handleTouchEventWith(ev, handler) {
        for (let i = 0; i < ev.changedTouches.length; i++) {
            let touch = ev.changedTouches[i];
            handler.call(this, this.relativeEventPos(touch), touch.identifier, ev);
        }
    }
@param {!TouchEvent} ev
@param {!function(!Point, *, !MouseEvent|!TouchEvent) : void} handler