code: function _wrapReadyCallback(onready, target) {
              return function(err) {
                  if (target && "function" === typeof target.callback) {
                      target.callback.call(target.context, err, this.target);
                  }
                  if (onready) {
                      if ("function" === typeof onready) {
                          onready(err, this.target);
                      }
                      else if ("function" === typeof onready.callback) {
                          onready.callback.call(onready.context, err, this.target);
                      }
                  }
              };
          }
docs: Method for wrapping the callback of iframe ready
  @param {Function} [onready] - the handler for iframe ready
  @param {Object} [target] - the target iframe configuration
  @returns {Function} handler function for messages
  @private