code: function getMaxZoomRatio(pointers) {
    var pointers2 = assign({}, pointers);
    var ratios = [];
    forEach(pointers, function (pointer, pointerId) {
      delete pointers2[pointerId];
      forEach(pointers2, function (pointer2) {
        var x1 = Math.abs(pointer.startX - pointer2.startX);
        var y1 = Math.abs(pointer.startY - pointer2.startY);
        var x2 = Math.abs(pointer.endX - pointer2.endX);
        var y2 = Math.abs(pointer.endY - pointer2.endY);
        var z1 = Math.sqrt(x1 * x1 + y1 * y1);
        var z2 = Math.sqrt(x2 * x2 + y2 * y2);
        var ratio = (z2 - z1) / z1;
        ratios.push(ratio);
      });
    });
    ratios.sort(function (a, b) {
      return Math.abs(a) < Math.abs(b);
    });
    return ratios[0];
  }
docs: Get the max ratio of a group of pointers.
  @param {string} pointers - The target pointers.
  @returns {number} The result ratio.