Show mounted ComponentKit views in Flipper

Summary:
Before this diff, Flipper showed *leaf* views created by ComponentKit, but not any intermediate views. Now we show both.

A new node type `SKComponentMountedView` is used for this purpose. Its descriptor `SKComponentMountedViewDescriptor` mostly delegates to its view's descriptor, but redirects back into ComponentKit for children.

Reviewed By: Andrey-Mishanin

Differential Revision: D21130997

fbshipit-source-id: b3c12ea7cc1200962b3ba7c269c48d68b1809948

iOS/Plugins/FlipperKitLayoutPlugin/FlipperKitLayoutComponentKitSupport/SKComponentLayoutWrapper.h

@@ -9,8 +9,35 @@
 
 #import <ComponentKit/CKComponentLayout.h>
 #import <ComponentKit/CKFlexboxComponent.h>
+#import <ComponentKit/CKOptional.h>
+#import <ComponentKit/CKVariant.h>
+
+#import <vector>
 
 @protocol CKInspectableView;
+@class SKComponentLayoutWrapper;
+@class SKComponentMountedView;
+
+// CK::Variant does not support Objective-C types unless they are boxed:
+struct SKLeafViewChild {
+  UIView* view;
+};
+struct SKMountedViewChild {
+  SKComponentMountedView* view;
+};
+
+/**
+ The children of a SKComponentLayoutWrapper may be:
+ - A single leaf view, which may have UIView children of its own.
+ - A single non-leaf view, if the component created a view; its children will be
+   the component's child components.
+ - An array of SKComponentLayoutWrappers, if the component did not create a
+   view.
+ */
+using SKComponentLayoutWrapperChildren = CK::Variant<
+    SKLeafViewChild,
+    SKMountedViewChild,
+    std::vector<SKComponentLayoutWrapper*>>;
 
 @interface SKComponentLayoutWrapper : NSObject
 
@@ -18,11 +45,11 @@
 @property(nonatomic, readonly) NSString* identifier;
 @property(nonatomic, readonly) CGSize size;
 @property(nonatomic, readonly) CGPoint position;
-@property(nonatomic, readonly) std::vector<SKComponentLayoutWrapper*> children;
+@property(nonatomic, readonly) SKComponentLayoutWrapperChildren children;
 @property(nonatomic, weak, readonly) id<CKInspectableView> rootNode;
-// Null for layouts which are not direct children of a CKFlexboxComponent
-@property(nonatomic, readonly) BOOL isFlexboxChild;
-@property(nonatomic, readonly) CKFlexboxComponentChild flexboxChild;
+/** CK::none for components that are not the child of a CKFlexboxComponent. */
+@property(nonatomic, readonly) CK::Optional<CKFlexboxComponentChild>
+    flexboxChild;
 
 + (instancetype)newFromRoot:(id<CKInspectableView>)root
                   parentKey:(NSString*)parentKey;