[BUG] Previous `State`'s `Heartbeat` keeps running even after Next `State` Starts!

[Errorbot1122]

It seems that the previous State's OnHeartbeat and (maybe) OnLeave functions sometimes continue running a little, even while the next State's OnStart and (maybe) OnHeartbeat functions are running! This seems to happen because of your use of task.defer. This is not good as, most would rightfully assume that the OnStart function would only run, after the last state has left!

[Errorbot1122]

After doing some more research, this seems to be happening when a function has some delay like with a wait, or a path:ComputeAsync.

The culprit is most likely line 274 of init.lua

RobloxStateMachine/src/StateMachine/init.lua

Lines 255 to 275 in 2f7113a

	local previousState: State = nil
	self._trove:Add(RunService.Heartbeat:Connect(function(deltaTime: number)
	if self._Destroyed then
	return
	end
	self:_CheckTransitions()
	local state = self:_GetCurrentStateObject()
	local firstFrame: boolean = state ~= previousState
	previousState = state
	if firstFrame then
	return
	end
	if not state or getmetatable(state).OnHeartbeat == state.OnHeartbeat then
	return
	end
	task.spawn(state.OnHeartbeat, state, self:GetData(), deltaTime)
	end))

Here, it spawns a completely new thread to call Heartbeat on every RunService:Heartbeat, however in the case we swapped states, we don't stop the older heartbeat threads (or wait for them), so the new states ones start running while the old ones still finish up. This can cause issues. (Kinda like Race Conditions...)

A fix could be to store all the active threads in a Heap like array (to prevent infinitely growing array). Each thread would be given a spot, and when the thread ends, it automatically removes it self. Allowing another thread to fill its place. Now when you swap States, you kill all the state's active threads, calling an OnCleanUp method to clean up any data that got abruptly destroyed.