Add ability to register additional attribute providers.

Summary:
We have some "internal-only" stuff we want to put in the attribute of a layout node.
This allows us to do so a way that keeps it out of open-source and avoid the clunky subclassing.

Reviewed By: adamjernst

Differential Revision: D24162047

fbshipit-source-id: 518ab9fbf0b4274076f5b0ae1b249e197563fa7f

iOS/Plugins/FlipperKitLayoutPlugin/FlipperKitLayoutComponentKitSupport/SKComponentLayoutDescriptor.h

@@ -11,10 +11,20 @@
 
 @class SKComponentLayoutWrapper;
 
+typedef SKNamed* (*SKAttributeGenerator)(SKComponentLayoutWrapper* node);
+
 @interface SKComponentLayoutDescriptor
     : SKNodeDescriptor<SKComponentLayoutWrapper*>
 
 + (void)registerSubDescriptor:(SKSubDescriptor)descriptor
                       forName:(NSString*)name;
 
+/**
+ Allows you to 'plug-in' additional logic to update the attribute
+ string displayed for a node.
+
+ You can return a `nil` object from this, it will be gracefully ignored.
+ */
++ (void)registerAttributeGenerator:(SKAttributeGenerator)generator;
+
 @end

iOS/Plugins/FlipperKitLayoutPlugin/FlipperKitLayoutComponentKitSupport/SKComponentLayoutDescriptor.mm

@@ -41,6 +41,18 @@ static std::vector<std::pair<NSString*, SKSubDescriptor>>& subDescriptors() {
   return d;
 }
 
+static std::vector<SKAttributeGenerator>& attributeGenerators() {
+  // Avoid a global constructor; we want to lazily initialize this when needed.
+  static std::vector<SKAttributeGenerator> d;
+  return d;
+}
+
++ (void)registerAttributeGenerator:(SKAttributeGenerator)generator {
+  if (generator) {
+    attributeGenerators().push_back(generator);
+  }
+}
+
 + (void)registerSubDescriptor:(SKSubDescriptor)descriptor
                       forName:(NSString*)name {
   if (name && descriptor) {
@@ -170,6 +182,17 @@ static std::vector<std::pair<NSString*, SKSubDescriptor>>& subDescriptors() {
                     newWithName:@"responder"
                       withValue:SKObject(NSStringFromClass(
                                     [node.component.nextResponder class]))]];
+  for (SKAttributeGenerator generator : attributeGenerators()) {
+    if (!generator) {
+      // technically, this could be nullptr, so lets be careful.
+      continue;
+    }
+
+    SKNamed* const attribute = generator(node);
+    if (attribute) {
+      [attributes addObject:attribute];
+    }
+  }
   return attributes;
 }