一.项目介绍

Fresco是Facebook提供的一个开源图片加载与管理库。它的功能很强大&＃xff0c;可以从网络、本地存储和Android资源文件中加载图片&＃xff0c;它完全自己负责图片加载与显示&＃xff0c;不需要你为细节去操心。
Fresco含有3级缓存设计&＃xff08;2级内存&＃xff0c;1级文件&＃xff09;。

Android2.3及以上的系统都可以使用Fresco。在Android5.0以下的系统上&＃xff0c;Fresco将图片放在一个特别的内存中&＃xff08;ashmem heap&＃xff09;&＃xff0c;而非Java Heap中&＃xff0c;而5.0及以上的系统由于对内存的管理比之前的版本优化很多&＃xff0c;所以图片缓存直接放在Java Heap中了。

二.简单用法&＃xff08;一个demo&＃xff09;

1. 添加依赖
2. 初始化Fresco
3. 编写布局
4. 指定Uri

布局中引入&＃xff1a;

<com.facebook.drawee.view.SimpleDraweeViewandroid:id&＃61;"&＃64;&＃43;id/my_image_view"android:layout_width&＃61;"120dp"android:layout_height&＃61;"wrap_content"fresco:viewAspectRatio&＃61;"1"fresco:fadeDuration&＃61;"300"fresco:actualImageScaleType&＃61;"fitCenter"fresco:placeholderImage&＃61;"&＃64;mipmap/ic_launcher"fresco:failureImage&＃61;"&＃64;mipmap/ic_launcher"fresco:roundAsCircle&＃61;"true"fresco:roundedCornerRadius&＃61;"10dp"fresco:roundTopLeft&＃61;"true"fresco:roundTopRight&＃61;"true"fresco:roundBottomLeft&＃61;"true"fresco:roundBottomRight&＃61;"true"fresco:roundingBorderWidth&＃61;"1dp"fresco:roundingBorderColor&＃61;"#00ffff"/>

testIv.setImageURI("https://ss0.bdstatic.com/5aV1bjqh_Q23odCf/static/superman/img/logo/logo_white_fe6da1ec.png");


三.Fresco中的一些概念

1.DraweeView

继承于 View, 负责图片的显示。一般情况下&＃xff0c;使用SimpleDraweeView 即可

2.ImageRequest

ImageRequest存储着Image Pipeline处理被请求图片所需要的有用信息(Uri、是否渐进式图片、是否返回缩略图、缩放、是否自动旋转等)。

3.监听下载事件

有时候我们需要监听图片显示的过程&＃xff0c;比如在失败&＃xff0c;中间过程&＃xff0c;成功时做一些事情。我们可以这么做&＃xff1a;

为SimpleDraweeView 指定一个 DraweeController
为DraweeController 指定一个 ControllerListener
在ControllerListener 的回调方法里处理 失败&＃xff0c;中间过程&＃xff0c;成功时的事情

Uri uri;
DraweeController controller &＃61; Fresco.newControllerBuilder()
.setControllerListener(controllerListener)
.setUri(uri);
.build();
mSimpleDraweeView.setController(controller);

上面的代码指定了一个 ControllerListener &＃xff0c;它包含一些回调方法&＃xff1a;

onFinalImageSet 加载完成
onIntermediateImageSet 加载中间过程
onFailure 加载失败

4.Fresco对各种Uri类型的资源的支持

Fresco 支持许多URI格式。见下表&＃xff1a;

类型 Scheme 示例
远程图片: http://, https:// HttpURLConnection 或者参考 使用其他网络加载方案
本地文件: file:// FileInputStream
Content provider: content:// ContentResolver
asset目录下的资源: asset:// AssetManager
res目录下的资源: res:// Resources.openRawResource

特别注意&＃xff1a;Fresco 不支持 相对路径的URI. 所有的URI都必须是绝对路径&＃xff0c;并且带上该URI的scheme。

四.总体设计

Fresco 是一个典型的 MVC 模型&＃xff0c;只不过把 Model 叫做 DraweeHierarchy。

M : DraweeHierarchy
V : DraweeView
C : DraweeController

五.核心模块分述

1.视图层DraweeView继承体系及各个类的作用

DraweeView
–| GenericDraweeView
——| SimpleDraweeView

DraweeView (Viewer)

获取和设置Hierarchy&＃43;Controller&＃xff0c;DraweeView的相关信息在DraweeHolder中
DraweeHolder是一个辅助的类&＃xff0c;解耦的设计方式&＃xff0c;将需要设置以及传递控制的信息&＃xff0c;全部交给DrawHolder来实现

/** This method is idempotent so it only has effect the first time it&＃39;s called */private void init(Context context) {if (mInitialised) {return;}mInitialised &＃61; true;mDraweeHolder &＃61; DraweeHolder.create(null, context);if (Build.VERSION.SDK_INT >&＃61; Build.VERSION_CODES.LOLLIPOP) {ColorStateList imageTintList &＃61; getImageTintList();if (imageTintList &＃61;&＃61; null) {return;}setColorFilter(imageTintList.getDefaultColor());}}

再查看剩余的DraweeView的程序&＃xff0c;发现其均将只是将相关事件传递给DraweeHolder&＃xff0c;这是一种解耦的设计方式&＃xff0c;以后就是不采用DraweeView&＃xff0c;采用其他的方式&＃xff0c;照样可以使用这套逻辑

GenericDraweeView

解析在xml中设置的属性

protected void inflateHierarchy(Context context, &＃64;Nullable AttributeSet attrs) {GenericDraweeHierarchyBuilder builder &＃61;GenericDraweeHierarchyInflater.inflateBuilder(context, attrs);setAspectRatio(builder.getDesiredAspectRatio());setHierarchy(builder.build());}

具体看下inflateHierarchy方法&＃xff0c;这个方法中主要做的是解析并设置xml属性&＃xff0c;这里采用的是建造者模式GenericDraweeHierarchyBuilder。

SimpleDraweeView

从外界设置ConrolllerBuilderSupplier
可以设置ImageUri
核心的业务逻辑位于DraweeView中
在控件初始化时&＃xff0c;初始化了一个DraweeHolder

我们看下它的setImageURI方法的实现&＃xff1a;

public void setImageURI(Uri uri, &＃64;Nullable Object callerContext) {DraweeController controller &＃61; mSimpleDraweeControllerBuilder.setCallerContext(callerContext).setUri(uri).setOldController(getController()).build();setController(controller);}

2.控制层DraweeController继承体系以及个各类的作用

DraweeController
–| AbstractDraweeController
—-| PipelineDraweeController

DraweeController:

获取和设置Hieraychy
view的各种事件通知过来&＃xff0c;controller来控制这些逻辑的操作&＃xff08;onAttach/onDetach/onTouchEvent/getAnimatable&＃xff09;

AbstractDraweeController

最关键的功能&＃xff1a; 实现了客户端向服务端的提交请求&＃xff0c;即向DataSource中注册观察者&＃xff0c;在有结果返回的时候&＃xff0c;在主线程通知客户端更新即可&＃xff0c;即设置Hierarychy的drawable即可
参照之前的分析方式&＃xff0c;仍然采用先构造&＃xff0c;然后具体方法的顺序

2.1 构造方法&＃xff0c;设置了UI线程池&＃xff0c;重试&＃xff0c;以及手势相关的信息

public AbstractDraweeController(DeferredReleaser deferredReleaser,Executor uiThreadImmediateExecutor,String id,Object callerContext) {mDeferredReleaser &＃61; deferredReleaser;mUiThreadImmediateExecutor &＃61; uiThreadImmediateExecutor;init(id, callerContext, true);}

private void init(String id, Object callerContext, boolean justConstructed) {mEventTracker.recordEvent(Event.ON_INIT_CONTROLLER);// cancel deferred releaseif (!justConstructed && mDeferredReleaser !&＃61; null) {mDeferredReleaser.cancelDeferredRelease(this);}// reinitialize mutable state (fetch state)mIsAttached &＃61; false;mIsVisibleInViewportHint &＃61; false;releaseFetch();mRetainImageOnFailure &＃61; false;// reinitialize optional componentsif (mRetryManager !&＃61; null) {mRetryManager.init();}if (mGestureDetector !&＃61; null) {mGestureDetector.init();mGestureDetector.setClickListener(this);}if (mControllerListener instanceof InternalForwardingListener) {((InternalForwardingListener) mControllerListener).clearListeners();} else {mControllerListener &＃61; null;}mControllerViewportVisibilityListener &＃61; null;// clear hierarchy and controller overlayif (mSettableDraweeHierarchy !&＃61; null) {mSettableDraweeHierarchy.reset();mSettableDraweeHierarchy.setControllerOverlay(null);mSettableDraweeHierarchy &＃61; null;}mControllerOverlay &＃61; null;// reinitialize constant stateif (FLog.isLoggable(FLog.VERBOSE)) {FLog.v(TAG, "controller %x %s -> %s: initialize", System.identityHashCode(this), mId, id);}mId &＃61; id;mCallerContext &＃61; callerContext;}


2.2 具体方法&＃xff0c;在这里做分析时&＃xff0c;我们重点关注图片如何获取&＃xff0c;因而我们关注的核心方法是onAttach()&＃xff0c;在这里实现了图片请求的机制&＃xff0c;以及图片获取到如何回调&＃xff0c;如何显示到UI层的控制&＃xff0c;在下面的程序中&＃xff0c;看到核心的设置的方法是submitRequest()

&＃64;Overridepublic 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 &＃61; true;if (!mIsRequestSubmitted) {submitRequest();}}

此处以第一次请求为例&＃xff0c;这样分析比较简单&＃xff0c;查看下面的方法&＃xff0c;在请求时&＃xff0c;设置请求的进度为0&＃xff0c;获取到数据源(DataSource)&＃xff0c;然后给数据源注册观察者(DataSubscriber)&＃xff0c;先查看下面的SubmitRequest方法

protected void submitRequest() {final T closeableImage &＃61; getCachedImage();if (closeableImage !&＃61; null) {mDataSource &＃61; null;mIsRequestSubmitted &＃61; true;mHasFetchFailed &＃61; false;mEventTracker.recordEvent(Event.ON_SUBMIT_CACHE_HIT);getControllerListener().onSubmit(mId, mCallerContext);onNewResultInternal(mId, mDataSource, closeableImage, 1.0f, true, true);return;}mEventTracker.recordEvent(Event.ON_DATASOURCE_SUBMIT);getControllerListener().onSubmit(mId, mCallerContext);mSettableDraweeHierarchy.setProgress(0, true);mIsRequestSubmitted &＃61; true;mHasFetchFailed &＃61; false;mDataSource &＃61; getDataSource();if (FLog.isLoggable(FLog.VERBOSE)) {FLog.v(TAG,"controller %x %s: submitRequest: dataSource: %x",System.identityHashCode(this),mId,System.identityHashCode(mDataSource));}final String id &＃61; mId;final boolean wasImmediate &＃61; mDataSource.hasResult();final DataSubscriber dataSubscriber &＃61;new BaseDataSubscriber() {&＃64;Overridepublic void onNewResultImpl(DataSource dataSource) {// isFinished must be obtained before image, otherwise we might set intermediate result// as final image.boolean isFinished &＃61; dataSource.isFinished();float progress &＃61; dataSource.getProgress();T image &＃61; dataSource.getResult();if (image !&＃61; null) {onNewResultInternal(id, dataSource, image, progress, isFinished, wasImmediate);} else if (isFinished) {onFailureInternal(id, dataSource, new NullPointerException(), /* isFinished */ true);}}&＃64;Overridepublic void onFailureImpl(DataSource dataSource) {onFailureInternal(id, dataSource, dataSource.getFailureCause(), /* isFinished */ true);}&＃64;Overridepublic void onProgressUpdate(DataSource dataSource) {boolean isFinished &＃61; dataSource.isFinished();float progress &＃61; dataSource.getProgress();onProgressUpdateInternal(id, dataSource, progress, isFinished);}};mDataSource.subscribe(dataSubscriber, mUiThreadImmediateExecutor);}

到了这里&＃xff0c;一次请求已经完成了&＃xff0c;请求的结果会在回调中执行&＃xff0c;但是请求是如何生成的呢&＃xff1f;我们并没有看到具体发送请求的逻辑&＃xff0c;这个疑问我们先记录下来&＃xff08;暂且标记为Q1&＃xff09;。先来看看对于请求结果是如何处理的&＃xff0c;以新的一次请求结果为例&＃xff0c;onNewResultImpl&＃xff08;&＃xff09;方法&＃xff0c;而onNewResultImpl方法&＃xff0c;以image不为空为例&＃xff0c;最终会调用AbstractDraweeController.onNewResultInternal()方法。下面我们来看看&＃xff0c;是如何处理这次新的请求的结果。

1.判断是否是想要的数据源&＃xff0c;即查看数据信息是否是当前请求的信息&＃xff0c;如果不是&＃xff0c;直接释放了资源
2.如果是想要的数据源&＃xff0c;创建对应的drawable&＃xff0c;设置当前显示的drawable&＃xff0c;释放之前缓存的drawable对象和Image对象

private void onNewResultInternal(String id,DataSource dataSource,&＃64;Nullable T image,float progress,boolean isFinished,boolean wasImmediate) {// ignore late callbacks (data source that returned the new result is not the one we expected)if (!isExpectedDataSource(id, dataSource)) {logMessageAndImage("ignore_old_datasource &＃64; onNewResult", image);releaseImage(image);dataSource.close();return;}mEventTracker.recordEvent(isFinished ? Event.ON_DATASOURCE_RESULT : Event.ON_DATASOURCE_RESULT_INT);// create drawableDrawable drawable;try {drawable &＃61; createDrawable(image);} catch (Exception exception) {logMessageAndImage("drawable_failed &＃64; onNewResult", image);releaseImage(image);onFailureInternal(id, dataSource, exception, isFinished);return;}T previousImage &＃61; mFetchedImage;Drawable previousDrawable &＃61; mDrawable;mFetchedImage &＃61; image;mDrawable &＃61; drawable;try {// set the new imageif (isFinished) {logMessageAndImage("set_final_result &＃64; onNewResult", image);mDataSource &＃61; null;mSettableDraweeHierarchy.setImage(drawable, 1f, wasImmediate);getControllerListener().onFinalImageSet(id, getImageInfo(image), getAnimatable());// IMPORTANT: do not execute any instance-specific code after this point} else {logMessageAndImage("set_intermediate_result &＃64; onNewResult", image);mSettableDraweeHierarchy.setImage(drawable, progress, wasImmediate);getControllerListener().onIntermediateImageSet(id, getImageInfo(image));// IMPORTANT: do not execute any instance-specific code after this point}} finally {if (previousDrawable !&＃61; null && previousDrawable !&＃61; drawable) {releaseDrawable(previousDrawable);}if (previousImage !&＃61; null && previousImage !&＃61; image) {logMessageAndImage("release_previous_result &＃64; onNewResult", previousImage);releaseImage(previousImage);}}}

好了&＃xff0c;就是获取到图像后续的操作&＃xff0c;这个其实就是我们UI的操作&＃xff0c;分析到此即可&＃xff0c;其他的情况&＃xff0c;我们参照这个分析的方式分析即可。下面我们来解决一下之前的Q1问题,数据源的请求是如何发送出去的&＃xff0c;这个问题就比较复杂了&＃xff0c;我们需要通过至少四篇的博客来分析这个请求的过程。

PipelineDraweeController

(以PipelineDraweeController为例)在通过builder.build()创建Controller的过程中,会调用obtainDataSourceSupplier来获取所需的DataSourceSupplier,进而在请求图片(submitRequest)的过程中获取当前的DataSource. 而通过在DataSource中订阅DataSourceSubscriber,使得请求到的数据在改变时能够通过controller将获取到的图片或者中间结果传递到DraweeHierarchy中,最终显示出来(…写得有点绕…x_x),对这个过程有个大概的了解有助于于之后DataSource模块进行连接。

PipelineDraweeController: Fresco默认的实现,也就是SimpleDraweeView中使用的, 用来桥接image pipeline和 SettableDraweeHierarchy

3.模型层DraweeHierachy继承体系以及各个类的作用

DraweeHierachy

用于获取顶层的drawable

先来看看DraweeHierachy的源码&＃xff0c;发现其为接口&＃xff0c;并且只有一个方法&＃xff0c;就是用于获取顶层的Drawable

public interface DraweeHierarchy {/*** Returns the top level drawable in the corresponding hierarchy. Hierarchy should always have* the same instance of its top level drawable.* &＃64;return top level drawable*/Drawable getTopLevelDrawable();
}

SettableDraweeHierachy&＃xff1a;

图像可以被重置
图像可以设置进度
设置失败
设置重试
设置controllerOverlay
在理解获取顶层的Drawable时&＃xff0c;需要首先理解Drawable的继承结构

public interface SettableDraweeHierarchy extends DraweeHierarchy {public void reset();public void setImage(Drawable drawable, float progress, boolean immediate);public void setProgress(float progress, boolean immediate);public void setFailure(Throwable throwable);public void setRetry(Throwable throwable);public void setControllerOverlay(Drawable drawable);
}

4.Fresco初始化的过程

ImagePipelineFactory\ImagePipelineConfig

ImagePipelineConfig这里使用了建造者模式&＃xff0c;为Fresco提供非常多的可配置选项。

这其中配置了比较核心的几项

mBitmapMemoryCacheParamsSupplier 内存缓存数据的策略
mCacheKeyFactory 缓存键值对的获取
mExecutorSupplier 获取本地读写线程池&＃xff0c;网络数据线程池&＃xff0c;解码线程池&＃xff0c;以及后台线程池
mImageDecoder 解码器
网络数据获取器
......
ImagePipeLineConfig是一个比较核心的类&＃xff0c;通过这个&＃xff0c;我们可以得知&＃xff0c;Freco初始化时&＃xff0c;配置了大量的策略&＃xff0c;可配置项很多&＃xff0c;也就让我们的使用更加灵活和易于拓展

5.Fresco客户端与服务端的交互

以上我们分析了一遍&＃xff0c;但是并未提到网络请求的位置的方式&＃xff0c;具体的网络请求是在哪里呢&＃xff1f;

我们回去看AbstractDraweeController.submitRequest()方法&＃xff0c;当时我们并未说明getDataSource()方法是如何实现的。

&＃64;Overrideprotected DataSource> getDataSource() {if (FLog.isLoggable(FLog.VERBOSE)) {FLog.v(TAG, "controller %x: getDataSource", System.identityHashCode(this));}return mDataSourceSupplier.get();}


那么,这里就要考虑Supplier是从哪里初始化的,然后做了怎样的操作,查看当前的mDataSourceSupplier赋值的地方,得知,初始化的地方有两种方式,一种是构造,一种是直接的initialize,initialize的方法注释中已经告诉我们,这个是controller在detach的时候调用的,所以我们只需要查看构造即可

PipelineDraweeController的构造方法&＃xff1a;

public PipelineDraweeController(Resources resources,DeferredReleaser deferredReleaser,AnimatedDrawableFactory animatedDrawableFactory,Executor uiThreadExecutor,MemoryCache memoryCache,Supplier>> dataSourceSupplier,String id,CacheKey cacheKey,Object callerContext) {super(deferredReleaser, uiThreadExecutor, id, callerContext);mResources &＃61; resources;mAnimatedDrawableFactory &＃61; animatedDrawableFactory;mMemoryCache &＃61; memoryCache;mCacheKey &＃61; cacheKey;init(dataSourceSupplier);}


private void init(Supplier>> dataSourceSupplier) {mDataSourceSupplier &＃61; dataSourceSupplier;}

那么现在需要考虑的就是构造方法在哪里进行的调用,Alt&＃43;F7快捷键查看调用的地方,发现只有一处地方,就是我们的PipelineDraweeControllerFactory.newController()的方法,而这个方法我们之前已经分析过了
,现在稍微总结一下:
就是说每次在生成请求的时候,即AbstractDraweeController在submitRequest的时候,都会去向Supplier中获取到数据源DataSource,这个数据源是在AbstractDraweeControllerBuilder.getDataSourceSupplierForRequest()中的get方法中获取到的数据源,所以已经将服务端的supplier的生成和客户端的supplier的使用已经结合了起来.
但是还是没看到请求是怎么发送出去的,然后请求回来的数据是如何更新UI的.
在前面我们已经分析了请求回来的数据是如何更新的,这个是通过给DataSource订阅观察者,然后去更新UI数据,这个具体的细节,我们之后再分析,这个已经超出了我们要分析的如何发送请求的范围

在AbstractDraweeControllerBuider的buildController方法&＃xff1a;

/** Builds a regular controller. */protected AbstractDraweeController buildController() {AbstractDraweeController controller &＃61; obtainController();controller.setRetainImageOnFailure(getRetainImageOnFailure());controller.setContentDescription(getContentDescription());controller.setControllerViewportVisibilityListener(getControllerViewportVisibilityListener());maybeBuildAndSetRetryManager(controller);maybeAttachListeners(controller);return controller;}


obtainController的具体实现在PipelineDraweeControllerBuilder中&＃xff1a;

protected PipelineDraweeController obtainController() {DraweeController oldController &＃61; getOldController();PipelineDraweeController controller;if (oldController instanceof PipelineDraweeController) {controller &＃61; (PipelineDraweeController) oldController;controller.initialize(obtainDataSourceSupplier(),generateUniqueControllerId(),getCacheKey(),getCallerContext());} else {controller &＃61; mPipelineDraweeControllerFactory.newController(obtainDataSourceSupplier(),generateUniqueControllerId(),getCacheKey(),getCallerContext());}return controller;}

其中调用到了AbstractDraweeControllerBuilder的obtainDataSourceSupplier方法&＃xff1a;

/** Gets the top-level data source supplier to be used by a controller. */protected Supplier> obtainDataSourceSupplier() {if (mDataSourceSupplier !&＃61; null) {return mDataSourceSupplier;}Supplier> supplier &＃61; null;// final image supplier;if (mImageRequest !&＃61; null) {supplier &＃61; getDataSourceSupplierForRequest(mImageRequest);} else if (mMultiImageRequests !&＃61; null) {supplier &＃61; getFirstAvailableDataSourceSupplier(mMultiImageRequests, mTryCacheOnlyFirst);}// increasing-quality supplier; highest-quality supplier goes firstif (supplier !&＃61; null && mLowResImageRequest !&＃61; null) {List>> suppliers &＃61; new ArrayList<>(2);suppliers.add(supplier);suppliers.add(getDataSourceSupplierForRequest(mLowResImageRequest));supplier &＃61; IncreasingQualityDataSourceSupplier.create(suppliers);}// no image requests; use null data source supplierif (supplier &＃61;&＃61; null) {supplier &＃61; DataSources.getFailedDataSourceSupplier(NO_REQUEST_EXCEPTION);}return supplier;}

具体这个请求是怎么发送出去的呢?之前有讲到在DraweeController的onAttach中获取了dataSource并且订阅了观察者,用于处理datasource返回的结果.来看一下实现中这个dataSource到底是怎么获取的。

我们再看一下getDataSourceSupplierForRequest方法&＃xff1a;

/** Creates a data source supplier for the given image request. */protected Supplier> getDataSourceSupplierForRequest(final REQUEST imageRequest,final CacheLevel cacheLevel) {final Object callerContext &＃61; getCallerContext();return new Supplier>() {&＃64;Overridepublic DataSource get() {return getDataSourceForRequest(imageRequest, callerContext, cacheLevel);}&＃64;Overridepublic String toString() {return Objects.toStringHelper(this).add("request", imageRequest.toString()).toString();}};}

这里调用到了getDataSourceForRequest方法&＃xff0c;该方法在PipelineDraweeControllerBuilder中

&＃64;Overrideprotected DataSource> getDataSourceForRequest(ImageRequest imageRequest,Object callerContext,CacheLevel cacheLevel) {return mImagePipeline.fetchDecodedImage(imageRequest,callerContext,convertCacheLevelToRequestLevel(cacheLevel));}

这里主要是通过调用了mImagePipeline.fetchDecodedImage方法&＃xff0c;可见是通过Pipeline来获取的这个datasource.其内部是发起了一个submitFetchRequest返回一个DataSource.Pipeline也是Fresco一个重要的组成部分。下面会单独说&＃xff1a;

6.Pipeline模块

简单来说pipeline就是实现了三级缓存,解码,变形等等,完成了提供可呈现图片的所有工作.Facebook官方中已经说明,ImagePipeline负责完成加载图像,并且将结果反馈(以回调或者说观察者的方式)出来.

这里先解释几个概念

Producer: 为了实现业务的隔离而设计的接口; 将pipeline中需要进行的每一项任务作为一个producer,通常将前一个producer作为参数传递个下一个producer,从而实现面向接口的业务模块剪的隔离.
Consumer: producer生产的结果会最终传递到consumer中,再通过实现了DataSource接口的适配器通知外部

public DataSource> fetchDecodedImage(ImageRequest imageRequest,Object callerContext,ImageRequest.RequestLevel lowestPermittedRequestLevelOnSubmit) {try {Producer> producerSequence &＃61;mProducerSequenceFactory.getDecodedImageProducerSequence(imageRequest);return submitFetchRequest(producerSequence,imageRequest,lowestPermittedRequestLevelOnSubmit,callerContext);} catch (Exception exception) {return DataSources.immediateFailedDataSource(exception);}}

我们去具体的看mProducerSequenceFactory中的代码&＃xff1a;

/*** 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.* &＃64;param imageRequest the request to submit* &＃64;return a DataSource representing the pending decoded image(s)*/
public DataSource> fetchDecodedImage(ImageRequest imageRequest,Object callerContext) {try {//有两块,下面的代码会分别进行分析//1.首先获取 producerSequence: 解码图片的请求队列Producer> producerSequence &＃61;mProducerSequenceFactory.getDecodedImageProducerSequence(imageRequest); //2.return submitFetchRequest(producerSequence,imageRequest,ImageRequest.RequestLevel.FULL_FETCH,callerContext);} catch (Exception exception) {return DataSources.immediateFailedDataSource(exception);}
}//******************************
//1.第一步 getDecodedImageProducerSequence
//******************************
//返回一个用于请求 解码图片的队列,可见这个队列是和imageRequest相关的,源码中,imageRequest是一个不可改变的JavaBean,其中包含了所有Pipeline请求图片所需要的所有信息.
public Producer> getDecodedImageProducerSequence(ImageRequest imageRequest) {//继续看getBasicDecodedImageSequence的实现Producer> pipelineSequence &＃61;getBasicDecodedImageSequence(imageRequest);if (imageRequest.getPostprocessor() !&＃61; null) {return getPostprocessorSequence(pipelineSequence);} else {return pipelineSequence;}
}//从这里就可以清楚地看到,针对不同的uri类型生成了不同的FetchSequence也就是Producer
private Producer> getBasicDecodedImageSequence(ImageRequest imageRequest) {Preconditions.checkNotNull(imageRequest);Uri uri &＃61; imageRequest.getSourceUri();//判空Preconditions.checkNotNull(uri, "Uri is null.");//是否是网络请求if (UriUtil.isNetworkUri(uri)) {return getNetworkFetchSequence();} else if (UriUtil.isLocalFileUri(uri)) {//是否是本地videoif (MediaUtils.isVideo(MediaUtils.extractMime(uri.getPath()))) {return getLocalVideoFileFetchSequence();} else {//本地图片return getLocalImageFileFetchSequence();}} else if (UriUtil.isLocalContentUri(uri)) {return getLocalContentUriFetchSequence();} else if (UriUtil.isLocalAssetUri(uri)) {return getLocalAssetFetchSequence();} else if (UriUtil.isLocalResourceUri(uri)) {return getLocalResourceFetchSequence();} else if (UriUtil.isDataUri(uri)) {return getDataFetchSequence();} else {//throw 异常...}
}


至此我们终于看到具体的网络请求和三级缓存的位置了。

六.阅读体会&优缺点&改进意见

采用了大量的设计模式
采用了生产者消费者模式
面向接口编程的思想非常浓
MVC设计思想

在该代码阅读过程中&＃xff0c;我参考了很多资料&＃xff0c;最终才对这个流程稍微的有点认识&＃xff0c;这个开源项目代码完整度非常高&＃xff0c;必须站在一个比较高的设计思考上去思考才能理解得更好一些&＃xff0c;我的目的也仅仅是对主流程实现的理解&＃xff0c;其他很多的细节点并未去阅读&＃xff0c;有赖于未来需要时再去进一步的学习。

参考资料

Fresco简介
http://blog.v5.cn/2015/10/30/fresco%E7%AE%80%E4%BB%8B/

Fresco用法总结基础篇
http://www.lai18.com/content/9608860.html

Fresco之强大之余的痛楚
http://www.jianshu.com/p/5364957dcf49

Fresco源码解析 - Hierarchy / View / Controller
http://blog.csdn.net/feelang/article/details/45126421

Fresco 源码分析(一) DraweeView-DraweeHierarchy-DraweeController(MVC) DraweeView的分析
http://www.cnblogs.com/pandapan/p/4634563.html

Fresco的一点研究
http://frankls.cn/2015/12/02/study-of-fresco