main_thread_check.lldb

# Add this to ~/.lldbinit; Then run: `main-check SomeClass someSelector:doesThing:`
command regex main-check 's/(.+) +(.+)/expr -lobjc -- (void)__main_thread_add_check_for_selector((void*)objc_getClass("%1"), @selector(%2))/'

MainThreadCheck.m

void addMainThreadCheck(Class cls, SEL selector) {
#if DEBUG
    void *symbol = dlsym(RTLD_DEFAULT, "__main_thread_add_check_for_selector");
    if (!symbol) {
        return;
    }
    void (*addCheck)(Class, SEL) = (__typeof__(addCheck))symbol;
    addCheck(cls, selector);
#endif
}

MainThreadCheck.swift

func addMainThreadCheck(_ cls: AnyClass, _ selector: Selector) {
#if DEBUG
    let RTLD_DEFAULT = UnsafeMutableRawPointer(bitPattern: -2)
    guard let symbol = dlsym(RTLD_DEFAULT, "__main_thread_add_check_for_selector") else {
        return
    }
    typealias Signature = @convention(c) (AnyClass, Selector) -> Void
    let addCheck = unsafeBitCast(symbol, to: Signature.self)
    addCheck(cls, selector)
#endif
}

FWIW seems like this method doesn't work anymore. Registering the selector works but it never fires the checker

Thanks for commenting. Is the checker enabled in the scheme's Run > Diagnostics?

Yep. Without that you hit the return codepath