dist/drag.js
function Drag(parent, options) {
        _classCallCheck(this, Drag);

        options = options || {};

        var _this = _possibleConstructorReturn(this, (Drag.__proto__ || Object.getPrototypeOf(Drag)).call(this, parent));

        _this.moved = false;
        _this.wheelActive = utils.defaults(options.wheel, true);
        _this.wheelScroll = options.wheelScroll || 1;
        _this.reverse = options.reverse ? 1 : -1;
        _this.clampWheel = options.clampWheel;
        _this.factor = options.factor || 1;
        _this.xDirection = !options.direction || options.direction === 'all' || options.direction === 'x';
        _this.yDirection = !options.direction || options.direction === 'all' || options.direction === 'y';
        _this.parseUnderflow(options.underflow || 'center');
        _this.mouseButtons(options.mouseButtons);
        return _this;
    }
function Drag(parent, options) {
        _classCallCheck(this, Drag);

        options = options || {};

        var _this = _possibleConstructorReturn(this, (Drag.__proto__ || Object.getPrototypeOf(Drag)).call(this, parent));

        _this.moved = false;
        _this.wheelActive = utils.defaults(options.wheel, true);
        _this.wheelScroll = options.wheelScroll || 1;
        _this.reverse = options.reverse ? 1 : -1;
        _this.clampWheel = options.clampWheel;
        _this.factor = options.factor || 1;
        _this.xDirection = !options.direction || options.direction === 'all' || options.direction === 'x';
        _this.yDirection = !options.direction || options.direction === 'all' || options.direction === 'y';
        _this.parseUnderflow(options.underflow || 'center');
        _this.mouseButtons(options.mouseButtons);
        return _this;
    }
enable one-finger touch to drag
@private
@param {Viewport} parent
@param {object} [options]
@param {string} [options.direction=all] direction to drag (all, x, or y)
@param {boolean} [options.wheel=true] use wheel to scroll in y direction (unless wheel plugin is active)
@param {number} [options.wheelScroll=1] number of pixels to scroll with each wheel spin
@param {boolean} [options.reverse] reverse the direction of the wheel scroll
@param {boolean|string} [options.clampWheel] (true, x, or y) clamp wheel (to avoid weird bounce with mouse wheel)
@param {string} [options.underflow=center] (top/bottom/center and left/right/center, or center) where to place world if too small for screen
@param {number} [options.factor=1] factor to multiply drag to increase the speed of movement
@param {string} [options.mouseButtons=all] changes which mouse buttons trigger drag, use: 'all', 'left', right' 'middle', or some combination, like, 'middle-right'