joinBindings implementation

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 = {}
 
 --[[
@@ -32,13 +45,13 @@ function bindingPrototype:getValue()
 
 	--[[
 		If our source is another binding but we're not subscribed, we'll
-		return the mapped value from our upstream binding.
+		return the mapped value from our upstream binding(s).
 
 		This allows us to avoid subscribing to our source until someone
 		has subscribed to us, and avoid creating dangling connections.
 	]]
-	if internalData.upstreamBinding ~= nil and internalData.upstreamDisconnect == nil then
-		return internalData.valueTransform(internalData.upstreamBinding:getValue())
+	if internalData.upstreamBindingCount > 0 then
+		return internalData.valueTransform(self:__getValueFromUpstreamBindings())
 	end
 
 	return internalData.value
@@ -53,13 +66,49 @@ function bindingPrototype:map(valueTransform)
 	end
 
 	local binding = Binding.create(valueTransform(self:getValue()))
+	local internalData = binding[InternalData]
 
-	binding[InternalData].valueTransform = valueTransform
-	binding[InternalData].upstreamBinding = self
+	internalData.valueTransform = valueTransform
+
+	internalData.upstreamBindings.source = self
+	internalData.upstreamBindingCount = internalData.upstreamBindingCount + 1
 
 	return binding
 end
 
+--[[
+	Determines the final (not yet transformed) value from upstream bindings
+]]
+function bindingPrototype:__getValueFromUpstreamBindings()
+	local internalData = self[InternalData]
+	local newValue = mapBindingsToValues(internalData.upstreamBindings)
+
+	if not internalData.isJoinedBinding then
+		--[[
+			If this is not a joined binding, there will always only be one upstream
+			binding.
+
+			To ensure that joined bindings with a single upstream binding always
+			result in a table, we use the internal variable isJoinedBinding
+		]]
+		local _, value = next(newValue)
+		newValue = value
+	end
+
+	return newValue
+end
+
+--[[
+	Disconnects all connections to upstream bindings
+]]
+function bindingPrototype:__upstreamDisconnect()
+	local internalData = self[InternalData]
+
+	for _, disconnect in pairs(internalData.upstreamConnections) do
+		disconnect()
+	end
+end
+
 --[[
 	Update a binding's value. This is only accessible by Roact.
 ]]
@@ -80,13 +129,17 @@ function Binding.subscribe(binding, handler)
 
 	--[[
 		If this binding is mapped to another and does not have any subscribers,
-		we need to create a subscription to our source binding so that updates
+		we need to create subscriptions to our source bindings so that updates
 		get passed along to us
 	]]
-	if internalData.upstreamBinding ~= nil and internalData.subscriberCount == 0 then
-		internalData.upstreamDisconnect = Binding.subscribe(internalData.upstreamBinding, function(value)
-			Binding.update(binding, value)
-		end)
+	if internalData.upstreamBindingCount > 0 and internalData.subscriberCount == 0 then
+		local function upstreamCallback()
+			Binding.update(binding, binding:__getValueFromUpstreamBindings())
+		end
+
+		for _, upstreamBinding in pairs(internalData.upstreamBindings) do
+			table.insert(internalData.upstreamConnections, Binding.subscribe(upstreamBinding, upstreamCallback))
+		end
 	end
 
 	local disconnect = internalData.changeSignal:subscribe(handler)
@@ -111,9 +164,8 @@ function Binding.subscribe(binding, handler)
 			If our subscribers count drops to 0, we can safely unsubscribe from
 			our source binding
 		]]
-		if internalData.subscriberCount == 0 and internalData.upstreamDisconnect ~= nil then
-			internalData.upstreamDisconnect()
-			internalData.upstreamDisconnect = nil
+		if internalData.subscriberCount == 0 then
+			binding:__upstreamDisconnect()
 		end
 	end
 end
@@ -132,8 +184,10 @@ function Binding.create(initialValue)
 			subscriberCount = 0,
 
 			valueTransform = identity,
-			upstreamBinding = nil,
-			upstreamDisconnect = nil,
+			isJoinedBinding = false,
+			upstreamBindings = {},
+			upstreamConnections = {},
+			upstreamBindingCount = 0,
 		},
 	}
 
@@ -146,4 +200,23 @@ 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 = Binding.create(mapBindingsToValues(bindings))
+	local internalData = joinedBinding[InternalData]
+
+	internalData.isJoinedBinding = true
+
+	for key, binding in pairs(bindings) do
+		internalData.upstreamBindings[key] = binding
+		internalData.upstreamBindingCount = internalData.upstreamBindingCount + 1
+	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 = joinedBinding:getValue()
+			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",