Request animation frame

define(function () {
    function Gomo(options, callback) {
        if (typeof options === 'function') {
            callback = options;
            options = {};
        }

        var duration = options.duration || 500;
        var easing = this.easing[options.easing || 'easeInOut'];
        var start = null;
        var finish = null;
        var that = this;
        var isRunning = this.isRunning = false;

        function step(timestamp) {
            if (start === null) { 
                start = timestamp;
                finish = start + duration;
                isRunning = true;
            }
            var elapsed = timestamp - start;
            var fraction = timestamp > finish ? 1 : elapsed / duration;
            callback(easing(fraction, elapsed, duration), that);

            if (fraction < 1) {
                that.requestId = requestAnimationFrame(step);
            } else {
                isRunning = false;
                if (options.finish) {
                    options.finish();
                }
            }
        }

        this.requestId = requestAnimationFrame(step);
    }

    Gomo.prototype.stop = function() {
        if (this.requestId) {
            cancelAnimationFrame(this.requestId);
        }
    };

    Gomo.prototype.easing = {
        // fraction, elapsed, duration
        'linear': function(x) { return x; },
        'easeInOut': function(x) { return (-Math.cos(x * Math.PI) / 2) + 0.5; },
        'easeOutBounce': function (x, t, d) {
            if ((t/=d) < (1/2.75)) {
                return (7.5625*t*t);
            } else if (t < (2/2.75)) {
                return (7.5625*(t-=(1.5/2.75))*t + 0.75);
            } else if (t < (2.5/2.75)) {
                return (7.5625*(t-=(2.25/2.75))*t + 0.9375);
            } else {
                return (7.5625*(t-=(2.625/2.75))*t + 0.984375);
            }
        }
    };

    return Gomo;
});