[ASNetworkImageNode] Assert if both URL and image are set (#2782)

* [ASNetworkImageNode] Assert if both URL and image are set

Here's one other idea for addressing this problem. Essentially,
we assert if you've set both the URL and the image. I've played around
with Pinterest and there's only one case where we hit this (the transition).

So I've also added another method (which is a bummer, it's weird I know)
but there's one good reason to add this method, ephemeralImage, which is
the user doesn't have to manually clear it out like they would if they
used defaultImage to save memory.

Putting this up for discussion.

* Fix comment

* Oh yeah, @dynamic, thanks @adlai

* Remove ephemeralImage

Author: garrettmoon

AsyncDisplayKit/ASNetworkImageNode.h

@@ -51,17 +51,18 @@ NS_ASSUME_NONNULL_BEGIN
 /**
  * The image to display.
  *
- * @discussion Setting an image to the image property of an ASNetworkImageNode will cause it to act like a plain
- * ASImageNode if a URL is not set as well. As soon as the URL is set the ASNetworkImageNode will act like an 
- * ASNetworkImageNode and the image property will be managed internally. This means the image property will be cleared
- * out and replaced by the placeholder (<defaultImage>) image while loading and the final image after the new image 
- * data was downloaded and processed. If you want to use a placholder image, use the defaultImage property 
- * instead.
+ * @discussion By setting an image to the image property the ASNetworkImageNode will act like a plain ASImageNode.
+ * As soon as the URL is set the ASNetworkImageNode will act like an ASNetworkImageNode and the image property
+ * will be managed internally. This means the image property will be cleared out and replaced by the placeholder 
+ * (<defaultImage>) image while loading and the final image after the new image data was downloaded and processed.
+ * If you want to use a placholder image functionality use the defaultImage property instead.
  */
 @property (nullable, nonatomic, strong) UIImage *image;
 
 /**
- * A placeholder image to display while the URL is loading.
+ * A placeholder image to display while the URL is loading. This is slightly different than placeholderImage in the
+ * ASDisplayNode superclass as defaultImage will *not* be displayed synchronously. If you wish to have the image
+ * displayed synchronously, use @c placeholderImage.
  */
 @property (nullable, nonatomic, strong, readwrite) UIImage *defaultImage;
 

AsyncDisplayKit/ASNetworkImageNode.mm

@@ -114,7 +114,12 @@ - (void)setImage:(UIImage *)image
 {
   ASDN::MutexLocker l(__instanceLock__);
 
-  _imageWasSetExternally = (image != nil && _URL == nil);
+  _imageWasSetExternally = (image != nil);
+  if (_imageWasSetExternally) {
+    ASDisplayNodeAssertNil(_URL, @"Directly setting an image on an ASNetworkImageNode causes it to behave like an ASImageNode instead of an ASNetworkImageNode. If this is what you want, set the URL to nil first.");
+    [self _cancelDownloadAndClearImage];
+    _URL = nil;
+  }
 
   [self _setImage:image];
 }
@@ -134,6 +139,8 @@ - (void)setURL:(NSURL *)URL resetToDefault:(BOOL)reset
   {
     ASDN::MutexLocker l(__instanceLock__);
 
+    ASDisplayNodeAssert(_imageWasSetExternally == NO, @"Setting a URL to an ASNetworkImageNode after setting an image changes its behavior from an ASImageNode to an ASNetworkImageNode. If this is what you want, set the image to nil first.");
+    
     _imageWasSetExternally = NO;
 
     if (ASObjectIsEqual(URL, _URL)) {