Update the macOS-specify comment and explain the reason of the `imageViewLayer` method. Update macOS 10.14 behavior.

SDWebImage/SDAnimatedImageView.m

@@ -61,18 +61,12 @@ static NSUInteger SDDeviceFreeMemory() {
 #else
 @property (nonatomic, strong) CADisplayLink *displayLink;
 #endif
-// Layer-backed NSImageView use a subview of `NSImageViewContainerView` to do actual layer rendering. We use this layer instead of `self.layer` during animated image rendering.
-#if SD_MAC
-@property (nonatomic, strong, readonly) CALayer *imageViewLayer;
-#endif
 
 @end
 
 @implementation SDAnimatedImageView
 #if SD_UIKIT
 @dynamic animationRepeatCount;
-#else
-@dynamic imageViewLayer;
 #endif
 
 #pragma mark - Initializers
@@ -239,7 +233,7 @@ static NSUInteger SDDeviceFreeMemory() {
         
         [self.layer setNeedsDisplay];
 #if SD_MAC
-        [self.layer displayIfNeeded]; // macOS's imageViewLayer is not equal to self.layer. But `[super setImage:]` will impliedly mark it needsDisplay. We call `[self.layer displayIfNeeded]` to immediately refresh the imageViewLayer to avoid flashing
+        [self.layer displayIfNeeded]; // macOS's imageViewLayer may not equal to self.layer. But `[super setImage:]` will impliedly mark it needsDisplay. We call `[self.layer displayIfNeeded]` to immediately refresh the imageViewLayer to avoid flashing
 #endif
     }
 }
@@ -312,13 +306,6 @@ static NSUInteger SDDeviceFreeMemory() {
 }
 #endif
 
-#if SD_MAC
-- (CALayer *)imageViewLayer {
-    NSView *imageView = objc_getAssociatedObject(self, NSSelectorFromString(@"_imageView"));
-    return imageView.layer;
-}
-#endif
-
 #pragma mark - Life Cycle
 
 - (void)dealloc
@@ -681,6 +668,18 @@ static NSUInteger SDDeviceFreeMemory() {
 }
 
 #if SD_MAC
+// Layer-backed NSImageView optionally optimize to use a subview to do actual layer rendering.
+// When the optimization is turned on, it calls `updateLayer` instead of `displayLayer:` to update subview's layer.
+// When the optimization it turned off, this return nil and calls `displayLayer:` directly.
+- (CALayer *)imageViewLayer {
+    NSView *imageView = imageView = objc_getAssociatedObject(self, NSSelectorFromString(@"_imageView"));
+    if (!imageView) {
+        // macOS 10.14
+        imageView = objc_getAssociatedObject(self, NSSelectorFromString(@"_imageSubview"));
+    }
+    return imageView.layer;
+}
+
 - (void)updateLayer
 {
     if (_currentFrame) {