Rework patch visualizer with many fixes and improvements (#726)

- Reconciler now has precommit and postcommit hooks for patch applying
  - This is used to compute a patch tree snapshot precommit and update the
tree metadata postcommit
- PatchVisualizer can now display Removes that happened during sync
  - It was previously missing because the removed objects no longer
existed so it couldn't get any info on them (This is resolved because
the info is gotten in precommit, before the instance was removed)
- PatchVisualizer now shows Old and New values instead of just Incoming
during sync
  - (Still displays Current and Incoming during confirmation)
  - This is much more useful, since you now see what the changes were and
not just which things were changed
- PatchVisualizer displays clarifying message when initial sync has no
changes instead of just showing a blank box
- Objects in the tree UI no longer get stuck expanded when the next
patch has the same instance but different info on it
- Objects in the tree UI correctly become selectable after their
instance is added and unclickable when removed during sync

plugin/src/Reconciler/init.lua

@@ -3,6 +3,9 @@
 	and mutating the Roblox DOM.
 ]]
 
+local Packages = script.Parent.Parent.Packages
+local Log = require(Packages.Log)
+
 local applyPatch = require(script.applyPatch)
 local hydrate = require(script.hydrate)
 local diff = require(script.diff)
@@ -14,13 +17,63 @@ function Reconciler.new(instanceMap)
 	local self = {
 		-- Tracks all of the instances known by the reconciler by ID.
 		__instanceMap = instanceMap,
+		__precommitCallbacks = {},
+		__postcommitCallbacks = {},
 	}
 
 	return setmetatable(self, Reconciler)
 end
 
+function Reconciler:hookPrecommit(callback: (patch: any, instanceMap: any) -> ()): () -> ()
+	table.insert(self.__precommitCallbacks, callback)
+	Log.trace("Added precommit callback: {}", callback)
+
+	return function()
+		-- Remove the callback from the list
+		for i, cb in self.__precommitCallbacks do
+			if cb == callback then
+				table.remove(self.__precommitCallbacks, i)
+				Log.trace("Removed precommit callback: {}", callback)
+				break
+			end
+		end
+	end
+end
+
+function Reconciler:hookPostcommit(callback: (patch: any, instanceMap: any, unappliedPatch: any) -> ()): () -> ()
+	table.insert(self.__postcommitCallbacks, callback)
+	Log.trace("Added postcommit callback: {}", callback)
+
+	return function()
+		-- Remove the callback from the list
+		for i, cb in self.__postcommitCallbacks do
+			if cb == callback then
+				table.remove(self.__postcommitCallbacks, i)
+				Log.trace("Removed postcommit callback: {}", callback)
+				break
+			end
+		end
+	end
+end
+
 function Reconciler:applyPatch(patch)
-	return applyPatch(self.__instanceMap, patch)
+	for _, callback in self.__precommitCallbacks do
+		local success, err = pcall(callback, patch, self.__instanceMap)
+		if not success then
+			Log.warn("Precommit hook errored: {}", err)
+		end
+	end
+
+	local unappliedPatch = applyPatch(self.__instanceMap, patch)
+
+	for _, callback in self.__postcommitCallbacks do
+		local success, err = pcall(callback, patch, self.__instanceMap, unappliedPatch)
+		if not success then
+			Log.warn("Postcommit hook errored: {}", err)
+		end
+	end
+
+	return unappliedPatch
 end
 
 function Reconciler:hydrate(virtualInstances, rootId, rootInstance)
@@ -31,4 +84,4 @@ function Reconciler:diff(virtualInstances, rootId)
 	return diff(self.__instanceMap, virtualInstances, rootId)
 end
 
-return Reconciler
+return Reconciler