Have image nodes draw into opaque contexts automatically if possible (#…

…1432)

* Have image nodes draw into opaque contexts if the image is opaque and it fills the context

* Call backingSize once

Source/ASImageNode.mm

@@ -473,9 +473,17 @@ + (UIImage *)createContentsForkey:(ASImageNodeContentsKey *)key drawParameters:(
     return nil;
   }
 
+  // If the image is opaque, and the draw rect contains the bounds rect, we can use an opaque context.
+  UIImage *image = key.image;
+  const CGRect imageDrawRect = key.imageDrawRect;
+  const CGRect contextBounds = { CGPointZero, key.backingSize };
+  const BOOL imageIsOpaque = ASImageAlphaInfoIsOpaque(CGImageGetAlphaInfo(image.CGImage));
+  const BOOL imageFillsContext = CGRectContainsRect(imageDrawRect, contextBounds);
+  const BOOL contextIsOpaque = (imageIsOpaque && imageFillsContext) || key.isOpaque;
+
   // Use contentsScale of 1.0 and do the contentsScale handling in boundsSizeInPixels so ASCroppedImageBackingSizeAndDrawRectInBounds
   // will do its rounding on pixel instead of point boundaries
-  ASGraphicsBeginImageContextWithOptions(key.backingSize, key.isOpaque, 1.0);
+  ASGraphicsBeginImageContextWithOptions(contextBounds.size, contextIsOpaque, 1.0);
 
   BOOL contextIsClean = YES;
 
@@ -503,13 +511,12 @@ + (UIImage *)createContentsForkey:(ASImageNodeContentsKey *)key drawParameters:(
   // upon removal of the object from the set when the operation completes.
   // Another option is to have ASDisplayNode+AsyncDisplay coordinate these cases, and share the decoded buffer.
   // Details tracked in https://github.com/facebook/AsyncDisplayKit/issues/1068
-
-  UIImage *image = key.image;
-  BOOL canUseCopy = (contextIsClean || ASImageAlphaInfoIsOpaque(CGImageGetAlphaInfo(image.CGImage)));
+
+  BOOL canUseCopy = (contextIsClean || imageIsOpaque);
   CGBlendMode blendMode = canUseCopy ? kCGBlendModeCopy : kCGBlendModeNormal;
 
   @synchronized(image) {
-    [image drawInRect:key.imageDrawRect blendMode:blendMode alpha:1];
+    [image drawInRect:imageDrawRect blendMode:blendMode alpha:1];
   }
 
   if (context && key.didDisplayNodeContentWithRenderingContext) {