How to check the actual frame rate (FPS) with Corona SDK

EDIT: This example has a follow-up in this blog post containing improved and object-oriented source code for the frame rate calculator.

In Corona SDK you set the target frame rate in config.lua to either 30 or 60 FPS (Frames Per Second). Then during application execution you can get this value through display.fps. However, remember that this is only the target frame rate, and the actual frame rate is something completely different. The actual FPS will fluctuate during program execution and can be significantly lower than the target FPS in case your app has too much to handle at any given moment.

I couldn't find any built-in way to get the actual FPS, so I decided to roll my own very simple solution to handle this. My frame rate calculator is just a Lua function that sets up an enter frame listener which then counts frames and calculates FPS based on time passed between the frames. The function startFrameRateCalculator accepts a single argument which is a callback function with a single parameter: the actual FPS.

Below is the source code for a little demo of this that spawns lots of colorful rectangles, moving from the center of the display towards the edges. The FPS value is displayed in a label, and the label is updated in the callback function injected to the startFrameRateCalculator function. If you run the program it should look something like this:

Actual FPS calculation demo

This was just a quick hack to test the idea, so feel free to improve the source code anyway you want. 

	display.setStatusBar(display.HiddenStatusBar)
	local function startFrameRateCalculator(callbackFunction)
	local lastTimestampMs
	local frameCounter = 0
	Runtime:addEventListener("enterFrame", function()
	frameCounter = frameCounter + 1
	local currentTimestampMs = system.getTimer()
	if (not lastTimestampMs) then
	lastTimestampMs = currentTimestampMs
	end
	-- Calculate actual fps approximately four times every second
	if (frameCounter >= (display.fps / 4)) then
	local deltaMs = currentTimestampMs - lastTimestampMs
	local fps = frameCounter / (deltaMs / 1000)
	frameCounter = 0
	lastTimestampMs = currentTimestampMs
	callbackFunction(fps)
	end
	end)
	end
	local function createSquares(numberOfSquares)
	local sizePixels = 2
	local squares = {}
	for i = 1, numberOfSquares do
	local square = display.newRect(display.contentCenterX, display.contentCenterY, sizePixels, sizePixels)
	local random = math.random
	square:setFillColor(random(), random(), random())
	square:toBack()
	squares[#squares + 1] = square
	end
	return squares
	end
	------------------------------------------------------------------
	math.randomseed( os.time() )
	-- Setup a label to display the FPS value
	local fpsLabel = display.newText({
	x = display.contentCenterX,
	y = 20,
	fontSize = 72,
	font = native.systemFontBold,
	text = "FPS: " .. math.round(display.fps)
	})
	fpsLabel:setFillColor(1,1,0)
	-- Start calculating FPS, and provide a callback function to update the label with current FPS value
	startFrameRateCalculator(function(fps)
	fpsLabel.text = "FPS: " .. math.round(fps)
	end)
	-- A simple way to be able to randomize where the squares should move
	local moveTo = {
	{
	x = function() return math.random(0, display.contentWidth) end,
	y = function() return 0 end,
	},
	{
	x = function() return display.contentWidth end,
	y = function() return math.random(0, display.contentHeight) end,
	},
	{
	x = function() return math.random(0, display.contentWidth) end,
	y = function() return display.contentHeight end,
	},
	{
	x = function() return 0 end,
	y = function() return math.random(0, display.contentHeight) end,
	}
	}
	-- Endlessly create new squares in the center of the display, and send them off towards the edges of the display
	timer.performWithDelay(10, function()
	local numberOfSquares = 40
	local squares = createSquares(numberOfSquares)
	for i = 1, #squares do
	local moveToFunctions = moveTo[math.random(1, #moveTo)]
	local square = squares[i]
	square.alpha = 0
	transition.to(square, {
	x = moveToFunctions.x(),
	y = moveToFunctions.y(),
	rotation = math.random(-360, 360),
	time = 500,
	xScale = 20,
	yScale = 20,
	alpha = 1,
	onComplete = function()
	display.remove(square)
	end
	})
	end
	end, 0)

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