open是Swift 3中的新访问级别，随实现一起引入 的

open is a new access level in Swift 3, introduced with the implementation of

• SE-0117允许区分公共访问和公共可覆盖性

自2016年8月7日起，Swift 3快照即可提供该功能， 以及Xcode 8 beta 6.

It is available with the Swift 3 snapshot from August 7, 2016, and with Xcode 8 beta 6.

简而言之:

• open类可在其外部可访问和可子类 定义模块. open类成员可以访问 ，并且 定义模块之外的 overridable .
• public类可可访问，但不可子类 定义模块. public类成员可以访问 ，但是 在定义模块之外的不可覆盖.

• An open class is accessible and subclassable outside of the defining module. An open class member is accessible and overridable outside of the defining module.
• A public class is accessible but not subclassable outside of the defining module. A public class member is accessible but not overridable outside of the defining module.

所以open是public以前的版本 Swift版本和public的访问受到限制. 或者，就像克里斯·拉特纳(Chris Lattner)所说的那样 SE-0177:允许区分公共访问权限和公共可重写权限:

So open is what public used to be in previous Swift releases and the access of public has been restricted. Or, as Chris Lattner puts it in SE-0177: Allow distinguishing between public access and public overridability:

开放"现在只是比公众更公开"，提供了一个非常简单和干净的模型.

"open" is now simply "more public than public", providing a very simple and clean model.

在您的示例中，open var hashValue是可访问的属性，可以在NSObject子类中覆盖.

In your example, open var hashValue is a property which is accessible and can be overridden in NSObject subclasses.

有关更多示例和详细信息，请参阅

For more examples and details, have a look at SE-0117.