code: function(opacity) {
                  this.callBase({
                      beginValue: function(target) {
                          return target.getOpacity();
                      },
                      finishValue: opacity,
                      setter: function(target, value) {
                          target.setOpacity(value);
                      }
                  });
              }
docs: @constructor
  @for kity.OpacityAnimator
  @grammar new kity.OpacityAnimator(opacity)
  
  @param  {Number} opacity 目标透明度，取值范围 0 - 1