add gs

RCCService2018/content/LuaPackages/RoactImpl/getDefaultPropertyValue.lua

@@ -0,0 +1,54 @@
+--[[
+	Attempts to get the default value of a given property on a Roblox instance.
+
+	This is used by the reconciler in cases where a prop was previously set on a
+	primitive component, but is no longer present in a component's new props.
+
+	Eventually, Roblox might provide a nicer API to query the default property
+	of an object without constructing an instance of it.
+]]
+
+local Symbol = require(script.Parent.Symbol)
+
+local Nil = Symbol.named("Nil")
+local _cachedPropertyValues = {}
+
+local function getDefaultPropertyValue(className, propertyName)
+	local classCache = _cachedPropertyValues[className]
+
+	if classCache then
+		local propValue = classCache[propertyName]
+
+		-- We have to use a marker here, because Lua doesn't distinguish
+		-- between 'nil' and 'not in a table'
+		if propValue == Nil then
+			return true, nil
+		end
+
+		if propValue ~= nil then
+			return true, propValue
+		end
+	else
+		classCache = {}
+		_cachedPropertyValues[className] = classCache
+	end
+
+	local created = Instance.new(className)
+	local ok, defaultValue = pcall(function()
+		return created[propertyName]
+	end)
+
+	created:Destroy()
+
+	if ok then
+		if defaultValue == nil then
+			classCache[propertyName] = Nil
+		else
+			classCache[propertyName] = defaultValue
+		end
+	end
+
+	return ok, defaultValue
+end
+
+return getDefaultPropertyValue