您可以使用setViewControllers(_:animated:) 重新排列视图控制器堆栈。您无需执行任何特殊操作即可使后退按钮正常工作。导航控制器根据其viewControllers 数组中的第二项（如果有第二项）设置后退按钮，并在更新viewControllers 数组时更新后退按钮。
我会这样做。首先，我们向UIViewController 添加一个方法来询问它是否是特定userId 的视图控制器。由于大多数视图控制器不是（也不能是）正确的视图控制器，它只返回false：
extension UIViewController {
func isViewControllerForUserId(userId: Int) -> Bool {
return false
}
}
然后我们重写MessagesViewController 中的这个方法，在适当的时候返回true：
extension MessagesViewController {
override func isViewControllerForUserId(userId: Int) -> Bool {
return self.userId == userId
}
}
现在，为了显示特定用户的视图控制器，我们在导航控制器的堆栈中搜索现有视图控制器。我们采取的行动取决于我们是否找到它：
func showMessageForUserId(userId: Int) {
if let index = navController.viewControllers.indexOf({ $0.isViewControllerForUserId(userId) }) {
navController.moveToTopOfNavigationStack(viewControllerAtIndex: index)
} else {
pushNewViewControllerForUserId(userId)
}
}
如果我们没有找到，我们创建一个新的视图控制器并推送它：
private func pushNewViewControllerForUserId(userId: Int) {
let vc = self.storyboard?.instantiateViewControllerWithIdentifier("MessagesViewController") as! MessagesViewController
vc.userId = userId
self.navController.pushViewController(vc, animated: true)
}
如果我们确实找到了，我们用这个方法把它移到导航栈的顶部：
extension UINavigationController {
func moveToTopOfNavigationStack(viewControllerAtIndex index: Int) {
var stack = viewControllers
if index == stack.count - 1 {
// nothing to do because it's already on top
return
}
let vc = stack.removeAtIndex(index)
if (reorderingIsBuggy) {
setViewControllers(stack, animated: false)
}
stack.append(vc)
setViewControllers(stack, animated: true)
}
private var reorderingIsBuggy: Bool {
// As of iOS 9.3 beta 3, `UINavigationController` drops the prior top-of-stack
// when you use `setViewControllers(_:animated:)` to move a lower item to the
// top with animation. The workaround is to remove the lower item from the stack
// without animation, then add it to the top of the stack with animation. This
// makes it display a push animation instead of a pop animation and avoids
// dropping the prior top-of-stack.
return true
}
}