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Notes on Mobile Game Programming

Table of Contents:


  • Nearly every computer program ever written accepts input from the user, processes that input in some way, produces some output, and provides feedback to the user.
    • Game programming is no different.
    • Games forces developers to examine code optimization for the best user experience, something all web developers should consider.
  • Usually, game programs create event listeners that listen for user input via touch, the mouse, or keyboard, and a game loop that manages processing.
    • Once launched, the game loop continues until it encounters some end condition.
    • If end conditions aren't met, objects are rendered to the screen and the game continues.
  • In this project, the game will be simple, and called Rebound.
    • Rebound is a simple paddle and ball game where the player must keep the ball from reaching the bottom of the playing area.
    • The score is reflected at the bottom of the playing area. As the game progresses, difficulty is increased by moving the ball more quickly.
    • Rebound includes some simple collision detection routines to reward the player for using the middle of the paddle. (game buzzes)
    • It also includes some basic sound effects and a background music track, and a settings panel lets players decide if they want to hear the sounds and start a new game.

Part 1: Button Chaser

View the game here!

  • Before implementing all the features of the Rebound game, we need to add some tools to our toolset.
    • You need to crawl before we can walk.
    • Let's start by creating a simple game.
  • Button Chaser is a Whac-A-Mole type game where the player needs to tap a button on screen that is randomly placed.
    • The button will move once per second, and the score is increased each time the button is tapped.
    • The game ends after 30 seconds, and the player is informed that the game is over.
let score = 0;
let iterations = 0;
let aWidth, aHeight, timer;

// Executes after all of the resources for the page are downloaded.
window.addEventListener("load", setGameAreaBounds);

function setGameAreaBounds() {
    aWidth = innerWidth;
    aHeight = innerHeight;
    // Taking other element sizes into account:
    aWidth -= 22;
    aHeight -= 97;

    document.getElementById("game-area").style.width = aWidth + 'px';
    document.getElementById("game-area").style.height = aHeight + 'px';
    document.getElementById("dot").addEventListener("click", detectHit);

    // Make sure the dot stays within the
    // right and bottom of the gaming area:
    aWidth -= 74;
    aHeight -= 74;



function detectHit() {
    score += 1;
    document.getElementById("score-label").innerHTML = "Score: " + score;

function moveDot() {

    // Allowing the dot to appear randomly
    // within the available area:
    let x = Math.floor(Math.random() * aWidth);
    let y = Math.floor(Math.random() * aHeight);

    // Make sure the dot stays within the
    // left and top of the gaming area:
    if (x < 10) {
        x = 10;
    if (y < 10) {
        y = 10;

    document.getElementById("dot").style.left = x + "px";
    document.getElementById("dot") = y + "px";

    // Note: Not recursive, setTimeout returns
    // immediately and tells the browser to wait
    // the specified delay, and then execute the code.
    if (iterations < 10) {
        timer = setTimeout("moveDot()", 1000);
    } else {
        // Ending the game and disabling functionality:
        document.getElementById("score-label").innerHTML += " Game Over!";
        document.getElementById("dot").removeEventListener("click", detectHit);


Animation in JS

  • Animation on the web is very much like those flip books you created as a kid.
    • Eg. You place an individual image on the corner of each page in the book then flip through the book rapidly to see the animation.
    • If you flip rapidly enough, the animation is smooth.
    • If you don't flip rapidly enough, the animation is jerky.
    • Animators call the speed of the flipping, the frame rate.
  • Using JavaScript, you can manipulate the properties of an image to simulate motion.
    • You might change the source attribute of the image. Eg. Focusing on the horse and its rider.
    • The animation cycles through a set of images in a series, then repeats.
    • This is how changing the source attribute of an image element, works.
  • In JS, this technique is called replacement animation.
    • You might also change the images location on the screen.
    • For other types of animation, you may notice how the image doesn't change but the location of the image onscreen does.
    • Eg. The cactus follows a well-defined path in the animation. We'll explore
  • You'll be able to create a path for objects onscreen using mathematical formulas.
    • Eg. How the cactus sits behind the horse and rider when they occupy the same location onscreen.
    • This is how we can simulate depth in an animation.
  • You can manipulate an objects Z index to accomplish layering objects.
    • The cool part is that you can often combine these techniques to produce compelling animations.

Replacement Animation

View the animation here!

  • The general process for replacement animation is to create an array of images to cycle through, then swap the images fast enough so that the user perceives motion.
    • Just remember, there really isn't any motion taking place. You are simply controlling which frame the user is currently viewing.
  • It's customary to name the individual frames of an image set with the same prefix and then a frame counter for the number. Eg. ball0.gif
    • This construct works nicely to create an array to hold the images.
  • The good news is, we won't have to declare each individual image object.
    • Rather, because the names of the files are indexed, a simple loop does all the work.
let imgArray = new Array();

for (let i = 0; i < 24; i++) {
    // Using an image object forces
    // browser to preload each object,
    // thus removing latency.
    imgArray[i] = new Image();
    imgArray[i].src = "img/ball" + i + ".gif";

let counter = 0;

function rotate() {
    if (counter > imageArray.length - 1) {
        // Restarts and loops animation.
        counter = 0;
    document.getElementById("baseball").src = imgArray[counter];
    setTimeout("rotate()", 50);
// The load event fires after all of the resources have been downloaded.
// And that's important, because we're using the image array,
// storing image objects, that means all of the frames in the
// animation will be downloaded before anything gets going.
window.addEventListener("load", rotate);

Point-to-Point Animation

  • Point-to-point animation is done by calculating and objects coordinates on the screen using a mathematical formula.
    • In essence, the point-to-point animation keeps track of the objects top and left properties and then modifies their values according to the formula.
let currentX = 400;
let currentY = 100;

function animate() {
    document.getElementById("orb").style.left = currentX + "px";
    document.getElementById("orb").style.left = currentY + "px";

    currentX += 2;
    currentY += 2;

    if (currentX > 800) {
    setTimeout("animate()", 10);

window.addEventListener("load", animate);

JavaScript Coordinate System

  • The browser's viewable area is the canvas of the page, this is the known as the JavaScript Coordinate System.
  • The origin of the coordinate system is the top left of the page 0, 0.
    • This is the only point on the canvas that we can actually be sure exists.
    • Since users always have varying viewports, we cannot assume anything about the bottom or right values the screen.
  • When thinking of the coordinates, moving from right to left represents an increment in the x value of the pair.
    • Counter-intuitve to basic geometry, however, moving from top to bottom represents an increment in the y value of the pair.
  • These are the things that must be kept in mind when moving objects on the screen.

Boundary Collision Detection

  • Variables are required to hold the distance the ball travels between frames (delta).
let currentX = 400;
let currentY = 100;
let deltaX = 5;
let deltaY = 5;

function animate() {
    document.getElementById("orb").style.left = currentX + "px";
    document.getElementById("orb").style.left = currentY + "px";

    currentX += deltaX;
    currentY += deltaY;

    if ((currentX > 800 || currentX < 100) || (currentY > 600 || currentY < 100)) {
        deltaX *= -1;
        deltaY *= -1;
    setTimeout("animate()", 10);

window.addEventListener("load", animate);

Part 2: Rebound

Optimizing keyListener Function

function keyListener(e){
    if((e.keyCode == 37 || e.keyCode == 65) && paddleLeft > 0){
        paddleLeft -= pdx;
        if(paddleLeft < 0)
            paddleLeft = 0; = paddleLeft + 'px';
    if((e.keyCode == 39 || e.keyCode == 68) && paddleLeft < pWidth - 64){
        paddleLeft += pdx;
        if(paddleLeft > (pWidth - 64))
            paddleLeft = pWidth - 64; = paddleLeft + 'px';
function keyListener(e) {
    // Adding variable for element used multiple times:
    let key = e.keyCode;
    if((key == 37 || key== 65) && paddleLeft > 0) {
        paddleLeft -= pdx;
        if(paddleLeft < 0)
            paddleLeft = 0;
    } else if((key == 39 || key == 68) && paddleLeft < pWidth - 64) {
        paddleLeft += pdx;
        if(paddleLeft > (pWidth - 64))
            paddleLeft = pWidth - 64;
    // Moving duplicate code outside of loops: = paddleLeft + 'px';

setTimeout() vs. requestAnimationFrame

  • There are shortcomings with the usage of setTimeout() and similarly, setInterval().
    • Although convenient, they begin posing problems on mobile and lower-end devices.
    • They are resource intensive, taking up process cycles even when the animation isn't visible.
  • Both functions require a delay in milliseconds that's set by the developer before executing, or a number of frames per second.
    • When the browser can't accommodate this framerate, the animation is choppy and poor.
    • In other words, when the function is fired off, the browser isn't yet ready to redraw the screen, so the frame is skipped.
  • requestAnimationFrame, on the other hand, is a newer function that's optimized to run at the browser's peak speed.
    • This also allows browsers to suspend inactive or invisible animations.