="font-family: Arial, Helvetica, sans-serif;">rride</span>
protected PipelineDraweeController obtainController() {
DraweeController oldController = getOldController();
PipelineDraweeController controller;
if (oldController instanceof PipelineDraweeController) {
controller = (PipelineDraweeController) oldController;
controller.initialize(
obtainDataSourceSupplier(),
generateUniqueControllerId(),
getCallerContext());
} else {
controller = mPipelineDraweeControllerFactory.newController(
obtainDataSourceSupplier(),
generateUniqueControllerId(),
getCallerContext());
}
return controller;
}
/** Gets the top-level data source supplier to be used by a controller. */</span>
protected Supplier<DataSource<IMAGE>> obtainDataSourceSupplier() {
if (mDataSourceSupplier != null) {
return mDataSourceSupplier;
}
Supplier<DataSource<IMAGE>> supplier = null;
// final image supplier;
if (mImageRequest != null) {
supplier = getDataSourceSupplierForRequest(mImageRequest);//根据请求获取数据提供者
} else if (mMultiImageRequests != null) {
supplier = getFirstAvailableDataSourceSupplier(mMultiImageRequests);
}
// increasing-quality supplier; highest-quality supplier goes first
if (supplier != null && mLowResImageRequest != null) {
List<Supplier<DataSource<IMAGE>>> suppliers = Lists.newArrayListWithCapacity(2);
suppliers.add(supplier);
suppliers.add(getDataSourceSupplierForRequest(mLowResImageRequest));
supplier = IncreasingQualityDataSourceSupplier.create(suppliers);
}
// no image requests; use null data source supplier
if (supplier == null) {
supplier = DataSources.getFailedDataSourceSupplier(NO_REQUEST_EXCEPTION);
}
return supplier;
}
/** Creates a data source supplier for the given image request. */
protected Supplier<DataSource<IMAGE>> getDataSourceSupplierForRequest(REQUEST imageRequest){
return getDataSourceSupplierForRequest(imageRequest, /* bitmapCacheOnly */ false);
}
/** Creates a data source supplier for the given image request. */
protected Supplier<DataSource<IMAGE>> getDataSourceSupplierForRequest(final REQUEST imageRequest,
final boolean bitmapCacheOnly) {
final Object callerContext = getCallerContext();
return new Supplier<DataSource<IMAGE>>() {<span style="color: rgb(255, 0, 0);">//创建一个新的数据提供者</span>
@Override
public DataSource<IMAGE> get() {
return getDataSourceForRequest(imageRequest, callerContext, bitmapCacheOnly);//获取数据封装到数据提供者
}
@Override
public String toString() {
return Objects.toStringHelper(this).add("request", imageRequest.toString()).toString();
}
};
}
@Override
protected DataSource<CloseableReference<CloseableImage>> getDataSourceForRequest(
ImageRequest imageRequest,
Object callerContext,
boolean bitmapCacheOnly) {
if (bitmapCacheOnly) {
return mImagePipeline.fetchImageFromBitmapCache(imageRequest, callerContext);<span style="color: rgb(255, 0, 0);">//这里获取的是缓存的数据</span>
} else {
return mImagePipeline.fetchDecodedImage(imageRequest, callerContext);//这里获取的解码的imge数据，我们关心的是这个
}
}
@Override
public PipelineDraweeControllerBuilder get() {
return new PipelineDraweeControllerBuilder(
mContext,
mPipelineDraweeControllerFactory,
mImagePipeline,
mBoundControllerListeners);
}
private void init() {
Preconditions.checkNotNull(
sDraweeControllerBuilderSupplier,
"SimpleDraweeView was not initialized!");
//可以看到这个初始化的时候初始化了mSimpleDraweeControllerBuilder，是通过sDraweeControllerBuilderSupplier获取的
mSimpleDraweeControllerBuilder = sDraweeControllerBuilderSupplier.get();
}
我们明白了 mImagePipeline是怎么来后，核心是关注mImagePipeline.fetchDecodedImage(imageRequest, callerContext);这个方法是怎么将数据返回回来的，核心啊！！
/**
* Submits a request for execution and returns a DataSource representing the pending decoded
* image(s).
*
* The returned DataSource must be closed once the client has finished with it.
* @param imageRequest the request to submit
* @return a DataSource representing the pending decoded image(s)
*/
public DataSource<CloseableReference<CloseableImage>> fetchDecodedImage(
ImageRequest imageRequest,
Object callerContext) {
//Returns a sequence that can be used for a request for a decoded image
Producer<CloseableReference<CloseableImage>> producerSequence =
mProducerSequenceFactory.getDecodedImageProducerSequence(
imageRequest);//Q2
return submitFetchRequest(producerSequence, imageRequest, callerContext);
}
这里有两个方法，一个是返回了一个序列，暂时不明白做什么的，一个submitFetchRequest很容易明白，这个方法是返回数据的，我们将前面一个标志为Q2,等下再看，我们先看看这个submitFetchRequest是如何返回数据的：
<span style="font-family: Arial, Helvetica, sans-serif;"> private <T> DataSource<CloseableReference<T>> submitFetchRequest(</span>
Producer<CloseableReference<T>> producerSequence,
ImageRequest imageRequest,
Object callerContext) {
SettableProducerContext settableProducerContext = new SettableProducerContext(
imageRequest,
generateUniqueFutureId(),
mRequestListener,
callerContext,
/* isPrefetch */ false,
imageRequest.getProgressiveRenderingEnabled() ||
!UriUtil.isNetworkUri(imageRequest.getSourceUri()),
Priority.HIGH);
return CloseableProducerToDataSourceAdapter.create(
producerSequence,
settableProducerContext,
mRequestListener);
}
这里有个SettableProducerContext，看了下注释,大概是提供上下文一样的东西，方便取消请求，先不管。
/** * ProducerContext that allows the client to cancel an image request in-flight. */ @ThreadSafe public class SettableProducerContext implements ProducerContext
那么核心就是这个方法了：
return CloseableProducerToDataSourceAdapter.create(
producerSequence,
settableProducerContext,
mRequestListener);
/**
* DataSource<CloseableReference<T>> backed by a Producer<CloseableReference<T>>
*
* @param <T>
*/
@ThreadSafe
public class CloseableProducerToDataSourceAdapter<T>
extends AbstractProducerToDataSourceAdapter<CloseableReference<T>> {
public static <T> DataSource<CloseableReference<T>> create(
Producer<CloseableReference<T>> producer,
SettableProducerContext settableProducerContext,
RequestListener listener) {
return new CloseableProducerToDataSourceAdapter<T>(
producer, settableProducerContext, listener);
}
protected AbstractProducerToDataSourceAdapter(
Producer<T> producer,//这是刚才传递进去的Q2
SettableProducerContext settableProducerContext,
RequestListener requestListener) {
mSettableProducerContext = settableProducerContext;
mRequestListener = requestListener;
mRequestListener.onRequestStart(
settableProducerContext.getImageRequest(),
mSettableProducerContext.getCallerContext(),
mSettableProducerContext.getId(),
mSettableProducerContext.isPrefetch());
//这个createConsumer()是最终返回数据处理的消费者
producer.produceResults(createConsumer(), settableProducerContext);
}private Consumer<T> createConsumer() {
return new BaseConsumer<T>() {
@Override//新的数据回调</span>
protected void onNewResultImpl(@Nullable T newResult, boolean isLast) {
AbstractProducerToDataSourceAdapter.this.onNewResultImpl(newResult, isLast);
}
@Override//数据获取失败回调</span>
protected void onFailureImpl(Throwable throwable) {
AbstractProducerToDataSourceAdapter.this.onFailureImpl(throwable);
}
@Override//请求取消回调
protected void onCancellationImpl() {
AbstractProducerToDataSourceAdapter.this.onCancellationImpl();
}
};
}
protected void onNewResultImpl(@Nullable T result, boolean isLast) {
if (super.setResult(result, isLast)) {
if (isLast) {
mRequestListener.onRequestSuccess(//是调用
mSettableProducerContext.getImageRequest(),
mSettableProducerContext.getId(),
mSettableProducerContext.isPrefetch());
}
}
}
private void onFailureImpl(Throwable throwable) {
if (super.setFailure(throwable)) {
mRequestListener.onRequestFailure(//mRequestListener将结果返回，成功或者失败
mSettableProducerContext.getImageRequest(),
mSettableProducerContext.getId(),
throwable,
mSettableProducerContext.isPrefetch());
}
}
这里我们发现了，Q2的producer的 producer.produceResults的方法传递进去一个Consumer，然后会自动将数据通过Consumer返回回来，接着通过requestlistener回调回去。这里可以看到了，实现上返回数据回来的还是Q2的那个方法，这里还有个疑问，这个requestlistener是如何将数据返回出去的呢？我们先解决一下这个疑问。
毫无疑问，在这个方法里传进了这个requestlistsner
return CloseableProducerToDataSourceAdapter.create(
producerSequence,
settableProducerContext,
mRequestListener);
public ImagePipeline(
ProducerSequenceFactory producerSequenceFactory,
Set<RequestListener> requestListeners,
Supplier<Boolean> isPrefetchEnabledSupplier,
MemoryCache<BitmapMemoryCacheKey, CloseableImage, Void> bitmapMemoryCache,
MemoryCache<CacheKey, PooledByteBuffer, Void> encodedMemoryCache,
CacheKeyFactory cacheKeyFactory) {
mIdCounter = new AtomicLong();
mProducerSequenceFactory = producerSequenceFactory;
mRequestListener = new ForwardingRequestListener(requestListeners);
mIsPrefetchEnabledSupplier = isPrefetchEnabledSupplier;
mBitmapMemoryCache = bitmapMemoryCache;
mEncodedMemoryCache = encodedMemoryCache;
mCacheKeyFactory = cacheKeyFactory;
}public class ForwardingRequestListener implements RequestListener {
private static final String TAG = "ForwardingRequestListener";
private final List<RequestListener> mRequestListeners;
public ForwardingRequestListener(
Set<RequestListener> requestListeners) {
mRequestListeners = Lists.newArrayListWithCapacity(requestListeners.size());
for (RequestListener requestListener : requestListeners) {
mRequestListeners.add(requestListener);
}
}
通过源码很容易看出来ForwardingRequestListener 维持了一个请求列表，统一管理所有请求监听。ImagePipelineFactory是实行单例模式，这个ImagePipelineFactory是在Fresco初始化时进行初始化的，它也是维持着一个ImagePipeline，就是说通过它一个维持一个请求的监听集合。这里我有点搞不明白，为什么一个image请求所有请求监听都要回调：
@Override
public void onRequestSuccess(ImageRequest request, String requestId, boolean isPrefetch) {
final int numberOfListeners = mRequestListeners.size();
for (int i = 0; i < numberOfListeners; ++i) {
RequestListener listener = mRequestListeners.get(i);
try {
listener.onRequestSuccess(request, requestId, isPrefetch);
} catch (Exception exception) {
// Don't punish the other listeners if we're given a bad one.
onException("InternalListener exception in onRequestSuccess", exception);
}
}
}
说明那么多基本都通了，还有一点就是Q2问题，它是如何发送请求并获取的数据的呢，这大概是我们最想知道的吧。
我们看Q2 那个方法:
//Returns a sequence that can be used for a request for a decoded image
Producer<CloseableReference<CloseableImage>> producerSequence =
mProducerSequenceFactory.getDecodedImageProducerSequence(
imageRequest);//Q2/**
* Returns a sequence that can be used for a request for a decoded image.
*
* @param imageRequest the request that will be submitted
* @return the sequence that should be used to process the request
*/
public Producer<CloseableReference<CloseableImage>> getDecodedImageProducerSequence(
ImageRequest imageRequest) {
Producer<CloseableReference<CloseableImage>> pipelineSequence =
getBasicDecodedImageSequence(imageRequest);
if (imageRequest.getPostprocessor() != null) {
return getPostprocessorSequence(pipelineSequence);//这是上个进程的数据
} else {
return pipelineSequence;
}
}private Producer<CloseableReference<CloseableImage>> getBasicDecodedImageSequence(
ImageRequest imageRequest) {
...
Uri uri = imageRequest.getSourceUri();
if (UriUtil.isNetworkUri(uri)) {
return getNetworkFetchSequence();
} else if (UriUtil.isLocalFileUri(uri)) {
if (MediaUtils.isVideo(MediaUtils.extractMime(uri.getPath()))) {
return getLocalVideoFileFetchSequence();
} else {
return getLocalImageFileFetchSequence();
}
} else if (UriUtil.isLocalContentUri(uri)) {
return getLocalContentUriFetchSequence();
} else if (UriUtil.isLocalAssetUri(uri)) {
return getLocalAssetFetchSequence();
} else if (UriUtil.isLocalResourceUri(uri)) {
return getLocalResourceFetchSequence();
} else {
throw new RuntimeException(
"Unsupported image type! Uri is: " + uri.toString().substring(0, 30));
}
}
/**
* swallow result if prefetch -> bitmap cache get -> wait if scrolling ->
* background thread hand-off -> multiplex -> bitmap cache -> decode -> multiplex ->
* encoded cache -> disk cache -> (webp transcode) -> network fetch.
*/
private synchronized Producer<CloseableReference<CloseableImage>> getNetworkFetchSequence() {
if (mNetworkFetchSequence == null) {
mNetworkFetchSequence =
newBitmapCacheGetToDecodeSequence(getCommonNetworkFetchToEncodedMemorySequence());
}
return mNetworkFetchSequence;
}
重点来了，我们知道我们请求并获取数据的过程是逆向的，就是提交请求-----判断有没有缓存-------没有缓存网络获取-----获取成功返回。Fresco巧妙的用了两个东西，producer和consumer，我们看官方解析：
/** * Building block for image processing in the image pipeline. * * <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. * * <p> Producer supports multiple values and streaming. * * @param <T> */ public interface Producer<T>
/**
* Consumes data produced by {@link Producer}.<T>
*
* <p> The producer uses this interface to notify its client when new data is ready or an error
* occurs. Execution of the image request is structured as a sequence of Producers. Each one
* consumes data produced by producer preceding it in the sequence.
*
* <p>For example decode is a producer that consumes data produced by the disk cache get producer.
*
* <p> The consumer is passed new intermediate results via onNewResult(isLast = false) method. Each
* consumer should expect that one of the following methods will be called exactly once, as the very
* last producer call:
* <ul>
* <li> onNewResult(isLast = true) if producer finishes successfully with a final result </li>
* <li> onFailure if producer failed to produce a final result </li>
* <li> onCancellation if producer was cancelled before a final result could be created </li>
* </ul>
*
* <p> Implementations of this interface must be thread safe, as callback methods might be called
* on different threads.
*
* @param <T>
*/
public interface Consumer<T>那么我们大概就可以猜测了它的流程类似就是producer1-----》producer12--------》producer3--------》producer4，它们之间传递consumer进行数据传递，按照这样的思路看看源码对不对。
前面我们看到了getNetworkFetchSequence()这个方法，这个方法就是获取网络数据的，
/**
* swallow result if prefetch -> bitmap cache get -> wait if scrolling ->
* background thread hand-off -> multiplex -> bitmap cache -> decode -> multiplex ->
* encoded cache -> disk cache -> (webp transcode) -> network fetch.
*/
private synchronized Producer<CloseableReference<CloseableImage>> getNetworkFetchSequence() {
if (mNetworkFetchSequence == null) {
mNetworkFetchSequence =
newBitmapCacheGetToDecodeSequence(getCommonNetworkFetchToEncodedMemorySequence());
}
return mNetworkFetchSequence;
}private Producer<CloseableReference<CloseableImage>> newBitmapCacheGetToDecodeSequence(
Producer<CloseableReference<PooledByteBuffer>><span style="color: rgb(255, 0, 0);"> nextProducer</span>) {
DecodeProducer decodeProducer = mProducerFactory.newDecodeProducer(nextProducer);
return newBitmapCacheGetToBitmapCacheSequence(decodeProducer);
}
我们看下一个方法：
/**
* Bitmap cache get <pre name="code" class="java">/**
* multiplex -> encoded cache -> disk cache -> (webp transcode) -> network fetch.
*/
private synchronized Producer<CloseableReference<PooledByteBuffer>>
getCommonNetworkFetchToEncodedMemorySequence() {
if (mCommonNetworkFetchToEncodedMemorySequence == null) {
mCommonNetworkFetchToEncodedMemorySequence =
newEncodedCacheMultiplexToTranscodeSequence(mNetworkFetchProducer, /* isLocal */false);
if (mResizeAndRotateEnabledForNetwork) {
mCommonNetworkFetchToEncodedMemorySequence =
newResizeAndRotateImagesSequence(mCommonNetworkFetchToEncodedMemorySequence);
}
}
return mCommonNetworkFetchToEncodedMemorySequence;
}这里的流程可以总结为：获取bitmap memory cache-------》启动ExecutorService进行后台处理-------》同一key多路请求合并成一个--------》获取解码了的bitmap memory cache
我们返回看getCommonNetworkFetchToEncodedMemorySequence()：
/**
* multiplex -> encoded cache -> disk cache -> (webp transcode) -> network fetch.
*/
private synchronized Producer<CloseableReference<PooledByteBuffer>>
getCommonNetworkFetchToEncodedMemorySequence() {
if (mCommonNetworkFetchToEncodedMemorySequence == null) {
mCommonNetworkFetchToEncodedMemorySequence =
newEncodedCacheMultiplexToTranscodeSequence(mNetworkFetchProducer, /* isLocal */false);
if (mResizeAndRotateEnabledForNetwork) {
mCommonNetworkFetchToEncodedMemorySequence =
newResizeAndRotateImagesSequence(mCommonNetworkFetchToEncodedMemorySequence);
}
}
return mCommonNetworkFetchToEncodedMemorySequence;
}
/**
* encoded cache multiplex -> encoded cache -> (disk cache) -> (webp transcode)
* @param nextProducer next producer in the sequence
* @param isLocal whether the image source is local or not
* @return encoded cache multiplex to webp transcode sequence
*/
private Producer<CloseableReference<PooledByteBuffer>>
newEncodedCacheMultiplexToTranscodeSequence(
Producer<CloseableReference<PooledByteBuffer>> nextProducer,
boolean isLocal) {
if (Build.VERSION.SDK_INT < Build.VERSION_CODES.JELLY_BEAN_MR2) {
nextProducer = mProducerFactory.newWebpTranscodeProducer(nextProducer);//这里进行网络获取数据
}
if (!isLocal) {
nextProducer = mProducerFactory.newDiskCacheProducer(nextProducer);//如果需要缓存就硬盘就进行缓存
}
EncodedMemoryCacheProducer encodedMemoryCacheProducer =
mProducerFactory.newEncodedMemoryCacheProducer(nextProducer);
return mProducerFactory.newEncodedCacheKeyMultiplexProducer(encodedMemoryCacheProducer);
}其实最最简单的概括就是本文最上面的那张图了，事实上它又不止那么简单，这个详细说明也蛮多的，这里我只说个大概的思路，应该是没错的。那么还有它如何通过consumer进行数据传递，这也是蛮有意思，我想再下次具体详细看这个部分。
简单总结一下这个流程，Fresco是如何只使用setImageURI（）后自动完成了获取数据的：
DraweeView采用的 是典型的mvc模式，DraweeHierarchy 负责的是跟显示相关的，DraweeController 负责的是后台相关的，DraweeHolder主要是统筹两者的，Fresco的初始化方法执行后会初始化相关配置。以SimpleDraweeView为例，它的controller是PipelineDraweeController，Fresco在初始化时初始化一个PipelineDraweeControllerBuilderSupplier来提供这个controller，DraweeHolder主要通过controller启动后台服务获取数据，setController方法执行后后调用controller的onAttach()方法，这个方法会提交请求并获得数据，后将数据返回给ui线程，Fresco数据是封装成Supplier<DataSource<IMAGE>> 的形式返回，而真正将数据传递回来的是通过ImagePipeline，这个ImagePipeline会在PipelineDraweeControllerBuilderSupplier的构造器初始化创建一个，它会在Fresco初始化方法中进行初始化，SimpleDraweeView初始化方法中会调用PipelineDraweeControllerBuilderSupplier的get的方法将ImagePipeline返回，ImagePipeline有一个RequestListsner的成员变量，它管理这个一个监听列表，请求监听通过这个RequestListsner返回，而正确去获取数据的时候是启动一个后台服务进行数据获取的，整个数据获取有分多个阶段，Fresco通过producer划分这些阶段，比如缓存获取阶段、编码阶段、网络获取阶段，它会一个一个阶段去获取数据，如果producer1没有获取到数据就交给producer2进行获取数据以此类推下去，producer之间通过consumer返回数据，最终将数据返回到ui线程去。
画了个粗略的图：
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原文地址：http://blog.csdn.net/u014614038/article/details/51480072
