在Fresco源码解析 – 初始化过程分析章节中，我们分析了Fresco的初始化过程，两个initialize方法中都用到了 ImagePipelineFactory类。
ImagePipelineFactory.initialize(context);会创建一个所有参数都使用默认值的ImagePipelineConfig来初始化ImagePipeline。
ImagePipelineFactory.initialize(imagePipelineConfig)会首先用 imagePipelineConfig创建一个ImagePipelineFactory的实例 – sInstance。
sInstance = new ImagePipelineFactory(imagePipelineConfig);
然后，初始化Drawee时，在PipelineDraweeControllerBuilderSupplier的构造方法中通过 ImagePipelineFactory.getInstance()获取这个实例。
Fresco.java
(Context context) { sDraweeControllerBuilderSupplier = new PipelineDraweeControllerBuilderSupplier(context); SimpleDraweeView.initialize(sDraweeControllerBuilderSupplier);}
PipelineDraweeControllerBuilderSupplier.java
public PipelineDraweeControllerBuilderSupplier(Context context) { this(context, ImagePipelineFactory.getInstance());}public PipelineDraweeControllerBuilderSupplier(Context context,ImagePipelineFactory imagePipelineFactory) { this(context, imagePipelineFactory, null);}
PipelineDraweeControllerBuilderSupplier还有一个构造方法，就是 this(context, imagePipelineFactory, null)调用的构造方法。
public PipelineDraweeControllerBuilderSupplier(Context context,ImagePipelineFactory imagePipelineFactory,Set<ControllerListener> boundControllerListeners) { mContext = context; mImagePipeline = imagePipelineFactory.getImagePipeline(); mPipelineDraweeControllerFactory = new PipelineDraweeControllerFactory(context.getResources(),DeferredReleaser.getInstance(),imagePipelineFactory.getAnimatedDrawableFactory(),UiThreadImmediateExecutorService.getInstance()); mBoundControllerListeners = boundControllerListeners;}
其中，mImagePipeline = imagePipelineFactory.getImagePipeline()用于获取ImagePipeline的实例。
ImagePipelineFactory.java
public ImagePipeline getImagePipeline() { if (mImagePipeline == null) {mImagePipeline =new ImagePipeline(getProducerSequenceFactory(),mConfig.getRequestListeners(),mConfig.getIsPrefetchEnabledSupplier(),getBitmapMemoryCache(),getEncodedMemoryCache(),mConfig.getCacheKeyFactory()); } return mImagePipeline;}
可以看出mImagePipeline是一个单例，，构造ImagePipeline时用到的mConfig就是本片最开始讲到的 ImagePipelineConfig imagePipelineConfig。
经过这个过程，一个ImagePipeline就被创建好了，下面我们具体解析一下ImagePipeline的每个参数。
因为ImagePipelineFactory用ImagePipelineConfig来创建一个ImagePipeline，我们首先分析一下ImagePipelineConfig的源码。
{ private final Supplier<MemoryCacheParams> mBitmapMemoryCacheParamsSupplier; private final CacheKeyFactory mCacheKeyFactory; private final Context mContext; private final Supplier<MemoryCacheParams> mEncodedMemoryCacheParamsSupplier; private final ExecutorSupplier mExecutorSupplier; private final ImageCacheStatsTracker mImageCacheStatsTracker; private final AnimatedDrawableUtil mAnimatedDrawableUtil; private final AnimatedImageFactory mAnimatedImageFactory; private final ImageDecoder mImageDecoder; private final Supplier<Boolean> mIsPrefetchEnabledSupplier; private final DiskCacheConfig mMainDiskCacheConfig; private final MemoryTrimmableRegistry mMemoryTrimmableRegistry; private final NetworkFetcher mNetworkFetcher; private final PoolFactory mPoolFactory; private final ProgressiveJpegConfig mProgressiveJpegConfig; private final Set<RequestListener> mRequestListeners; mResizeAndRotateEnabledForNetwork; private final DiskCacheConfig mSmallImageDiskCacheConfig; private final PlatformBitmapFactory mPlatformBitmapFactory; // other methods}
上图可以看出，获取图像的第一站是Memeory Cache，然后是Disk Cache，最后是Network，而Memory和Disk都是缓存在本地的数据，MemoryCacheParams就用于表示它们的缓存策略。
MemoryCacheParams.java
/** * Pass arguments to control the cache’s behavior in the constructor. * * @param maxCacheSize The maximum size of the cache, in bytes. * @param maxCacheEntries The maximum number of items that can live in the cache. * @param maxEvictionQueueSize The eviction queue is an area of memory that stores items ready *for eviction but have not yet been deleted. This is the maximum *size of that queue in bytes. * @param maxEvictionQueueEntries The maximum number of entries in the eviction queue. * @param maxCacheEntrySize The maximum size of a single cache entry. */ public MemoryCacheParams(int maxCacheSize,int maxCacheEntries,int maxEvictionQueueSize,int maxEvictionQueueEntries,int maxCacheEntrySize) {this.maxCacheSize = maxCacheSize;this.maxCacheEntries = maxCacheEntries;this.maxEvictionQueueSize = maxEvictionQueueSize;this.maxEvictionQueueEntries = maxEvictionQueueEntries;this.maxCacheEntrySize = maxCacheEntrySize; }
关于每个参数的作用，注释已经写得很清楚，不再赘述。
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