Fresco源码解析 - 创建一个ImagePipeline（一） - android

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

private static void initializeDrawee(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 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。

public class ImagePipelineConfig {  private final Supplier mBitmapMemoryCacheParamsSupplier;  private final CacheKeyFactory mCacheKeyFactory;  private final Context mContext;  private final Supplier mEncodedMemoryCacheParamsSupplier;  private final ExecutorSupplier mExecutorSupplier;  private final ImageCacheStatsTracker mImageCacheStatsTracker;  private final AnimatedDrawableUtil mAnimatedDrawableUtil;  private final AnimatedImageFactory mAnimatedImageFactory;  private final ImageDecoder mImageDecoder;  private final Supplier mIsPrefetchEnabledSupplier;  private final DiskCacheConfig mMainDiskCacheConfig;  private final MemoryTrimmableRegistry mMemoryTrimmableRegistry;  private final NetworkFetcher mNetworkFetcher;  private final PoolFactory mPoolFactory;  private final ProgressiveJpegConfig mProgressiveJpegConfig;  private final Set mRequestListeners;  private final boolean 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。

CacheKeyFactory会为ImageRequest创建一个索引 - CacheKey。

/** * Factory methods for creating cache keys for the pipeline. */public interface CacheKeyFactory {   /**   * @return {@link CacheKey} for doing bitmap cache lookups in the pipeline.   */  public CacheKey getBitmapCacheKey(ImageRequest request);   /**   * @return {@link CacheKey} for doing encoded image lookups in the pipeline.   */  public CacheKey getEncodedCacheKey(ImageRequest request);   /**   * @return a {@link String} that unambiguously indicates the source of the image.   */  public Uri getCacheKeySourceUri(Uri sourceUri);}

ExecutorSupplier会根据ImagePipeline的使用场景获取不同的Executor。

public interface ExecutorSupplier {   /** Executor used to do all disk reads, whether for disk cache or local files. */  Executor forLocalStorageRead();   /** Executor used to do all disk writes, whether for disk cache or local files. */  Executor forLocalStorageWrite();   /** Executor used for all decodes. */  Executor forDecode();   /** Executor used for all image transformations, such as transcoding, resizing, and rotating. */  Executor forTransform();   /** Executor used for background operations, such as postprocessing. */  Executor forBackground();}

ImageCacheStatsTracker 作为 Cache 埋点工具，可以统计Cache的各种操作数据。

public interface ImageCacheStatsTracker {   /** Called whenever decoded images are put into the bitmap cache. */  public void onBitmapCachePut();   /** Called on a bitmap cache hit. */  public void onBitmapCacheHit();   /** Called on a bitmap cache miss. */  public void onBitmapCacheMiss();   /** Called whenever encoded images are put into the encoded memory cache. */  public void onMemoryCachePut();   /** Called on an encoded memory cache hit. */  public void onMemoryCacheHit();   /** Called on an encoded memory cache hit. */  public void onMemoryCacheMiss();   /**   * Called on an staging area hit.   *   * The staging area stores encoded images. It gets the images before they are written   * to disk cache.   */  public void onStagingAreaHit();   /** Called on a staging area miss hit. */  public void onStagingAreaMiss();   /** Called on a disk cache hit. */  public void onDiskCacheHit();   /** Called on a disk cache miss. */  public void onDiskCacheMiss();   /** Called if an exception is thrown on a disk cache read. */  public void onDiskCacheGetFail();   /**   * Registers a bitmap cache with this tracker.   *   * 
Use this method if you need access to the cache itself to compile your stats.   */  public void registerBitmapMemoryCache(CountingMemoryCache<?, ?> bitmapMemoryCache);   /**   * Registers an encoded memory cache with this tracker.   *   * Use this method if you need access to the cache itself to compile your stats.   */  public void registerEncodedMemoryCache(CountingMemoryCache<?, ?> encodedMemoryCache);}

剩下的几个参数与 Drawable 关联比较大，我们下一篇再分析。

Fresco源码解析 - 创建一个ImagePipeline（一）

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