ScaleFusion.lua

local Workspace = game:GetService("Workspace")
local GuiService = game:GetService("GuiService")

local Fusion = require("Fusion")
local Janitor = require("Janitor")

local TopInset, BottomInset = GuiService:GetGuiInset()

export type Properties = {
	Scale: number?,
	Size: Vector2,
}

type RequiredProperties = {
	Scale: number,
	Size: Vector2,
}

local function Llama_Dictionary_join(TableA, TableB)
	local NewTable = {}
	for Index, Value in next, TableA do
		NewTable[Index] = Value
	end

	for Index, Value in next, TableB do
		NewTable[Index] = Value
	end

	return NewTable
end

local DEFAULT_PROPERTIES = {Scale = 1}

local function Scale(BaseProperties: Properties)
	local Properties = Llama_Dictionary_join(DEFAULT_PROPERTIES, BaseProperties) :: RequiredProperties
	local CurrentScale = Fusion.State(1)

	local ScaleJanitor = Janitor.new()
	local CurrentCamera = Workspace.CurrentCamera

	local function Update()
		local CurrentSize = Properties.Size
		local ViewportSize = CurrentCamera.ViewportSize - TopInset + BottomInset
		CurrentScale:set(1 / math.max(CurrentSize.X / ViewportSize.X, CurrentSize.Y / ViewportSize.Y))
	end

	Update()
	ScaleJanitor:Add(CurrentCamera:GetPropertyChangedSignal("ViewportSize"):Connect(Update), "Disconnect", "ViewportSize")
	ScaleJanitor:Add(Workspace:GetPropertyChangedSignal("CurrentCamera"):Connect(function()
		CurrentCamera = Workspace.CurrentCamera
		ScaleJanitor:Add(CurrentCamera:GetPropertyChangedSignal("ViewportSize"):Connect(Update), "Disconnect", "ViewportSize")
	end), "Disconnect")

	local UIScale = Fusion.New("UIScale") {
		Scale = Fusion.Computed(function()
			return CurrentScale:get() * Properties.Scale
		end);
	} :: UIScale

	ScaleJanitor:LinkToInstance(UIScale)
	return UIScale
end

return Scale