Rot.js tutorial, part 2

This is the second part of a rot.js tutorial.

The Player Character

Time to make some interesting interactive shinies! First, the player needs a decent representation. It would be sufficient to use a plain JS object to represent the player, but it is generally more robust to define the player via its constructor function and instantialize it.

By this time, you probably got used to the fact that some variable names start with an underscore. This is a relatively common technique of marking them private. JavaScript does not offer true private variables, so this underscore-based nomenclature is just our useful way of marking stuff as "internal".

We would like to place the player on some spare floor tile: let's use exactly the same technique we used in Part 1 of this tutorial to place the boxes: just pick one free location from our list.

var Player = function(x, y) {
    this._x = x;
    this._y = y;
    this._draw();
}

Player.prototype._draw = function() {
    Game.display.draw(this._x, this._y, "@", "#ff0");
}

Game.player = null;

Game._generateMap = function() {
    /* ...previous stuff... */
    this._createPlayer(freeCells);
};

Game._createPlayer = function(freeCells) {
    var index = Math.floor(ROT.RNG.getUniform() * freeCells.length);
    var key = freeCells.splice(index, 1)[0];
    var parts = key.split(",");
    var x = parseInt(parts[0]);
    var y = parseInt(parts[1]);
    this.player = new Player(x, y);
};

Preparing the game turn engine

There will be two entities taking turns in our game: the Player Character and Pedro (The Enemy). To make things simple, let's just alternate their turns evenly. But even in this simple case, we can use the ROT.Engine timing framework to our advantage.

How does this work? First, the ROT.Scheduler.Simple will be fed with all available actors - this component will take care about fair turn scheduling. The ROT.Engine will then use this scheduler to automatically call relevant actors in a loop.

It is very important to embrace the fact that everything is asynchronous in the world of client-side JavaScript: there are basically no blocking calls. This eliminates the possibility of having a simple while loop as our main timing/scheduling instrument. Fortunately, the ROT.Engine is well prepared for this.

Creating the engine is just a matter of adding a few lines to our code:

Game.engine = null;

Game.init = function() {
    var scheduler = new ROT.Scheduler.Simple();
    scheduler.add(this.player, true);
    this.engine = new ROT.Engine(scheduler);
    this.engine.start();
}

Interaction between actors and the engine

There is a tight symbiotic relationship between the engine and its actors. When running, the engine repeatedly picks a proper actor (using the scheduler) and calls the actor's act() method. Actors are allowed to interrupt this loop (when waiting asynchronously, for example) by calling ROT.Engine::lock and resume it (ROT.Engine::unlock).

It is possible to have multiple lock levels (the lock is recursive); this allows for complex chaining of asynchronous calls. Fortunately, this won't be needed in our simple game.

So, what is an actor? Any JS object with the act method.

Player.prototype.act = function() {
    Game.engine.lock();
    /* wait for user input; do stuff when user hits a key */
    window.addEventListener("keydown", this);
}

Player.prototype.handleEvent = function(e) {
    /* process user input */
}

We are using somewhat uncommon (but very useful!) technique of assigning event handlers: we pass a JS object as a second argument to the addEventListener call. Such object (this, in this case) must have the handleEvent method, which will be called once the event ("keydown") occurs.

Working with the keyboard and moving the player around

There is one last bit remaining to implement: detect the pressed key, decide whether it is valid and move the player accordingly.

Our event handler (handleEvent) gets executed with one argument: the Event object. Its keyCode property is a number code of the key being pressed. Let's create a mapping of allowed key codes (this code sample uses numpad keys, but it is trivial to extend it to other layouts as well):

var keyMap = {};
keyMap[38] = 0;
keyMap[33] = 1;
keyMap[39] = 2;
keyMap[34] = 3;
keyMap[40] = 4;
keyMap[35] = 5;
keyMap[37] = 6;
keyMap[36] = 7;

Numeric values are not chosen randomly: they correspond to directional constants in rot.js (8-topology, clockwise, starting in top-left - the same as CSS does).

We need to perform a two-step validation of user input:

1. If the key code is not present in keyMap, the user pressed a key which we cannot handle
2. If the key code is present, we need to check whether the PC can move in that direction

To convert a directional constant (0..7) to a map coordinates, we can use the ROT.DIRS set of topological diffs:

Player.prototype.handleEvent = function(e) {
    if (!(e.keyCode in direction)) { return; }

    var direction = keyMap[e.keyCode];
    var diff = ROT.DIRS[8][direction];
    var newX = this._x + diff[0];
    var newY = this._y + diff[1];

    var newKey = newX + "," + newY;
    if (!(newKey in Game.map)) { return; } /* cannot move in this direction */
}

The actual move is performed in two steps - redrawing the old position and redrawing the new position. After that, we remove our keyboard listener (the turn has ended!) and - importantly - resume the game engine (unlock()).

Player.prototype.handleEvent = function(e) {
    /* ...previous stuff... */

    Game.display.draw(this._x, this._y, Game.map[this._x+","+this._y]);
    this._x = newX;
    this._y = newY;
    this._draw();
    window.removeEventListener("keydown", this);
    Game.engine.unlock();
}

And that's all for part 2. The whole working code is available at jsfiddle.net. Feel free to continue in Rot.js tutorial, part 3.