Merge 9100db1 into 41af74d

src/Binding.lua

@@ -11,6 +11,19 @@ local function identity(value)
 	return value
 end
 
+--[[
+	Maps a table of bindings to their respective values. Used in Binding.join.
+]]
+local function mapBindingsToValues(bindings)
+	local values = {}
+
+	for key, binding in pairs(bindings) do
+		values[key] = binding:getValue()
+	end
+
+	return values
+end
+
 local Binding = {}
 
 --[[
@@ -146,4 +159,33 @@ function Binding.create(initialValue)
 	return binding, setter
 end
 
+--[[
+	Creates a new binding which updates when any of the upstream bindings
+	updates, which can be further mapped into any value. This function will
+	be exposed to users of Roact.
+]]
+function Binding.join(bindings)
+	local joinedBinding, setter = Binding.create(mapBindingsToValues(bindings))
+	local internalData = joinedBinding[InternalData]
+
+	local upstreamConnections = {}
+	internalData.upstreamConnections = upstreamConnections
+
+	internalData.upstreamDisconnect = function()
+		for _, disconnect in pairs(upstreamConnections) do
+			disconnect()
+		end
+	end
+
+	local function updateBinding()
+		setter(mapBindingsToValues(bindings))
+	end
+
+	for key, binding in pairs(bindings) do
+		upstreamConnections[key] = Binding.subscribe(binding, updateBinding)
+	end
+
+	return joinedBinding
+end
+
 return Binding

src/Binding.spec.lua

@@ -21,6 +21,47 @@ return function()
 		end)
 	end)
 
+	describe("Binding.join", function()
+		it("should properly output values", function()
+			local binding1 = Binding.create(1)
+			local binding2 = Binding.create(2)
+
+			local joinedBinding = Binding.join({
+				binding1,
+				binding2,
+			})
+
+			local bindingValue = joinedBinding:getValue()
+			expect(bindingValue).to.be.a("table")
+			expect(bindingValue[1]).to.equal(1)
+			expect(bindingValue[2]).to.equal(2)
+		end)
+
+		it("should update when any one of the subscribed bindings updates", function()
+			local binding1, update1 = Binding.create(1)
+			local binding2, update2 = Binding.create(2)
+
+			local joinedBinding = Binding.join({
+				binding1,
+				binding2,
+			})
+
+			local spy = createSpy()
+			Binding.subscribe(joinedBinding, spy.value)
+
+			expect(spy.callCount).to.equal(0)
+			update1(3)
+			expect(spy.callCount).to.equal(1)
+			update2(4)
+			expect(spy.callCount).to.equal(2)
+
+			local bindingValue = spy.values[1]
+			expect(bindingValue).to.be.a("table")
+			expect(bindingValue[1]).to.equal(3)
+			expect(bindingValue[2]).to.equal(4)
+		end)
+	end)
+
 	describe("Binding object", function()
 		it("should provide a getter and setter", function()
 			local binding, update = Binding.create(1)

src/init.lua

@@ -22,6 +22,7 @@ local Roact = strict {
 	Portal = require(script.Portal),
 	createRef = require(script.createRef),
 	createBinding = Binding.create,
+	joinBindings = Binding.join,
 
 	Change = require(script.PropMarkers.Change),
 	Children = require(script.PropMarkers.Children),

src/init.spec.lua

@@ -7,6 +7,7 @@ return function()
 			createFragment = "function",
 			createRef = "function",
 			createBinding = "function",
+			joinBindings = "function",
 			mount = "function",
 			unmount = "function",
 			update = "function",