[React Native] Update code comments about layout-only view

Libraries/Components/View/View.js

@@ -44,6 +44,12 @@ var AccessibilityTraits = [
   'pageTurn',
 ];
 
+// <<<<< WARNING >>>>>
+// If adding any properties to View that could change the way layout-only status
+// works on iOS, make sure to update ReactNativeViewAttributes.js and
+// RCTShadowView.m (in the -[RCTShadowView isLayoutOnly] method).
+// <<<<< WARNING >>>>>
+
 /**
  * The most fundamental component for building UI, `View` is a
  * container that supports layout with flexbox, style, some touch handling, and

Libraries/ReactNative/ReactNativeViewAttributes.js

@@ -26,7 +26,11 @@ ReactNativeViewAttributes.UIView = {
   onMagicTap: true,
   collapsible: true,
 
-  // If any below are set, view should not be collapsible!
+  // If editing layout-only view attributes, make sure
+  // -[RCTShadowView isLayoutOnly] in RCTShadowView.m
+  // is up-to-date! If any property below is set, the
+  // view should not be collapsible, but this is done
+  // on the native side.
   onMoveShouldSetResponder: true,
   onResponderGrant: true,
   onResponderMove: true,

React/Views/RCTShadowView.m

@@ -439,12 +439,15 @@ static void RCTProcessMetaProps(const float metaProps[META_PROP_COUNT], float st
       @"flex",
     };
     layoutKeys = [NSSet setWithObjects:layoutKeyStrings count:sizeof(layoutKeyStrings)/sizeof(*layoutKeyStrings)];
+    // layoutKeys are the only keys whose presence does not reject layout-only status.
 
     static NSString *const specialKeyStrings[] = {
       @"accessible",
       @"collapsible",
     };
     specialKeys = [NSSet setWithObjects:specialKeyStrings count:sizeof(specialKeyStrings)/sizeof(*specialKeyStrings)];
+    // specialKeys are keys whose presence does not indicate whether layout-only or not
+    // their values must be tested below
   }
 
   NSNumber *collapsible = self.allProps[@"collapsible"];