Fresco Image Pipeline初始化分析

1. Fresco 相关介绍：
   Image Pipeline 简单配置

2. Fresco 初始化过程：
   1.创建ImagePipelineConfig 对象,配置好初始化相关的参数
   2.调用ImagePipelineFactory 工厂方法后去ImagePipeling对象

/**
 * Factory class for the image pipeline.
 *
 * <p>This class constructs the pipeline and its dependencies from other libraries.
 *
 * <p>As the pipeline object can be quite expensive to create, it is strongly
 * recommended that applications create just one instance of this class
 * and of the pipeline.
 */
@NotThreadSafe
public class ImagePipelineFactory {

  private static ImagePipelineFactory sInstance = null;

  /** Gets the instance of {@link ImagePipelineFactory}. */
  public static ImagePipelineFactory getInstance() {
    return Preconditions.checkNotNull(sInstance, "ImagePipelineFactory was not initialized!");
  }

  /** Initializes {@link ImagePipelineFactory} with default config. */
  public static void initialize(Context context) {
    initialize(ImagePipelineConfig.newBuilder(context).build());
  }

  /** Initializes {@link ImagePipelineFactory} with the specified config. */
  public static void initialize(ImagePipelineConfig imagePipelineConfig) {
    sInstance = new ImagePipelineFactory(imagePipelineConfig);
  }

  /** Shuts {@link ImagePipelineFactory} down. */
  public static void shutDown() {
    if (sInstance != null) {
      sInstance.getBitmapMemoryCache().removeAll(AndroidPredicates.<CacheKey>True());
      sInstance.getEncodedMemoryCache().removeAll(AndroidPredicates.<CacheKey>True());
      sInstance = null;
    }
  }

  private final ImagePipelineConfig mConfig;



  //由于这个工厂方法是单例类所以这个构造方法最好应该是private 类型，否则不符合单例模式
  public ImagePipelineFactory(ImagePipelineConfig config) {
    mConfig = Preconditions.checkNotNull(config);
  }
}