Have ASNetworkImageNode report whether images were cached or not (#639)

* Have ASNetworkImageNode report whether images were cached or not.

* Update changelog

* Add fileURL case

Source/ASNetworkImageNode.h

@@ -130,6 +130,21 @@ NS_ASSUME_NONNULL_BEGIN
 
 
 #pragma mark -
+
+typedef NS_ENUM(NSInteger, ASNetworkImageSource) {
+  ASNetworkImageSourceUnspecified = 0,
+  ASNetworkImageSourceSynchronousCache,
+  ASNetworkImageSourceAsynchronousCache,
+  ASNetworkImageSourceFileURL,
+  ASNetworkImageSourceDownload,
+};
+
+/// A struct that carries details about ASNetworkImageNode's image loads.
+typedef struct {
+  /// The source from which the image was loaded.
+  ASNetworkImageSource imageSource;
+} ASNetworkImageNodeDidLoadInfo;
+
 /**
  * The methods declared by the ASNetworkImageNodeDelegate protocol allow the adopting delegate to respond to
  * notifications such as finished decoding and downloading an image.
@@ -137,6 +152,18 @@ NS_ASSUME_NONNULL_BEGIN
 @protocol ASNetworkImageNodeDelegate <NSObject>
 @optional
 
+/**
+ * Notification that the image node finished downloading an image, with additional info.
+ * If implemented, this method will be called instead of `imageNode:didLoadImage:`.
+ *
+ * @param imageNode The sender.
+ * @param image The newly-loaded image.
+ * @param info Misc information about the image load.
+ *
+ * @discussion Called on a background queue.
+ */
+- (void)imageNode:(ASNetworkImageNode *)imageNode didLoadImage:(UIImage *)image info:(ASNetworkImageNodeDidLoadInfo)info;
+
 /**
  * Notification that the image node finished downloading an image.
  *