the_camera_challenge.lua

local camera, cameraChanged
local viewport, viewportChanged

-- Challenge:
--   1. Make `camera' always point to the the current or last known camera.
--     a. Problem: CurrentCamera can be nil.
--     b. Guarantee that `camera' is never nil.
--   2. Make `viewport' always be the current actual size of the viewport.
--     a. Problem: ViewportSize can falsely report (1,1) before it gets fixed up by RenderPrepare.
--     b. Guarantee that `viewport' is never (1,1).
--   3. Yield until the above guarantees are met.
--   4. Don't leak memory.

do
	local viewportChangedRaw

	local cameraChangedBindable = Instance.new('BindableEvent')
	local viewportChangedBindable = Instance.new('BindableEvent')
	cameraChanged = cameraChangedBindable.Event
	viewportChanged = viewportChangedBindable.Event

	local function updateViewport()
		local newViewport = camera.ViewportSize
		if newViewport ~= viewport and newViewport.X > 1 and newViewport.Y > 1 then
			local oldViewport = viewport
			viewport = newViewport
			viewportChangedBindable:Fire(oldViewport, newViewport)
		end
	end

	local function updateCamera()
		local newCamera = workspace.CurrentCamera or camera
		if camera ~= newCamera then
			local oldCamera = camera
			camera = newCamera
			cameraChangedBindable:Fire(oldCamera, newCamera)
			if viewportChangedRaw then
				viewportChangedRaw:Disconnect()
			end
			viewportChangedRaw = camera:GetPropertyChangedSignal('ViewportSize'):Connect(updateViewport)
			updateViewport()
		end
	end

	workspace:GetPropertyChangedSignal('CurrentCamera'):Connect(updateCamera)
	updateCamera()

	if not viewport then
		viewportChanged:Wait()
	end
end

-- Tests
print(camera)
print(viewport)
viewportChanged:Connect(function(oldViewport, newViewport)
	print('Viewport changed from', oldViewport, 'to', newViewport)
end)
cameraChanged:Connect(function(oldCamera, newCamera)
	print('Camera changed from', oldCamera, 'to', newCamera)
end)

Both OOP and Rx solution here:

https://gist.github.com/Quenty/803e0a6f36804e327c60347e52803359