Tutorials

Making a Simple Game - Part 3

You can find the full project here. If you haven't see Part 1 and Part 2 read them first.

The Game script & Input

These two scripts game.js and input.js are attached the root entity in the scene, called "Game". Scripts are generally executed in the order they are encountered in the hierarchy so it's easiest to attach any non-Entity specific scripts the first Entity. You can also manage the loading order of Scripts in the Settings panel of the Editor, to load scripts first without attaching them to an Entity.

game.js

var Game = pc.createScript('game');

Game.attributes.add('uiMenu', {type: 'entity'});
Game.attributes.add('uiInGame', {type: 'entity'});
Game.attributes.add('uiGameOver', {type: 'entity'});
Game.attributes.add('audio', {type: 'entity'});

Game.STATE_MENU = 'menu';
Game.STATE_INGAME = 'ingame';
Game.STATE_GAMEOVER = 'gameover';

// initialize code called once per entity
Game.prototype.initialize = function() {
    this._state = Game.STATE_MENU;
    this._score = 0;

    this.setResolution();

    window.addEventListener("resize", this.setResolution.bind(this));

    // listen to events from the UI
    this.app.on("ui:start", this.start, this);
    this.app.on("ui:reset", this.reset, this);
};

Game.prototype.setResolution = function () {
    // if the screen width is less than 640
    // fill the whole window
    // otherwise
    // use the default setting

    var w = window.screen.width;
    var h = window.screen.height;

    if (w < 640) {
        this.app.setCanvasResolution(pc.RESOLUTION_AUTO, w, h);
        this.app.setCanvasFillMode(pc.FILLMODE_FILL_WINDOW);
    }
};

// Call this to move from MENU to INGAME
Game.prototype.start = function () {
    this._state = Game.STATE_INGAME;
    this.app.fire("game:start");
    this.uiMenu.enabled = false;
    this.uiInGame.enabled = true;

    this.audio.sound.play("music");
};

// Call this to move from INGAME to GAMEOVER
Game.prototype.gameOver = function () {
    this._state = Game.STATE_GAMEOVER;
    this.app.fire("game:gameover");
    this.uiInGame.enabled = false;
    this.uiGameOver.enabled = true;

    this.audio.sound.stop();
    this.audio.sound.play("gameover");
};

// Call this to move from GAMEOVER to MENU
Game.prototype.reset = function () {
    this.app.fire("game:reset");
    this.resetScore();
    this._state = Game.STATE_MENU;
    this.uiGameOver.enabled = false;
    this.uiMenu.enabled = true;

    this.audio.sound.stop();
};

// return the current score
Game.prototype.getScore = function () {
    return this._score;
};

// add a value to the score
Game.prototype.addScore = function (v) {
    this._score += v;
    this.app.fire("game:score", this._score);
};

// reset the score
Game.prototype.resetScore = function () {
    this._score = 0;
    this.app.fire("game:score", this._score);
};

Game State

The game script manages the overall state of the game, it exposes some methods to alter the game state and fires events to alert other code that the game state has changed.

We've divided the game up into three main states: Menu, In Game and Game Over. The game script provides the methods to transition between each state, start(), gameOver() and reset(). Each one sets the _state variable to remember which state we're in; fires an application event to alert other scripts to state changes; switches user interface elements on and off; and manages the state of the music or game over sound effect.

These state change methods will be called from other scripts when the appropriate trigger events occur. For example, the gameOver() method is called by ball.js when the ball goes off the bottom of the screen.

Application Events

Let's pause to take a look at the way the game script fires events on the application.

this.app.fire("game:start")

Events are an extremely useful way to communicate form one script to many other scripts. The way an event works is that an object (in this case this.app) chooses to "fire" an event. Any other code that has access to the object can choose to listen to one or more events on this object and the code will be notified when the event is fired.

One of the issues with this is that the code needs access to the object in order to start listening to the event. This is why application events are so useful. Every script in PlayCanvas has access to this.app. That makes it useful to act as a central communications hub between any other scripts.

We have chosen to adopt a namespace pattern to make events clearer and avoid clashes. To listen for the game:start event from above. You would use this code:

this.app.on("game:start", function () {
    console.log("game:start event was fired");
}, this)

Scoring

The game script also manages the current score. It exposes methods that are used to modify the score and also fires events to let other code know that the score has changed.

Resolution

Finally the game script handles the initial choice of resolution to make sure the main canvas is the correct size on both mobile and desktop. On mobile (defined by a screen less than 640 pixels wide) the game simply fills the entire screen. On desktop we use the predefined resolution set in the project settings.

input.js

The input script listens for input from the mouse or touchscreen, normalizes the input from the two into a general purpose "tap" and communicates with the rest of the application that a tap has occurred.

var Input = pc.createScript('input');

Input.attributes.add('ball', {type: 'entity'});
Input.attributes.add('camera', {type: 'entity'});
Input.attributes.add('ballRadius', {type: 'number', default: 0.5});

Input.worldPos = new pc.Vec3();

// initialize code called once per entity
Input.prototype.initialize = function() {

    var self = this;

    this._paused = true;

    // Listen for game events so we know whether to respond to input
    this.app.on("game:start", function () {
        self._paused = false;
    });
    this.app.on("game:gameover", function () {
        self._paused = true;
    });

    // set up touch events if available
    if (this.app.touch) {
        this.app.touch.on("touchstart", this._onTouchStart, this);
    }

    // set up mouse events
    this.app.mouse.on("mousedown", this._onMouseDown, this);
};

Input.prototype._onTap = function (x, y) {
    var p = this.ball.getPosition();
    var camPos = this.camera.getPosition();
    var worldPos = Input.worldPos;

    // Get the position in the 3D world of the touch or click
    // Store the in worldPos variable.
    // This position is at the same distance away from the camera as the ball
    this.camera.camera.screenToWorld(x, y, camPos.z - p.z, worldPos);

    // get the distance of the touch/click to the ball
    var dx = (p.x - worldPos.x);
    var dy = (p.y - worldPos.y);

    // If the click is inside the ball, tap the ball
    var lenSqr = dx*dx + dy*dy;
    if (lenSqr < this.ballRadius*this.ballRadius) {
        this.ball.script.ball.tap(dx, dy);
    }
};

Input.prototype._onTouchStart = function (e) {
    if (this._paused) {
        return;
    }

    // respond to event
    var touch = e.changedTouches[0];
    this._onTap(touch.x, touch.y);

    // stop mouse events firing as well
    e.event.preventDefault();
};

Input.prototype._onMouseDown = function (e) {
    if (this._paused) {
        return;
    }

    // respond to event
    this._onTap(e.x, e.y);
};

First, in initialize we are setting up event listening. We listen for application events to determine if the game is in a paused state (that is in the menu or in the game over state). If the input is paused we don't respond to the taps. Next we listen for touch events (note, you must check if this.app.touch is available) and mouse events.

Touch Events

For touch events we take the first touch and pass through the screen co-ordinates. We also call preventDefault() on the browser event to stop the browser also generating a click event which it will do otherwise.

Mouse Events

On "mousedown" events we pass the screen co-ordinates through to the tap code. Note, that PlayCanvas ensures that touch and mouse events have the same coordinate system. This is not the case with normal browser events!

Taps

_onTap() takes a screen co-ordinate (x, y) works out if this has "hit" the ball and if so tells the ball code that it has been tapped.

this.camera.camera.screenToWorld(x, y, camPos.z - p.z, worldPos);

In detail, this function takes the screen co-ordinates (x, y) and asks the camera to convert them into a position in 3D space under that point on the screen. To do this, we need to supply a depth, as in how far away from the screen do you want the 3D point. In this case we get the 3D point at the same depth as the ball is.

We also pass in a vector Input.worldPos. It's important in PlayCanvas applications to avoid creating new objects, like calling new pc.Vec3() to create a new vector, in your update loops. The more memory allocations you do (by calling new) the more Garbage Collection the browser will have to do to clear up your allocations. Garbage Collection is a (comparitively slow) operation and will cause your game or application to stutter if it happens often.

In most cases, PlayCanvas will provide an option to pass in vector or similar option so that you can pre-allocate and re-use objects.

// get the distance of the touch/click to the ball
var dx = (p.x - worldPos.x);
var dy = (p.y - worldPos.y);

// If the click is inside the ball, tap the ball
var lenSqr = dx*dx + dy*dy;
if (lenSqr < this.ballRadius*this.ballRadius) {
    this.ball.script.ball.tap(dx, dy);
}

Once we have the the 3D point where we've just tapped, we test to see if it is overlapping with the ball. You'll see here we are testing the radius squared against the distance between the tap and the ball squared. This prevents us doing a slow Square Root operation everytime we test.

If the tap has hit the ball, we call the tap(dx, dy) function on the ball script we pass in the distance from the ball where the tap occurred. We'll use that in the Part 4.