validateProps

.gitmodules

@@ -3,4 +3,4 @@
 	url = https://github.com/LPGhatguy/lemur.git
 [submodule "modules/testez"]
 	path = modules/testez
-	url = https://github.com/Roblox/testez.git
+	url = https://github.com/Roblox/testez.git

docs/api-reference.md

@@ -304,4 +304,26 @@ end
 ```
 
 !!! note
-	`getDerivedStateFromProps` is a *static* lifecycle method. It does not have access to `self`, and must be a pure function.
+	`getDerivedStateFromProps` is a *static* lifecycle method. It does not have access to `self`, and must be a pure function.
+
+### validateProps
+```
+static validateProps(props) -> success, reason
+```
+
+Performs property validation. You can use this for type-checking properties; there is a [PropTypes](https://github.com/AmaranthineCodices/rbx-prop-types) library to assist in this.
+
+This function will only be called if the `propertyValidation` configuration option is set to `true`. If this function returns `false`, the error message it returns will be thrown in the output, along with a stack trace pointing to the current element.
+
+```lua
+function MyComponent.validateProps(props)
+	if props.requiredProperty == nil then
+		return false, "requiredProperty is required"
+	end
+
+	return false
+end
+```
+
+!!! note
+	`validateProps`, like `getDerivedStateFromProps`, is a *static* lifecycle method. It does not have access to `self`, and must be a pure function.

lib/Component.lua

@@ -77,6 +77,7 @@ function Component:extend(name)
 	end
 
 	class.__index = class
+	class._extendTraceback = debug.traceback()
 
 	setmetatable(class, {
 		__tostring = function(self)
@@ -309,6 +310,10 @@ function Component:_forceUpdate(newProps, newState)
 		end
 	end
 
+	if newProps then
+		self:_validateProps(newProps)
+	end
+
 	if self.willUpdate then
 		self._setStateBlockedReason = "willUpdate"
 		self:willUpdate(newProps or self.props, newState or self.state)
@@ -374,6 +379,8 @@ function Component:_mount(handle)
 
 	self._setStateBlockedReason = "render"
 
+	self:_validateProps(self.props)
+
 	local virtualTree
 	if GlobalConfig.getValue("componentInstrumentation") then
 		local startTime = tick()
@@ -424,4 +431,33 @@ function Component:_unmount()
 	self._handle = nil
 end
 
+--[[
+	Performs property validation, if it is enabled.
+]]
+function Component:_validateProps(props)
+	if not GlobalConfig.getValue("propertyValidation") then return end
+
+	local validator = self.validateProps
+
+	if validator == nil then return end
+	if typeof(validator) ~= "function" then
+		-- Hide as much as possible about error location, since this message
+		-- occurs from several possible call sites with different stack traces.
+		-- luacheck: ignore 6
+		error(("The value of validateProps must be a function (is a %q). Check the definition of the component extended at:\n%s"):format(
+			typeof(validator),
+			self._extendTraceback),
+		0)
+	end
+
+	local success, failureReason = validator(props)
+
+	if not success then
+		failureReason = failureReason or "<No failure reason was given by the validation function>"
+		error(("Property validation failed:\n%s\n%s"):format(
+			failureReason,
+			self:getElementTraceback() or Core._defaultSource), 0)
+	end
+end
+
 return Component

lib/Component.spec.lua

@@ -2,6 +2,7 @@ return function()
 	local Core = require(script.Parent.Core)
 	local Reconciler = require(script.Parent.Reconciler)
 	local Component = require(script.Parent.Component)
+	local GlobalConfig = require(script.Parent.GlobalConfig)
 
 	it("should be extendable", function()
 		local MyComponent = Component:extend("The Senate")
@@ -292,6 +293,106 @@ return function()
 		Reconciler.unmount(handle)
 	end)
 
+	describe("validateProps", function()
+		it("should be called if propertyValidation is set", function()
+			GlobalConfig.set({
+				propertyValidation = true,
+			})
+
+			local TestComponent = Component:extend("TestComponent")
+			local callCount = 0
+
+			TestComponent.validateProps = function(props)
+				callCount = callCount + 1
+				return true
+			end
+
+			function TestComponent:render()
+				return nil
+			end
+
+			local handle = Reconciler.mount(Core.createElement(TestComponent))
+			expect(callCount).to.equal(1)
+
+			handle = Reconciler.reconcile(handle, Core.createElement(TestComponent, {
+				foo = "bar",
+			}))
+			expect(callCount).to.equal(2)
+
+			Reconciler.unmount(handle)
+			expect(callCount).to.equal(2)
+
+			GlobalConfig.reset()
+		end)
+
+		it("should throw if the function returns false", function()
+			GlobalConfig.set({
+				propertyValidation = true,
+			})
+
+			local TestComponent = Component:extend("TestComponent")
+
+			TestComponent.validateProps = function(props)
+				return false
+			end
+
+			function TestComponent:render()
+				return nil
+			end
+
+			expect(function()
+				Reconciler.mount(Core.createElement(TestComponent))
+			end).to.throw()
+
+			GlobalConfig.reset()
+		end)
+
+		it("should throw if validateProps is not a function", function()
+			GlobalConfig.set({
+				propertyValidation = true,
+			})
+
+			local TestComponent = Component:extend("TestComponent")
+
+			TestComponent.validateProps = false
+
+			function TestComponent:render()
+				return nil
+			end
+
+			expect(function()
+				Reconciler.mount(Core.createElement(TestComponent))
+			end).to.throw()
+
+			GlobalConfig.reset()
+		end)
+
+		it("should not be run if typeChecking is false", function()
+			local TestComponent = Component:extend("TestComponent")
+			local callCount = 0
+
+			TestComponent.validateProps = function(props)
+				callCount = callCount + 1
+				return true
+			end
+
+			function TestComponent:render()
+				return nil
+			end
+
+			local handle = Reconciler.mount(Core.createElement(TestComponent))
+			expect(callCount).to.equal(0)
+
+			handle = Reconciler.reconcile(handle, Core.createElement(TestComponent, {
+				foo = "bar",
+			}))
+			expect(callCount).to.equal(0)
+
+			Reconciler.unmount(handle)
+			expect(callCount).to.equal(0)
+		end)
+	end)
+
 	describe("setState", function()
 		it("should throw when called in init", function()
 			local InitComponent = Component:extend("InitComponent")

lib/Config.lua

@@ -18,6 +18,8 @@ local defaultConfig = {
 	["elementTracing"] = false,
 	-- Enables instrumentation of shouldUpdate and render methods for Roact components
 	["componentInstrumentation"] = false,
+	-- Enables type checking with the validateProps property on stateful components.
+	["propertyValidation"] = false,
 }
 
 -- Build a list of valid configuration values up for debug messages.

lib/Core.lua

@@ -25,6 +25,9 @@ Core.None = Symbol.named("None")
 -- Marker used to specify that the table it is present within is a component.
 Core.Element = Symbol.named("Element")
 
+-- The default "stack traceback" if element tracing is not enabled.
+Core._defaultSource = "\n\t<Use Roact.setGlobalConfig with the 'elementTracing' key to enable detailed tracebacks>\n"
+
 --[[
 	Utility to retrieve one child out the children passed to a component.
 

lib/Reconciler.lua

@@ -34,8 +34,6 @@ local Symbol = require(script.Parent.Symbol)
 
 local isInstanceHandle = Symbol.named("isInstanceHandle")
 
-local DEFAULT_SOURCE = "\n\t<Use Roact.setGlobalConfig with the 'elementTracing' key to enable detailed tracebacks>\n"
-
 local function isPortal(element)
 	if type(element) ~= "table" then
 		return false
@@ -485,7 +483,7 @@ function Reconciler._setRbxProp(rbx, key, value, element)
 		local success, err = pcall(set, rbx, key, value)
 
 		if not success then
-			local source = element.source or DEFAULT_SOURCE
+			local source = element.source or Core._defaultSource
 
 			local message = ("Failed to set property %s on primitive instance of class %s\n%s\n%s"):format(
 				key,
@@ -504,7 +502,7 @@ function Reconciler._setRbxProp(rbx, key, value, element)
 		elseif key.type == Change then
 			Reconciler._singleEventManager:connectProperty(rbx, key.name, value)
 		else
-			local source = element.source or DEFAULT_SOURCE
+			local source = element.source or Core._defaultSource
 
 			-- luacheck: ignore 6
 			local message = ("Failed to set special property on primitive instance of class %s\nInvalid special property type %q\n%s"):format(
@@ -519,7 +517,7 @@ function Reconciler._setRbxProp(rbx, key, value, element)
 		-- Userdata values are special markers, usually created by Symbol
 		-- They have no data attached other than being unique keys
 
-		local source = element.source or DEFAULT_SOURCE
+		local source = element.source or Core._defaultSource
 
 		local message = ("Properties with a key type of %q are not supported\n%s"):format(
 			type(key),