Add react_recursiveDescription to UIView and debugging to RCTScrollView

Summary:
@public
RCTScrollView sometimes asserts with another contentView set. While this doesn't crash, it'd be good to know what's causing the assert. This will add a recursive description of the contentView.

Changelog: [iOS] [Changed] RCTScrollView: added debugging to help fix assert

Reviewed By: PeteTheHeat

Differential Revision: D15049869

fbshipit-source-id: 5431de7764881922327c6c0a3bdd392668396b58

React/Views/ScrollView/RCTScrollView.m

@@ -476,7 +476,7 @@ static inline void RCTApplyTransformationAccordingLayoutDirection(UIView *view,
   } else
 #endif
   {
-    RCTAssert(_contentView == nil, @"RCTScrollView may only contain a single subview");
+    RCTAssert(_contentView == nil, @"RCTScrollView may only contain a single subview, the already set subview looks like: %@", [_contentView react_recursiveDescription]);
     _contentView = view;
     RCTApplyTransformationAccordingLayoutDirection(_contentView, self.reactLayoutDirection);
     [_scrollView addSubview:view];

React/Views/UIView+React.h

@@ -113,4 +113,10 @@
  */
 @property (nonatomic, readonly) UIView *reactAccessibilityElement;
 
+/**
+ * Used in debugging to get a description of the view hierarchy rooted at
+ * the current view.
+ */
+- (NSString *)react_recursiveDescription;
+
 @end

React/Views/UIView+React.m

@@ -297,4 +297,28 @@
   return self;
 }
 
+#pragma mark - Debug
+
+- (void)react_addRecursiveDescriptionToString:(NSMutableString *)string atLevel:(NSUInteger)level
+{
+  for (NSUInteger i = 0; i < level; i++) {
+    [string appendString:@"   | "];
+  }
+
+  [string appendString:self.description];
+  [string appendString:@"\n"];
+
+  for (UIView *subview in self.subviews) {
+    [subview react_addRecursiveDescriptionToString:string atLevel:level + 1];
+  }
+}
+
+- (NSString *)react_recursiveDescription
+{
+  NSMutableString *description = [NSMutableString string];
+  [self react_addRecursiveDescriptionToString:description atLevel:0];
+  return description;
+}
+
 @end
+