Swift 4 Access Control(访问控制)Open, Public, Internal, Fileprivate, Private
Accesscontrolrestrictsaccesstopartsofyourcodefromcodeinothersourcefilesandmodules.Thisfeatureenablesyoutohidetheimplementationdetailsofyourcode,andtospecifyapreferredinterfacethroughwhichthatcodecanbe