iOS overriding a stored property

[grandM]

I wondered why a stored property can only be overriden by the use of a getter?

For instance this code would work:

override var supportedInterfaceOrientations: UIInterfaceOrientationMask {
        return .all
    }

but this would not work:

override var supportedInterfaceOrientations: UIInterfaceOrientationMask = .all

I also read that UIInterfaceOrientationMask is depreciated and UITraitCollection and UITraitEnvironment APIs should be used instead. Does anyone know how these are being used to enable all interfaceOrientations?

What is a bit mask by the way?

Thanks for the insights!