Add plugin half of script-only, existing-instance-only, two way sync

plugin/src/ApiContext.lua

@@ -1,4 +1,5 @@
 local Http = require(script.Parent.Parent.Http)
+local Log = require(script.Parent.Parent.Log)
 local Promise = require(script.Parent.Parent.Promise)
 
 local Config = require(script.Parent.Config)
@@ -153,6 +154,23 @@ function ApiContext:read(ids)
 		end)
 end
 
+function ApiContext:write(patch)
+	local url = ("%s/write"):format(self.__baseUrl)
+	local body = Http.jsonEncode({
+		sessionId = self.__sessionId,
+		patch = patch,
+	})
+
+	return Http.post(url, body)
+		:andThen(rejectFailedRequests)
+		:andThen(Http.Response.json)
+		:andThen(function(body)
+			Log.info("Write response: {:?}", body)
+
+			return body
+		end)
+end
+
 function ApiContext:retrieveMessages()
 	local url = ("%s/api/subscribe/%s"):format(self.__baseUrl, self.__messageCursor)
 

plugin/src/DevSettings.lua

@@ -25,6 +25,14 @@ local VALUES = {
 			[Environment.Test] = true,
 		},
 	},
+	ExperimentalTwoWaySync = {
+		type = "BoolValue",
+		values = {
+			[Environment.User] = false,
+			[Environment.Dev] = false,
+			[Environment.Test] = false,
+		},
+	},
 }
 
 local CONTAINER_NAME = "RojoDevSettings" .. Config.codename
@@ -132,6 +140,10 @@ function DevSettings:shouldTypecheck()
 	return getValue("TypecheckingEnabled")
 end
 
+function DevSettings:twoWaySyncEnabled()
+	return getValue("ExperimentalTwoWaySync")
+end
+
 function _G.ROJO_DEV_CREATE()
 	DevSettings:createDevSettings()
 end

plugin/src/Reconciler.lua

@@ -176,6 +176,23 @@ function Reconciler:applyPatch(patch)
 	end
 end
 
+--[[
+	Transforms a value into one that can be sent over the network back to the
+	Rojo server.
+
+	This operation can fail, and so it returns bool, value.
+]]
+function Reconciler:encodeApiValue(value)
+	if typeof(value) == "string" then
+		return true, {
+			Type = "String",
+			Value = value,
+		}
+	end
+
+	return false
+end
+
 --[[
 	Transforms a value encoded by rbx_dom_weak on the server side into a value
 	usable by Rojo's reconciler, potentially using RbxDom.
@@ -302,7 +319,6 @@ function Reconciler:__hydrateInternal(apiInstances, id, instance, hydratePatch)
 			local decodedValue = self:__decodeApiValue(virtualValue)
 
 			if existingValue ~= decodedValue then
-				Log.warn("Diff! {:?} vs {:?}", existingValue, decodedValue)
 				changedProperties[propertyName] = virtualValue
 			end
 		end

plugin/src/ServeSession.lua

@@ -2,6 +2,7 @@ local Log = require(script.Parent.Parent.Log)
 local Fmt = require(script.Parent.Parent.Fmt)
 local t = require(script.Parent.Parent.t)
 
+local DevSettings = require(script.Parent.DevSettings)
 local InstanceMap = require(script.Parent.InstanceMap)
 local Reconciler = require(script.Parent.Reconciler)
 local strict = require(script.Parent.strict)
@@ -47,10 +48,16 @@ local validateServeOptions = t.strictInterface({
 function ServeSession.new(options)
 	assert(validateServeOptions(options))
 
-	local instanceMap = InstanceMap.new()
+	-- Declare self ahead of time to capture it in a closure
+	local self
+	local function onInstanceChanged(instance, propertyName)
+		self:__onInstanceChanged(instance, propertyName)
+	end
+
+	local instanceMap = InstanceMap.new(onInstanceChanged)
 	local reconciler = Reconciler.new(instanceMap)
 
-	local self = {
+	self = {
 		__status = Status.NotStarted,
 		__apiContext = options.apiContext,
 		__reconciler = reconciler,
@@ -101,6 +108,45 @@ function ServeSession:stop()
 	self:__stopInternal()
 end
 
+function ServeSession:__onInstanceChanged(instance, propertyName)
+	if not DevSettings:twoWaySyncEnabled() then
+		return
+	end
+
+	local instanceId = self.__instanceMap.fromInstances[instance]
+
+	if instanceId == nil then
+		Log.warn("Ignoring change for instance {:?} as it is unknown to Rojo", instance)
+		return
+	end
+
+	local update = {
+		id = instanceId,
+		changedProperties = {},
+	}
+
+	if propertyName == "Name" then
+		update.changedName = instance.Name
+	else
+		local success, encoded = self.__reconciler:encodeApiValue(instance[propertyName])
+
+		if not success then
+			Log.warn("Could not sync back property {:?}.{}", instance, propertyName)
+			return
+		end
+
+		update.changedProperties[propertyName] = encoded
+	end
+
+	local patch = {
+		removed = {},
+		added = {},
+		updated = {update},
+	}
+
+	self.__apiContext:write(patch)
+end
+
 function ServeSession:__initialSync(rootInstanceId)
 	return self.__apiContext:read({ rootInstanceId })
 		:andThen(function(readResponseBody)