Introducing -[RCTShadowView canHaveSubviews]

Summary:
Override `canHaveSubviews` in RCTShadowView subclass to disallow any nested content.
For now, this prop will be checked only in DEV mode for performance reasons.

Reviewed By: javache

Differential Revision: D5189083

fbshipit-source-id: 87087dd806e1fd7320128dab969b13642174f81c

React/Views/RCTShadowView.h

@@ -217,9 +217,22 @@ typedef void (^RCTApplierBlock)(NSDictionary<NSNumber *, UIView *> *viewRegistry
              absolutePosition:(CGPoint)absolutePosition;
 
 /**
- * Return whether or not this node acts as a leaf node in the eyes of Yoga.
+ * Returns whether or not this view can have any subviews.
+ * Adding/inserting a child view to leaf view (`canHaveSubviews` equals `NO`)
+ * will throw an error.
+ * Return `NO` for components which must not have any descendants
+ * (like <Image>, for example.)
+ * Defaults to `YES`. Can be overridden in subclasses.
+ * Don't confuse this with `isYogaLeafNode`.
+ */
+- (BOOL)canHaveSubviews;
+
+/**
+ * Returns whether or not this node acts as a leaf node in the eyes of Yoga.
  * For example `RCTShadowText` has children which it does not want Yoga
  * to lay out so in the eyes of Yoga it is a leaf node.
+ * Defaults to `NO`. Can be overridden in subclasses.
+ * Don't confuse this with `canHaveSubviews`.
  */
 - (BOOL)isYogaLeafNode;
 

React/Views/RCTShadowView.m

@@ -365,6 +365,11 @@ - (void)dealloc
   YGNodeFree(_yogaNode);
 }
 
+- (BOOL)canHaveSubviews
+{
+  return NO;
+}
+
 - (BOOL)isYogaLeafNode
 {
   return NO;
@@ -403,6 +408,8 @@ - (void)setTextComputed
 
 - (void)insertReactSubview:(RCTShadowView *)subview atIndex:(NSInteger)atIndex
 {
+  RCTAssert(!self.canHaveSubviews, @"Attempt to insert subview inside leaf view.");
+
   [_reactSubviews insertObject:subview atIndex:atIndex];
   if (![self isYogaLeafNode]) {
     YGNodeInsertChild(_yogaNode, subview.yogaNode, (uint32_t)atIndex);