Better_trampoline.swift

@inlinable func trampolineToMain(_ function: @escaping @autoclosure ()->()) -> Bool {
    if Thread.isMainThread {
        return false
    } else {
        DispatchQueue.main.async {
            function()
        }
        return true
    }
}

// Use

func doSmthWithUI(a:Int, b:String) {
    print("BEFORE \(Thread.isMainThread)")
    if trampolineToMain(self.doSmthWithUI(a: a, b: b)) {
        return
    }
    print("AFTER \(Thread.isMainThread) a=\(a) b=\(b)")
}

DispatchQueue.global(qos: .background).async {
    doSmthWithUI(a: 42, b: "I love Swift")
}