code: function handleDoubleTap(evt) {
          var zoomFactor = zoomFactor || 2;
          var node = zoomNode;
          var pos = getPosition(node);
          currentScale = currentScale == 1 ? zoomFactor : 1;
          node.style.webkitTransform = generateTranslate(0, 0, 0, currentScale);
          if (currentScale != 1) node.style.webkitTransformOrigin = generateTransformOrigin(evt.touches[0].pageX - pos.left, evt.touches[0].pageY - pos.top);
      }
docs: Double tao handle
  @param {object} evt