Friday 15 June 2018

Building a retro game with React.js. Part 3 - I Like To Move It

So with most of the graphical pieces in position, it's time to make things move around.

Again, starting with the easy stuff, I wanted the four directional keys to move the Player around. But in Frenzy, you can only move (as opposed to draw) along the boundaries of the game area and on lines you have already drawn. So if we look at my first iteration of the code in GameArea to handle a request to move the Player left, it's something like this:
update = () => {
  if (this.keyListener.isDown(this.keyListener.LEFT)) {

moveLeft = () => {
  if (this.canMove(Direction.LEFT)) {
       playerX : this.state.playerX -1
I ended up bundling quite a lot of smarts into the Direction enumeration in order to make the logic less "iffy" and more declarative. That one Direction.LEFT key encapsulates everything that is needed to check whether a) the player is on a line that has the correct orientation (horizontal) and b) there is room on that line to go further to the left.
A line looks like this:
[Orientation.HORIZONTAL, 0, 0, 478, 0], // startX, startY, endX, endY
and Direction looks like this:
export const Direction = {
  LEFT: {
    orientation: Orientation.HORIZONTAL,
    primaryCoord: (x, y) => y,
    lineToPrimaryCoord: (line) => line[2],
    secondaryCoord: (x, y) => x,
    testSecondary: (c, line) => c > Math.min(line[1], line[3])

My test for whether I can move in a certain direction is:
static canPlayerMoveOnExistingLine = (playerX, playerY, direction, lines) => {
  const candidates = lines.filter(line => {
    return (line[0] === direction.orientation)
  const pri = direction.primaryCoord(playerX, playerY);
  const primaryLines = candidates.filter(candidateLine => {
    return direction.lineToPrimaryCoord(candidateLine) === pri;

  if (primaryLines.length > 0) {
    const sec = direction.secondaryCoord(playerX, playerY);
    const found = primaryLines.find(line => {
      return direction.testSecondary(sec, line);

    return typeof found !== 'undefined';
  return false;
Declared static for ease of testing - easy and well worth doing for something like this where actually moving the player around is time-consuming and tedious. It's working well as it stands, although as we all know, naming things is hard. It's pretty easy to follow the process though. At this point I'm holding a lines array in this.state and doing filter and find operations on it as you can see above. We'll have to wait and see whether that will be fast enough. It may well be a good idea to keep a currentLine in state, as most of the time it will be unchanged from the last player movement. Next up, it's time to start drawing some new lines on the screen!

I am starting to build up some tremendous respect for the original author of this game; although often dismissed as "very simple" there are some tricky little elements to coding this game and I'm only just scratching the surface. To achieve the necessary performance on an 8-bit, 1MHz processor with RAM measured in the handfuls of kilobytes is super impressive. Assembly language would have been necessary for speed, making the development and debugging a real pain. I haven't even started thinking about how to do the "fill" operation once a line has been drawn and it encloses some arbitrary space, but I suspect the original developer "sniffed" the graphics buffer to see what was at each pixel location - a "luxury" I don't think I'll have!
Time Tracking
Up to about 6 hours now.

Monday 11 June 2018

Building a retro game with React.js. Part 2 - Screens

Continuing to nibble my way towards the hard parts, I've finished the basic designs of the welcome page, main game page and the paused-game page, including wiring up the key listeners so it all works as expected.

One thing I hadn't thought about was the behaviour of listening to the P key to pause and also resume. The first iteration of the code would flicker madly between the game page and the paused page whenever the P key was pressed - the game loop runs so fast that the app would transition multiple times between the two pages, because the isDown test would simply toggle the state of this.state.paused. I messed around for a while with storing the UNIX time when the key was first pressed, and comparing it to the current time, before realising I really just needed to know what "direction" was being requested, and then waiting for this "request" to finish before completing the transition.

debounce = (testCondition, key, newState) => {
  if (testCondition) {
    if (this.keyListener.isDown(key)) {
      // No. They are still holding it
      return true;
   return false;
handleKeysWhilePaused = () => {
  if (this.debounce(this.state.pauseRequested, P, {
    pauseRequested: false,
    paused: true
  })) {

  if (this.debounce(this.state.unpauseRequested,P, {
    unpauseRequested: false,
    paused: false
  })) {

  if (this.keyListener.isDown(Q)) {

  if (this.keyListener.isDown(P)) {
    this.setState({unpauseRequested: true});

handleGameplayKeys = () => {
  if (this.keyListener.isDown(P)) {
    this.setState({pauseRequested: true});

update = () => {
  if (this.state.pauseRequested || this.state.paused) {
  } else {
Effectively I've implemented an isUp(), which is surprisingly not in the react-game-kit library. Oh well, it's all good learning.
Time tracking
I'd estimate the total time spent on the game so far would be 3-4 hours.

Friday 8 June 2018

A New Old Thing; Building a retro game with React.js. Part 1 - Background and Setup

I've blogged before about entering the fast-paced world of React.js, after a couple of years I'm still (on the whole) enjoying my day job working with it. Over this period React has done a pretty good job of delivering the "maintainable large JavaScript application" promise, but in the apps I built we've seen a few problems that stemmed from our developers' differing levels of experience with design patterns, immutability concepts, higher-order functions and higher-order components.

At the risk of being immodest, I'm comfortable with those concepts - Design Patterns from waaaay back and the functional paradigms from my five-year (and counting) love affair with Scala. What I wanted to explore was - what would happen if I built a React app by myself, endeavouring to write the cleanest, purest software based upon the best starting-point we currently have? How productive could I be? How long would it take to build a working full app? How would maintenance go? How quickly could I add a new feature?

As my day job is basically building CRUD apps, I wanted to do something a lot more fun for this side-project. And what could be more fun than a game? (Mental note: ask people working at Electronic Arts...) There's also a pleasing circularity in building a game and documenting how I did it - back in my earliest days of having a computer, aged about 7, I would buy magazines with program listings and laboriously enter them, line-by-line, while marvelling at how anyone could really have been good enough to know how to do this.

The Game
I'll admit, I've never built a non-trivial game before, but I think attempting an 8-bit home computer game I remember fondly from my childhood, on top of 2018's best front-end technologies, should be about the right level of difficulty.

The game I'll be replicating is called Frenzy; a Micro Power publication for the BBC B, Acorn Electron and Commodore 64. My machine was the Electron - basically a low-cost little brother to the mighty Beeb; highly limited in RAM and CPU, titles for this platform usually needed substantial trimming from their BBC B donor games, despite using the same BBC BASIC language.

Check out the links above for more details and screenshots, but the game is basically a simplified version of "Qix" or "Kix" where the object is to fill in areas of the screen without being hit by one or more moving enemies.

Just for the hell of it, I'm going to place this game front-and-centre on my homepage at, which I just nuked for this purpose. The page is now a React app being served off a Play Scala backend as per my new-era architecture, and the key technologies I'm using so far are: I'm sure more will follow.

Initial Development
To develop the game, I decided to start from the start. The welcome page would need to be suitably old-skool but would force me to consider a few things:
  • What screen size should I be working to?
  • Can I get a suitably chunky, monospaced font?
  • Press Space to start sounds easy, but how do I make that actually work?
The original Frenzy must have operated in the BBC's graphical MODE 1 because it used a whopping 4 colours and the pixels were square. So that means the native resolution was 320x256. While it would be tempting to stick to that screen size and thus have it fit on every smartphone screen, I've decided to double things and target a 640x512 effective canvas.
Some searching for 8-bit fonts led me to "Press Start 2P" which, while intended to honour Namco arcade machines, is near enough to the chunky fonts I remember fondly from my childhood home computer that I can't go past it:
As a tiny nod to the present, the "screen" is actually slightly transparent and has a drop shadow - showing how far we've come in 34 years!
The final piece of the welcome screen was achieved by mounting the FrenzyGame component in a React-Game-Kit Loop and using the KeyListener to subscribe to the keys I care about - a quick perusal of the demo game showed me how to use it:
class FrenzyGame extends Component {

  constructor(props) {
    this.keyListener = new KeyListener();

    this.state = {
      gameOver: true  

  componentDidMount() {
    this.loopSubscriptionId = this.context.loop.subscribe(this.update);

  componentWillUnmount() {

  update() {
    if (this.state.gameOver) {
      if (this.keyListener.isDown(this.keyListener.SPACE)) {
        this.setState({ gameOver: false });


  render() {
    return this.state.gameOver 
      ? this.renderWelcomeScreen() 
      : this.renderGame();