How to represent ObjC enum AVAudioSessionPortOverride which has declaration of int and string using Dart ffi? - objective-c

Question

I'm working on a cross platform sound API for Flutter.
We're trying to stop using Objective C/Swift for the iOS portion of the API and we're using Dart ffi as a replacement.
ffi(foreign function interface) allows dart to call into an Obj C API.
This means we need to create a dart library which wraps the Obj C audio library.
Whilst doing this we encountered the AVAudioSessionPortOverride enum which has two declarations; AVAudioSessionPortOverrideSpeaker = 'spkr' and AVAudioSessionPortOverrideNone = 0.
I'm confused as to what's going on here as one of these declarations is an int whilst the other is a string.
I note that AVAudioSessionPortOverride extends an NSUInteger so how is the string being handled. Is it somehow being converted to an int? if so any ideas on how I would do this in dart?
Here's what we have so far:
class AVAudioSessionPortOverride extends NSUInteger {
const AVAudioSessionPortOverride(int value) : super(value);
static AVAudioSessionPortOverride None = AVAudioSessionPortOverride(0);
static const AVAudioSessionPortOverride Speaker =
AVAudioSessionPortOverride('spkr');
}

Answer 1

'spkr' is in fact an int. See e.g. How to convert multi-character constant to integer in C? for an explanation of how this obscure feature in C works.
That said, if you look at the Swift representation of the PortOverride enum, you'll see this:
/// For use with overrideOutputAudioPort:error:
public enum PortOverride : UInt {
/// No override. Return audio routing to the default state for the current audio category.
case none = 0
/// Route audio output to speaker. Use this override with AVAudioSessionCategoryPlayAndRecord,
/// which by default routes the output to the receiver.
case speaker = 1936747378
}
Also, see https://developer.apple.com/documentation/avfoundation/avaudiosession/portoverride/speaker
Accordingly, 0 and 1936747378 are the values you should use.

Answer 2

Look at this
NSLog(#"spkr = %x s = %x p = %x k = %x r = %x", 'spkr', 's', 'p', 'k', 'r' );
Apple is doing everything your lecturer warned you against. You can get away with this since the string is 4 chars (bytes) long. If you make it longer you'll get a warning. The string gets converted to an int as illustrated in the code snippet above. You could reverse it by accessing the four bytes one by one and printing them as a character.
Spoiler - it will print
spkr = 73706b72 s = 73 p = 70 k = 6b r = 72