onDown(pt, id, ev) {
        if (!isLeftClicking(ev)) {
            return;
        }
        if (this._grabPointerId !== undefined) {
            if (!this.canRegrab()) {
                return;
            }

            this._dropHandler(this._lastPos, this._lastEv);
        }

        this._grabPointerId = id;
        this._grabActivityTime = window.performance.now();
        this._lastPos = pt;
        this._lastEv = ev;

        this._grabHandler(pt, ev);
    }
@param {!Point} pt
@param {*} id
@param {!MouseEvent|!TouchEvent} ev