Swift native base class or NSObject

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Swift classes that are subclasses of NSObject:

  • are Objective-C classes themselves
  • use objc_msgSend() for calls to (most of) their methods
  • provide Objective-C runtime metadata for (most of) their method implementations

Swift classes that are not subclasses of NSObject:

  • are Objective-C classes, but implement only a handful of methods for NSObject compatibility
  • do not use objc_msgSend() for calls to their methods (by default)
  • do not provide Objective-C runtime metadata for their method implementations (by default)

Subclassing NSObject in Swift gets you Objective-C runtime flexibility but also Objective-C performance. Avoiding NSObject can improve performance if you don’t need Objective-C’s flexibility.

Edit:

With Xcode 6 beta 6, the dynamic attribute appears. This allows us to instruct Swift that a method should use dynamic dispatch, and will therefore support interception. 

public dynamic func foobar() -> AnyObject {
}

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