MyPrivateClass.h

// MyPrivateClass.h

@interface MyPrivateClass: NSObject

- (void) doSomethingInternalWithSecretAttribute:(NSInteger)attribute;

@end

MyPublicClass.swift

// MyPublicClass.swift

public class MyPublicClass {
    private let privateClass: MyPrivateClass // We need private ObjC member

    ...

    public func doSomething() {
        privateClass.doSomethingInternal(withSecretAttribute: 314)
    }
}