function keydown(event) {

        if (_this.enabled === false) return;

        window.removeEventListener('keydown', keydown);

        _prevState = _state;

        if (_state !== STATE.NONE) {

            return;

        } else if (event.keyCode === _this.keys[STATE.ROTATE] && !_this.noRotate) {

            _state = STATE.ROTATE;

        } else if (event.keyCode === _this.keys[STATE.ZOOM] && !_this.noZoom) {

            _state = STATE.ZOOM;

        } else if (event.keyCode === _this.keys[STATE.PAN] && !_this.noPan) {

            _state = STATE.PAN;

        }

    }
@author Eberhard Graether / http://egraether.com/
@author Mark Lundin 	/ http://mark-lundin.com
@author Simone Manini / http://daron1337.github.io
@author Luca Antiga 	/ http://lantiga.github.io