Support downcasting

[tmandry]

I just had to write this.. monstrosity!!

fn ns_screens(&self) -> Vec<NSScreenInfo> {
    NSScreen::screens(self.mtm)
        .iter()
        .flat_map(|s| {
            let desc = s.deviceDescription();
            let cg_id = match desc.get(ns_string!("NSScreenNumber")) {
                Some(val) if unsafe { msg_send![val, isKindOfClass:NSNumber::class() ] } => {
                    let number: &NSNumber = unsafe { std::mem::transmute(val) }; // :(
                    number.as_u32()
                }
                val => {
                    warn!(
                        "Could not get NSScreenNumber for screen with name {:?}: {:?}",
                        unsafe { s.localizedName() },
                        val,
                    );
                    return None;
                }
            };
            Some(NSScreenInfo {
                frame: s.frame(),
                visible_frame: s.visibleFrame(),
                cg_id,
            })
        })
        .collect()
}

Original attempt (segfault)

fn ns_screens(&self) -> Vec<NSScreenInfo> {
    NSScreen::screens(self.mtm)
        .iter()
        .flat_map(|s| {
            let value = s
                .deviceDescription()
                .get(ns_string!("NSScreenNumber"))
                .map(|val: &AnyObject| -> &'_ CFType { unsafe { std::mem::transmute(val) } });
            //  oh, nooo                               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
            let number = value.and_then(|v| v.downcast::<CFNumber>());
            let Some(cg_id) = number else {
                return None;
            };
            Some(NSScreenInfo {
                frame: s.frame(),
                visible_frame: s.visibleFrame(),
                cg_id: cg_id.to_i32().unwrap().try_into().unwrap(), // :(
            })
        })
        .collect()
}

CFType and CFNumber are from core_foundation, which supports downcasting via type id. Its CFNumber interface is weirdly incomplete though, hence all the unwraps.

EDIT: I thought this would work because of toll-free bridging, but it segfaults. So I went with the approach in #474, which is a bit nicer.

I see this is being worked on in #474 but thought this would lend some motivation.

Thanks for your work on this crate!

[madsmtm]

Heh, yeah, I have to do that dance in Winit too, honestly it's mostly because the deviceDescription API here is terrible.

But I hear what you're saying, and will probably use this issue to track progress, instead of the PR.

I thought this would work because of toll-free bridging, but it segfaults

Yeah, that's because you used the wrong core-foundation type; you can't convert to &'_ CFType, that is basically the same as &Id<AnyObject> - instead, you should've returned CFType directly, but done the conversion using something like CFType::wrap_under_get_rule(val as *const AnyObject as *const c_void) (core-foundation doesn't actually seem to expose a way to do this without the extra retain/release).