# Canvas spatial grid collision

You can see a working example here. Be sure to click on the canvas or the key inputs will not be detected!

Are there any ways I could improve upon this? Anything that should have been done differently? Any best practices I have not implemented?

$(document).ready(function() { window.addEventListener('keydown', keyPress, true); // event listener for keyboard presses function keyPress(evt) { if(engine.activeBlock) { // block is actively falling switch(evt.keyCode) { case 37: // left arrow engine.activeBlock.keyLeft(); break; case 39: // right arrow engine.activeBlock.keyRight(); break; case 40: // down arrow engine.activeBlock.keyDown(); break; } } } // block object function block() { this.xStart = function() { // random x coordinate of starting position of block var x = Math.floor(Math.random() * (engine.cellsX - 1 + 1)) + 1; // random number between 1 and cellsX return (x - 1) * engine.cellSize; }; this.x = this.xStart(); // x coordinate this.y = 0 - (engine.cellSize * 1.5); // y coordinate this.velocity = 1; // velocity of the block this.generateColor = function() { // generates a random color var colors = ['blue', 'red', 'green', 'black', 'orange']; // colors array return colors[Math.floor(Math.random() * colors.length)]; }; this.color = this.generateColor(); // color of the block this.keyLeft = function() { // key left if(!engine.collisionLeft()) { // check collisions left side this.x -= engine.cellSize; } }; this.keyRight = function() { // key right if(!engine.collisionRight()) { // check collisions right side this.x += engine.cellSize; } }; this.keyDown = function() { // key down if(this.y > 0) { // block is on screen this.velocity += 1; // increase block velocity } }; this.update = function() { // updates the blocks position this.y += this.velocity; // increase velocity engine.collisionBelow(); // check collisions below ctx.fillStyle = this.color; ctx.fillRect(this.x, this.y, engine.cellSize, engine.cellSize); }; } // engine object function engine() { this.cellSize = 100; // size of each cell this.cellsX = canvas.width / this.cellSize; // number of cells along the x axis this.cellsY = canvas.height / this.cellSize; // number of cells along the y axis this.totalCells = this.cellsX * this.cellsY; // total cells this.buildGrid = function() { // builds grid var arr = new Array(this.cellsX); for(var i = 0; i < arr.length; ++i) { arr[i] = new Array(this.cellsY); } return arr; }; this.grid = this.buildGrid(); // holds grid this.activeBlock = false; // where a falling block is placed this.getGridCoords = function(i) { // x coordinate of where a block should be placed in grid return Math.floor(i / this.cellSize); }; this.drawGrid = function() { // outputs the blocks stored in grid for(var i = 0; i < this.grid.length; ++i) { for(var i2 = 0; i2 < this.grid[i].length; ++i2) { if(this.grid[i][i2]) { // grid cell is occupied ctx.fillStyle = this.grid[i][i2].color; ctx.fillRect(this.grid[i][i2].x, this.grid[i][i2].y, this.cellSize, this.cellSize); } } } }; this.collisionBelow = function() { // check collisions below if(this.activeBlock.y > (canvas.height - this.cellSize)) { this.insertBlock(); // insert block into grid } else if(this.grid[this.getGridCoords(this.activeBlock.x)][this.getGridCoords(this.activeBlock.y) + 1]) { this.insertBlock(); // insert block into grid } }; this.collisionLeft = function() { // check collisions left if(this.activeBlock.x <= 0) { return true; } else if(this.grid[this.getGridCoords(this.activeBlock.x) - 1][this.getGridCoords(this.activeBlock.y)]) { return true; } else if(this.grid[this.getGridCoords(this.activeBlock.x) - 1][this.getGridCoords(this.activeBlock.y) - 1]) { return true; } else if(this.grid[this.getGridCoords(this.activeBlock.x) - 1][this.getGridCoords(this.activeBlock.y) + 1]) { return true; } }; this.collisionRight = function() { // check collisions right if(this.activeBlock.x >= (canvas.width - this.cellSize)) { return true; } else if(this.grid[this.getGridCoords(this.activeBlock.x) + 1][this.getGridCoords(this.activeBlock.y)]) { return true; } else if(this.grid[this.getGridCoords(this.activeBlock.x) + 1][this.getGridCoords(this.activeBlock.y) - 1]) { return true; } else if(this.grid[this.getGridCoords(this.activeBlock.x) + 1][this.getGridCoords(this.activeBlock.y) + 1]) { return true; } }; this.insertBlock = function() { // insert block into grid this.activeBlock.x = this.cellSize * Math.floor(this.activeBlock.x / this.cellSize); // floor x coordinate to cellSize multiple this.activeBlock.y = this.cellSize * Math.floor(this.activeBlock.y / this.cellSize); // floor y coordinate to cellSize multiple this.grid[this.getGridCoords(this.activeBlock.x)][this.getGridCoords(this.activeBlock.y)] = this.activeBlock; this.activeBlock = new block(); }; } var canvas =$('canvas')[0]; // holds the canvas
var ctx = canvas.getContext('2d'); // holds the canvas context
var engine = new engine(); // initiate the engine
engine.activeBlock = new block(); // add block

function loop() { // game loop
ctx.fillStyle = '#eee';
ctx.fillRect(0, 0, canvas.width, canvas.height);
engine.activeBlock.update(); // update active block
engine.drawGrid(); // draw the grid of blocks
}

// run loop
setInterval(loop, 20);
});


Haven't looked at the code in detail, but here are some thoughts on the overall structure:

Right now, there's some mixing of concerns and tight coupling going on. The engine calls update on the active block, but the block then calls back to the engine to check for collisions. In other words, the two are tightly coupled; they're dependent on each other, and its unclear where certain responsibilities lie.

The block is a pretty simple object, which can be reduced to simply being an x, y position, and a velocity. And it should probably stop there. The engine would be responsible for moving the block (the block object can do the calculations, though), checking collisions, accepting keyboard input, etc.. I.e. keep the block "dumb"; it doesn't need to know its context.

I'd also suggest making a similarly "dumb" grid object that the engine object can interrogate and instruct. I.e. the grid doesn't move blocks around, it simply keeps track of them for the engine. The grid could check for collisions, but it'd be up to the engine to ask.

Where exactly to draw the boundaries between the objects and their responsibilities is up to you (there are many ways to break it down), but the idea is to keep things decoupled when possible. I.e. blocks don't rely on a grid or an engine being there; the grid may or may not care about blocks being blocks, just that there's something in a given cell, etc.. Perhaps there are more things, you can extract and/or abstract and encapsulate in objects or constructors, while the engine sits in the middle acting as controller.

On a purely syntactical level, I'd move the methods of into the prototypes for the objects. That is, from this

function block() {
// instance variables ...
this.update = function () { ... }
}


to this

function Block() {  // CamelCase since it's a constructor
// instance variables ...
}

Block.prototype.update = function () {
...
};


This will make it easier to do prototypal inheritance, as the methods will actually be prototypal.

• Thank you this has helped! This is my first ever game so I just tried to throw something together. I will try and improve on it. – ShaShads Feb 25 '13 at 9:55