Your .swift files must import frameworks in order to use them, but we don't necessarily need to import the whole of Cocoa.
If you have, for example, a model class that has no need of interacting with any UI elements (and a model class should certainly not be interacting with UI elements), you don't need to import Cocoa, Foundation will suffice:
import Foundation
class MyFoundationClass: NSObject
{
var someDate = Date()
} So here, MyFoundationClass is using the NSObject class and the Date class, both of which require Foundation. If you comment out the import statement, you'll get two errors, with the compiler complaining it doesn't know what those two classes are:
This is in contrast to the MySwiftClass we created previously. That class declared only an Int property, which is part of Swift, not Foundation, and also declared no Foundation superclass. So there was no need to import Foundation.
If our class needs to use user interface classes, which are not included in Foundation...