CameraChallenge.lua

---
-- @classmod CameraChallenge
-- @author Quenty

local require = require(game:GetService("ReplicatedStorage"):WaitForChild("Nevermore"))

-- 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 `Goodviewport' always be the current actual size of the Goodviewport.
--     a. Problem: ViewportSize can falsely report (1,1) before it gets fixed up by RenderPrepare.
--     b. Guarantee that `Goodviewport' is never (1,1).
--   3. Yield until the above guarantees are met.
--   4. Don't leak memory.
-- See: https://gist.github.com/Fraktality/31bc9cb5abf2af4340cd3a6a775c616d

local Workspace = game:GetService("Workspace")

local BaseObject = require("BaseObject")
local Maid = require("Maid")
local Promise = require("Promise")
local ValueObject = require("ValueObject")

local CameraChallenge = setmetatable({}, BaseObject)
CameraChallenge.ClassName = "CameraChallenge"
CameraChallenge.__index = CameraChallenge

function CameraChallenge.new()
	local self = setmetatable(BaseObject.new(), CameraChallenge)
	
	self.LastCamera = ValueObject.new()
	self._maid:GiveTask(self.LastCamera)

	self.GoodViewport = ValueObject.new()
	self._maid:GiveTask(self.GoodViewport)

	self._maid:GiveTask(Workspace:GetPropertyChangedSignal("CurrentCamera"):Connect(function()
		self.LastCamera.Value = Workspace.CurrentCamera
	end))

	self._maid:GiveTask(self.LastCamera.Changed:Connect(function(...)
		self:_handleCameraChanged(...)
	end))

	self.LastCamera.Value = Workspace.CurrentCamera

	return self
end

function CameraChallenge:PromiseValidViewport()
	if self:HasValidViewport() then
		return Promise.resolved()
	end
	if self._maid._currentPromise  then
		return self._maid._currentPromise
	end

	local maid = Maid.new()
	
	local promise = Promise.new(function(resolve, reject)
		maid:GiveTask(self.LastCamera.Changed:Connect(function()
			if self:HasValidViewport() then
				resolve(self.LastCamera.Value, self.GoodViewport.Value)
			end
		end))

		maid:GiveTask(self.GoodViewport.Changed:Connect(function()
			if self:HasValidViewport() then
				resolve(self.LastCamera.Value, self.GoodViewport.Value)
			end
		end))

		maid:GiveTask(reject)
	end)

	self._maid._currentPromise = promise

	promise:Finally(function()
		self._maid._currentPromise = nil
		maid:DoCleaning()
	end)

	return promise
end

function CameraChallenge:HasValidViewport()
	return self.LastCamera.Value and self.GoodViewport.Value
end

function CameraChallenge:_handleCameraChanged(newCamera, oldCamera, maid)
	if not newCamera then
		self.GoodViewport.Value = nil
		return
	end

	maid:GiveTask(newCamera:GetPropertyChangedSignal("ViewportSize"):Connect(function()
		self:_updateViewport(newCamera)
	end))

	self:_updateViewport(newCamera)
end

function CameraChallenge:_updateViewport(camera)
	if camera.ViewportSize == Vector2.new(1, 1) then
		self.GoodViewport.Value = nil
	else
		self.GoodViewport.Value = camera.ViewportSize
	end
end

return CameraChallenge

RxCameraChallenge.lua

-- 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 `Goodviewport' always be the current actual size of the Goodviewport.
--     a. Problem: ViewportSize can falsely report (1,1) before it gets fixed up by RenderPrepare.
--     b. Guarantee that `Goodviewport' is never (1,1).
--   3. Yield until the above guarantees are met.
--   4. Don't leak memory.
-- See: https://gist.github.com/Fraktality/31bc9cb5abf2af4340cd3a6a775c616d

local Rx = require("Rx")
local RxInstanceUtils = require("RxInstanceUtils")

local function observeChallenge()
	return RxInstanceUtils.observeProperty(workspace, "CurrentCamera"):Pipe({
		Rx.where(function(value)
			return value ~= nil
		end);
		Rx.switchMap(function(camera)
			return RxInstanceUtils.observeProperty(camera, "ViewportSize"):Pipe({
				Rx.where(function(viewportSize)
					return viewportSize.x ~= 1 and viewportSize.y ~= 1
				end);
				Rx.map(function(viewport)
					return camera, viewport
				end)
			})
		end);
	})
end

observeChallenge():Subscribe(function(camera, viewport)
	-- this code runs once all conditions are met, and always a valid viewport and camera will be here.

	-- note that the camera may be removed/unparented, and we will not emit a new camera and viewport
	-- until the next valid viewport and camera are reported.
	-- this matches the expectations provided above (i.e. the last known/current valid camera)

	print(camera, viewport)
end)