How to access an internal Swift class in Objective-C within the same framework?

For me it just worked by checking: "Allow app extension API only". You find it by going to the project setting, select your target and then it is in the General tab under Deployment Info.

Can someone explain to me, why this does solve the problem?

As indicated in the docs, declarations marked with internal modifier don't appear in the generated header, so the compiler does not know about them and thus complaints. Of course, you could send messages using performSelector approach, but that's not convenient and bug-prone. We just need to help the compiler know that those declarations are there.

First, we need to use @objc attribute variant that allows you to specify name for your symbol in Objective-C:

// SwiftObject.swift

@objc(SWIFTYetAnotherSwiftObject)
internal class YetAnotherSwiftObject: NSObject {
    internal class func doSomething() {}
}

And then you just need to create @interface declaration with the methods you want to use in your code - so the compiler will be happy, and also apply SWIFT_CLASS macro with the symbol name you've specified earlier - so the linker would pick the actual implementation:

// SomeObject.m file

SWIFT_CLASS("SWIFTYetAnotherSwiftObject")
@interface YetAnotherSwiftObject : NSObject

+ (void)doSomething;

@end


@implementation SomeObject

- (void)someOtherMethod {
    [YetAnotherSwiftObject doSomething]; // Should work now !!!
}

@end

• I've used the interface declaration in .m file just for clarity, the better option would be to combine such declarations in .h file, and include it.
• By declaring methods in that interface we're making a promise to compiler, and it won't complain if you'll put there a method that does not exist (or with wrong signature, etc.) Obviously, you'll crash in runtime in that case - so be cautious.