Add getDerivedStateFromProps (#57)

lib/Component.lua

@@ -20,6 +20,24 @@ setState cannot be used currently, are you calling setState from any of:
 * the render function
 * the shouldUpdate function]]
 
+local function mergeState(currentState, partialState)
+	local newState = {}
+
+	for key, value in pairs(currentState) do
+		newState[key] = value
+	end
+
+	for key, value in pairs(partialState) do
+		if value == Core.None then
+			newState[key] = nil
+		else
+			newState[key] = value
+		end
+	end
+
+	return newState
+end
+
 --[[
 	Create a new Roact stateful component class.
 
@@ -76,6 +94,14 @@ function Component:extend(name)
 			self.state = {}
 		end
 
+		if class.getDerivedStateFromProps then
+			local partialState = class.getDerivedStateFromProps(props, self.state)
+
+			if partialState then
+				self.state = mergeState(self.state, partialState)
+			end
+		end
+
 		-- Now that state has definitely been set, we can now allow it to be changed.
 		self._canSetState = true
 
@@ -133,20 +159,7 @@ function Component:setState(partialState)
 		partialState = partialState(self.state, self.props)
 	end
 
-	local newState = {}
-
-	for key, value in pairs(self.state) do
-		newState[key] = value
-	end
-
-	for key, value in pairs(partialState) do
-		if value == Core.None then
-			newState[key] = nil
-		else
-			newState[key] = value
-		end
-	end
-
+	local newState = mergeState(self.state, partialState)
 	self:_update(self.props, newState)
 end
 
@@ -174,6 +187,22 @@ end
 function Component:_forceUpdate(newProps, newState)
 	self._canSetState = false
 
+	-- Compute new derived state.
+	-- Get the class - getDerivedStateFromProps is static.
+	local class = getmetatable(self)
+
+	-- Only update if newProps are given!
+	if newProps then
+		if class.getDerivedStateFromProps then
+			local derivedState = class.getDerivedStateFromProps(newProps, newState or self.state)
+
+			-- getDerivedStateFromProps can return nil if no changes are necessary.
+			if derivedState ~= nil then
+				newState = mergeState(newState or self.state, derivedState)
+			end
+		end
+	end
+
 	if self.willUpdate then
 		self:willUpdate(newProps or self.props, newState or self.state)
 	end

lib/Component.spec.lua

@@ -177,6 +177,45 @@ return function()
 		Reconciler.teardown(instance)
 	end)
 
+	it("should call getDerivedStateFromProps appropriately", function()
+		local TestComponent = Component:extend("TestComponent")
+		local getStateCallback
+
+		function TestComponent.getDerivedStateFromProps(newProps, oldState)
+			return {
+				visible = newProps.visible
+			}
+		end
+
+		function TestComponent:init(props)
+			self.state = {
+				visible = false
+			}
+
+			getStateCallback = function()
+				return self.state
+			end
+		end
+
+		function TestComponent:render() end
+
+		local handle = Reconciler.reify(Core.createElement(TestComponent, {
+			visible = true
+		}))
+
+		local state = getStateCallback()
+		expect(state.visible).to.equal(true)
+
+		handle = Reconciler.reconcile(handle, Core.createElement(TestComponent, {
+			visible = 123
+		}))
+
+		state = getStateCallback()
+		expect(state.visible).to.equal(123)
+
+		Reconciler.teardown(handle)
+	end)
+
 	describe("setState", function()
 		it("should throw when called in init", function()
 			local InitComponent = Component:extend("InitComponent")