Implement a private API in Swift

Using Swift
3

There's an experimental annotation you can apply to public methods, @_spi.

public struct MyStruct {
  public init() {}

  @_spi(Private) public func myPrivateFunc() {}
}

Clients that just import ThisFramework will have access to MyStruct and MyStruct.init, but if they want to call myPrivateFunc, they need to annotate their import with a corresponding @_spi annotation with the same Private "SPI tag"

@_spi(Private) import ThisFramework

let s = MyStruct()
s.myPrivateFunc()

You can use whatever SPI tag you'd like, and you can use as many as you'd like within the module.

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