Shimming in Swift

According to Apple engineer Elizabeth Reid "shimming" is when you use conditional compilation to reuse code between iOS and OS X. For example:

#if TARGET_OS_IPHONE
@import UIKit;
#define BaseView UIView
#else
@import AppKit;
#define BaseView NSView
#endif

@interface MyView : BaseView

@end


In the WWDC 2014 session [Sharing code between iOS and OS X][1] she also states:

> If you literally translate how you would shim with Objective-C, this
> will not compile in Swift.
>
> There are ways to shim your code in Swift.
>
> But it gets more complicated than your basic conditional compilation
> that we can use in Objective-C.

So, which are the ways to "shim your code" in Swift?


[1]:

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that would look like this in _Swift_:

#if os(iOS)
import UIKit
typealias BaseClass = UIView
#else
import AppKit
typealias BaseClass = NSView
#endif

//

class MyClass : BaseClass {

// ...

}