Roblox Instance Destroyed and Garbage Collected Example Code

OnDestroyed.lua

--[[
	Notable behavior:
	* ObjectValues will get their Value set to nil if:
		* The ObjectValue is in nil
		* its Value is in nil
		* its Value gets garbage collected because it has no string reference
	* When an object gets garbage collected, all of its connections get disconnected.
		* Connections that don't hold a reference to the object won't prevent garbage collection!!
	* When an object gets destroyed, connections are disconnected by the next Heartbeat
	
	We can make use of these behaviors to detect when an object is both destroyed and garbage collected
]]

local OnDestroyed = {}

function OnDestroyed.onInstanceGarbageCollectedSimple(instance, callback)
	local weakReference = Instance.new("ObjectValue")
	weakReference.Value = instance
	instance = nil
	coroutine.wrap(function()
		while weakReference.Value ~= nil do
			wait(5)
		end
		callback()
	end)()
end

-- The following will not detect if an object is already in nil and gets Destroyed:
function OnDestroyed.onInstanceDestroyedSimple(instance, callback)
	local connection
	connection = instance.AncestryChanged:Connect(function()
		game:GetService("RunService").Heartbeat:Wait()
		if not connection.Connected then
			callback()
		end
	end)
	
	instance = nil
end

function OnDestroyed.onInstanceDestroyedOrGarbageCollected(instance, callback)
	local weakReference = Instance.new("ObjectValue")
	weakReference.Value = instance
	instance = nil
	
	local changeIndex = 0
	local connection
	local function onChanged()
		changeIndex = changeIndex + 1
		
		-- Detects Destroyed and Garbage Collected:
		game:GetService("RunService").Heartbeat:Wait()
		if not connection.Connected then
			callback()
			return
		end
		
		-- Detects Garbage Collected:
		if not weakReference.Value:IsDescendantOf(game) then
			local thisChangeIndex = changeIndex
			while connection.Connected do
				wait(5)
				if changeIndex ~= thisChangeIndex then
					return
				end
			end
			callback()
		end
	end
	
	connection = weakReference.Value.AncestryChanged:Connect(onChanged)
	
	-- If the object is already in nil, we need to start running the GC detector:
	coroutine.wrap(onChanged)()
end

return OnDestroyed

Test.server.lua

local OnDestroyed = require(game.ReplicatedStorage.OnDestroyed)

local function testCase(name, setup)
	local f = Instance.new("Folder")
	setup(f)
	OnDestroyed.onInstanceDestroyedOrGarbageCollected(f, function()
		print("Test case \"" .. name .. "\" destroyed or garbage collected")
	end)
	f = nil
end

testCase("nil", function() end)

testCase("nil -> destroy", function(f)
	delay(2, function()
		f:Destroy()
	end)
end)

testCase("workspace -> destroy", function(f)
	f.Parent = workspace
	delay(2, function()
		f:Destroy()
	end)
end)

testCase("workspace -> nil", function(f)
	f.Parent = workspace
	delay(2, function()
		f.Parent = nil
	end)
end)

testCase("workspace -> nil -> workspace -> nil", function(f)
	f.Parent = workspace
	delay(2, function()
		f.Parent = nil
		wait(2)
		f.Parent = workspace
		wait(2)
		f.Parent = nil
	end)
end)

testCase("workspace -> nil -> workspace -> destroyed", function(f)
	f.Parent = workspace
	delay(2, function()
		f.Parent = nil
		wait(2)
		f.Parent = workspace
		wait(2)
		f:Destroy()
	end)
end)