Create a Retro Racing Game - Adding Interaction

This is the second installment in our Corona SDK Retro Racing game tutorial. In today’s tutorial, we’ll add to our interface and the game interaction. Read on!

Also available in this series:

1. Create a Retro Racing Game - Interface Creation
2. Create a Retro Racing Game - Adding Interaction

Where We Left Off. . .

Please be sure to check part 1 of the series to fully understand and prepare for this tutorial.

Step 1: Start Button Listeners

This function adds the necesary listeners to the TitleView buttons.

function startButtonListeners(action)
	if(action == 'add') then
		playBtn:addEventListener('tap', showGameView)
		creditsBtn:addEventListener('tap', showCredits)
	else
		playBtn:removeEventListener('tap', showGameView)
		creditsBtn:removeEventListener('tap', showCredits)
	end
end

Step 2: Show Credits

The credits screen is shown when the user taps the about button. A tap listener is added to the credits view to remove it.

function showCredits:tap(e)
	playBtn.isVisible = false
	creditsBtn.isVisible = false
	creditsView = display.newImage('credits.png', 0, display.contentHeight)
	
	lastY = titleBg.y
	transition.to(titleBg, {time = 300, y = (display.contentHeight * 0.5) - (titleBg.height + 50)})
	transition.to(creditsView, {time = 300, y = (display.contentHeight * 0.5) + 35, onComplete = function() creditsView:addEventListener('tap', hideCredits) end})
end

Step 3: Hide Credits

When the credits screen is tapped, it'll be tweened out of the stage and removed.

function hideCredits:tap(e)
	transition.to(creditsView, {time = 300, y = display.contentHeight + 25, onComplete = function() creditsBtn.isVisible = true playBtn.isVisible = true creditsView:removeEventListener('tap', hideCredits) display.remove(creditsView) creditsView = nil end})
	transition.to(titleBg, {time = 300, y = lastY});
end

Step 4: Show the Game View

When the Start button is tapped, the title view is tweened and removed revealing the game view. There are many parts involved in this view, so we'll split them over the next few steps.

function showGameView:tap(e)
	transition.to(titleView, {time = 300, x = -titleView.height, onComplete = function() startButtonListeners('rmv') display.remove(titleView) titleView = nil end})

Step 5: Place the Game Background

This code places the game background image in the stage.

-- Game Background
	
display.remove(bg)
gameBg = display.newImage('gameBg.png')

Step 6: Add the Car

Next, we add the Car image and place it in the center of the stage.

-- Car
	
car = display.newImage('car.png', 90, 412)

Step 7: Create the Highway

This part creates the Highway lines, this image is added twice to create a loop. One is visible on the screen while the other waits in the top part of the stage.

-- Highway
	
lines1 = display.newImage('lines.png', 0, -18)
lines2 = display.newImage('lines.png', 0, -558)
lines1:setReferencePoint(display.TopLeftReferencePoint)
lines2:setReferencePoint(display.TopLeftReferencePoint)
	
obstacles = display.newGroup()

Step 8: Add Text Fields

Now we place three textfields to display the Score, Level, and Speed.

-- TextFields
	
score = display.newText('0', 272, 38, native.systemFont, 17)
score:setTextColor(0)
	
level = display.newText('1', 272, 108, native.systemFont, 17)
level:setTextColor(0)
	
speed = display.newText('1', 272, 178, native.systemFont, 17)
speed:setTextColor(0)

Step 9: Create the Game Controls

Two buttons are added to use as controls for the car: left and right.

-- Direction Buttons
	
leftBtn = display.newImage('button.png', -2, 420)
leftBtn.name = 'l'
rightBtn = display.newImage('button.png', 260, 420)
rightBtn.name = 'r'

Step 10: Car Physics

Now we add the car to the physics engine, this way we can report collisions.

-- Physics
	
physics.addBody(car, 'dynamic')
	
gameListeners('add')

Step 11: Game Listeners

This function adds the necessary listeners to start the game logic.

This is a important part if you try to customize the game. The moveTimer controls the speed of the game. By default it is 100 milliseconds for Level 1, so level 2 may be 200 and so on.

The duration of the game is based in the number of obstacles that are presented in the level; this is defined by the oTimer. Modify the 15 value to create shorter or larger levels.

function gameListeners(action)
	if(action == 'add') then
		leftBtn:addEventListener('tap', moveCar)
		rightBtn:addEventListener('tap', moveCar)
		moveTimer = timer.performWithDelay(100, update, 0)--level speed
		oTimer = timer.performWithDelay(1400, addObstacle, 15)--level lenght
		car:addEventListener('collision', onCollision)
	else
		leftBtn:removeEventListener('tap', moveCar)
		rightBtn:removeEventListener('tap', moveCar)
		timer.cancel(moveTimer)
		moveTimer = nil
		timer.cancel(oTimer)
		oTimer = nil
		car:removeEventListener('collision', onCollision)
	end
end

Step 12: Car Movement

This function controls the car. It tests the button pressed and moves the car to the chosen position if available.

function moveCar(e)
	if(e.target.name == 'l' and car.x ~= 62) then
		car.x = car.x - 54
	elseif(e.target.name == 'r' and car.x ~= 170) then
		car.x = car.x + 54
	end
end

Step 13: Add Obstacle

Obstacles are added by a timer based on a randomly calculated position which can be left, center or right. This function create an obstacle, gives it physics and then is added to a Group to make its movement and removal easier.

function addObstacle(e)
	local rx = positionsTable[math.floor(math.random() * 3) + 1]
	local o = display.newImage('obstacle.png', rx, -52)
	
	physics.addBody(o, 'static')
	o.isSensor = true
	
	obstacles:insert(o)
	started = true
end

Step 14: Move the Highway

The update function is run by a timer to create the brick-by-brick effect. The next code loops the highway lines.

function update(e)
	-- Move Highway
	
	lines1.y = lines1.y + 16
	lines2.y = lines2.y + 16
	
	-- Reset highway position
	
	if(lines1.y >= 522) then
		lines1.y = -558
	elseif(lines2.y >= 522) then
		lines2.y = -558
	end

Step 15: Move Obstacles

This lines move the obstacles and remove the ones that are no longer visible. The score increases each time the car doesn't hit an obstacle.

	for i = 1, obstacles.numChildren do
		-- Move Obstacles
		
		if(obstacles[i] ~= nil) then
			obstacles[i].y = obstacles[i].y + 16
		end
		-- Remove offstage obstacles
		
		if(obstacles[i] ~= nil and obstacles[i].y >= display.contentHeight + 26) then
			display.remove(obstacles[i])
			score.text = tostring(tonumber(score.text) + 50)
		end
	end

Step 16: Level Completion

Here we check if all the obstacles defined are already out of the stage and call a win alert if true.

	-- Check for level complete
	
	if(started and obstacles.numChildren == 0) then
		alert()
	end
end

Step 17: Adding an Alert

The Collision function checks if the car is hit by an obstacle and calls a lose alert if true.

function onCollision(e)
	alert('lose')
end

function alert(msg)
	gameListeners('rmv')
	
	if(msg == 'lose') then
		alert = display.newImage('alert.png', 52, 229.5)
	else
		alert = display.newImage('complete.png', 18, 229.5)
	end
	
	transition.from(alert, {time = 300, xScale = 0.5, yScale = 0.5})
	physics.stop()
end

Step 18: Call the Main Function

In order to start the game, the Main function needs to be called. With the above code in place, we'll do that here:

Main()

Step 19: Add a Loading Screen

The Default.png file is an image that will be displayed right when you start the application while iOS loads the basic data to show the Main Screen. Add this image to your project source folder.

Step 20: Add an Icon

Using the graphics you created before you can now create a nice and good looking icon. The icon size for
the non-retina iPhone icon is 57x57px, but the retina version is 114x114px and the iTunes store requires a 512x512px version. I suggest creating the 512x512 version first and then scaling down for the other sizes.

It doesn't need to have the rounded corners or the transparent glare, iTunes and the iPhone will do that for you.

Step 21: Testing in the Simulator

It's time to do the final test. Open the Corona Simulator, browse to your project folder, and then click open. If everything works as expected, you are ready for the final step!

Step 22: Build

In the Corona Simulator go to File > Build and select your target device. Fill the required data and click build. Wait a few seconds and your app will be ready for device testing and/or submission for distribution!

Conclusion

Experiment with the final result and try to make your custom version of the game!

I hope you liked this tutorial series and found it helpful. Thank you for reading!