如何对optional of optional执行可选绑定？

Question

如何对optional of optional进行可选绑定？

例如，假设UIViewController的navigationController属性是optional of optional。我应该使用方法#1还是方法#2，或者还有第三种方法？

方法#1

if let navigationController = viewController.navigationController! {

}

或

方法#2

if let optionalNavigationController = viewController.navigationController {
   let navigationController = optionalNavigationController {

    }
}

Answer 1

还有另一种解决方案：

if let stillOptional = viewController.navigationController, let notOptional = stillOptional {
    //use notOptional here
}

Answer 2

if let navigationController = viewController?.navigationController

Answer 3

更多函数式编程解决方案：

if let a = optionalOfOptional?.map({$0}) {
    println(a)
}