init.lua

local LayoutController = {}
LayoutController.__index = LayoutController

local INVALID_ARG = [[bad argument #%d to 'LayoutUtil' (%s expected, got %s)]]
local camera = workspace.CurrentCamera

local function toOffset(udim, parentOffset)
	return (udim.Scale * parentOffset) + udim.Offset
end

local function absoluteSizeFromUDim2(childUDim2, parentAbsoluteSize)
	return Vector2.new(toOffset(childUDim2.X, parentAbsoluteSize.X), toOffset(childUDim2.Y, parentAbsoluteSize.Y))
end

local function addConstraint(object, absoluteSize)
	local constraint = object:FindFirstChildOfClass("UIAspectRatioConstraint")
		or Instance.new("UIAspectRatioConstraint")
	constraint.AspectRatio = absoluteSize.X / absoluteSize.Y
	constraint.Parent = object
end

--[=[
	Automatically inserts and calculates UIAspectRatioConstraints into the corresponding UIGridLayout/UIListLayout. The
	parent parameter is optional, but should be explicitly provided if the layout hasn't been parented. In order to
	calculate the AspectRatio, I need the size of the UILayout (or children of a UIListLayout) in offset. Therefor if the
	parent isn't a GuiObject or a Vector2, it will assume the parent size is the screens resolution.

	@param {UIGridLayout | UIListLayout} layout The UILayout to be applied.
	@param {GuiObject | Instance | Vector2} [parentObjectOrSize] The object (or Vector2) to be recognized as the parent
		in order to retrieve the AbsoluteSize (non-GuiObjects or nil, defaults to the screens resolution).
]=]

function LayoutController.new()
	local self = setmetatable({}, LayoutController)
	return self
end

function LayoutController:FixLayout(layout: UILayout, parentObjectOrSize: ScrollingFrame)
	local className = typeof(layout) == "Instance" and layout.ClassName
	assert(
		className == "UIGridLayout" or className == "UIListLayout",
		INVALID_ARG:format(1, "UIGridLayout or UIListLayout", className)
	)

	local parent = parentObjectOrSize or layout.Parent
	local parentAbsoluteSize
	if parent:IsA("GuiObject") then
		parentAbsoluteSize = parent.AbsoluteSize
	elseif typeof(parentObjectOrSize) == "Vector2" then
		parentAbsoluteSize = parentObjectOrSize
	else
		parentAbsoluteSize = camera.ViewportSize
	end

	parent.CanvasSize = UDim2.new(0, layout.AbsoluteContentSize.X, 0, layout.AbsoluteContentSize.Y)

	if layout.ClassName == "UIGridLayout" then
		addConstraint(layout, absoluteSizeFromUDim2(layout.CellSize, parentAbsoluteSize))
	else -- UIListLayout
		for _, child in ipairs(parent:GetChildren()) do
			if child:IsA("GuiObject") then
				addConstraint(child, absoluteSizeFromUDim2(child.Size, parentAbsoluteSize))
			end
		end
	end
end

local singleton = LayoutController.new()
return singleton