How to represent an optional Bool (Bool?) in Objective-C?

Question

I am trying to write a protocol in swift

@objc protocol RestAPIManagerDelegate {

      optional func credentialValidated(isValid: Bool?)
}

But I am getting following error:

'Method cannot be marked @objc because the type of the parameter cannot be represented in Objective-C'

Any suggestion?

Answer 1

It's OK to use NSNumber? instead, but there is another way. You can create OptionalBool enum and use it instead of Bool? for compatibility with Objective-C.

Creating OptionalBool enum in Swift code:

@objc enum OptionalBool: Int {
    case none
    case yes
    case no
}

@objc protocol RestAPIManagerDelegate {

    @objc optional func credentialValidated(isValid: OptionalBool)
    
}

Using OptionalBool in Objective-C code:

@interface RestAPIManagerHandler () <RestAPIManagerDelegate>
@end

@implementation RestAPIManagerHandler

- (void)credentialValidatedWithIsValid:(enum OptionalBool)isValid {
    switch (isValid) {
        case OptionalBoolYes:
            NSLog(@"TRUE");
            break;
            
        case OptionalBoolNo:
            NSLog(@"FALSE");
            break;
            
        case OptionalBoolNone:
            NSLog(@"NULL");
            break;
    }
}

@end

Answer 2

The problem is this type declaration:

`isValid: Bool?`

That is perfectly fine in Swift. But you cannot expose it to Objective-C, because Objective-C does not have any notion of an Optional BOOL - a BOOL in Objective-C is basically just a number, a primitive C datatype (what we call a scalar).

Here's another way of looking at it. In an interchange with Objective-C, you can use a Swift Optional anywhere that Objective-C can say nil - indeed, to a large extent Swift Optional exists exactly in order to deal with the possibility that Objective-C will say nil or that you might need to say nil to Objective-C. But a BOOL in Objective-C can never be nil - it can only be YES or NO (Swift true or false).

So you have three choices:

• Take away the @objc that exposes all this to Objective-C

• Remove the Optional and just declare that type a Bool

• Use an object type. For example, declare the type as AnyObject? (or NSNumber?). This will work because Swift will bridge a Bool to an NSNumber (including as it passes into an AnyObject), and Objective-C will deal just fine with an Optional AnyObject or Optional NSNumber because those are object types, not scalars.