Question

let viewControllers: [UIViewController] = self.navigationController!.viewControllers

for VC  in viewControllers  {            
    if (VC.isKind(of: HomeViewController.self)) {            
         bScreen = true
         self.navigationController?.popToViewController(VC, animated: true)
     }
}

if bScreen == false {
    let homeVC = HomeViewController()
    self.navigationController?.pushViewController(homeVC, animated: false)
}

我循环浏览导航控制器阵列以移动到HomeViewController.above代码工作正常。有些时候我正在崩溃，因为“致命错误：在解开一个可选值时意外发现了nil。”我知道这次崩溃的原因。请告诉我。帮助我如何检查视图控制器对象的nil值。任何帮助将不胜感激。谢谢提前

Answer 1

使用此代码。这对你有帮助。

Angularjs

Answer 2

- Swift 3 -

for vc in (self.navigationController?.viewControllers)! {
     if vc is HomeViewController {
        _ = self.navigationController?.popToViewController(vc, animated: true)
     }
}

Answer 3

let getCurrentVCIndex = self.navigationController?.viewControllers.indexOf({ (viewController) -> Bool in

if let _ = viewController as? HomeViewController {
    return true
}
return false
})


if  getCurrentVCIndex
 {
let HomeVC = self.navigationController?.viewControllers[getCurrentVCIndex!] as! HomeViewController

self.navigationController?.popToViewController(HomeVC, animated: true)
}
else
  {
 // use push
}

或使用

if  let  HomeVC =  self.navigationController?.viewControllers.filter({$0 is HomeViewController}).first
{
self.navigationController?.popToViewController(HomeVC!, animated: true)
}else
{
  // use push
}

Answer 4

如果允许/保护可选值以避免崩溃，最好使用它。

if let viewControllers: [UIViewController] = self.navigationController.viewControllers{

        for VC  in viewControllers  {

            if (VC.isKind(of: HomeViewController.self)) {

                bScreen = true
                self.navigationController?.popToViewController(VC, animated: true)
            }
        }

if bScreen == false
        {
            let homeVC = HomeViewController()
            self.navigationController?.pushViewController(homeVC, animated: false)
        }
}

Answer 5

永远不要直接使用!，直到你确定它不会是零。替换您的代码如下。您可以使用if let或guard let来展开选项。

    if let viewControllers: [UIViewController] = self.navigationController?.viewControllers {
                for VC  in viewControllers  {

                    if (VC.isKind(of: ViewController.self)) {

                        bScreen = true
                        self.navigationController?.popToViewController(VC, animated: true)
                    }
                }

                if bScreen == false
                {
                    let homeVC = ViewController()
                    self.navigationController?.pushViewController(homeVC, animated: false)
                }
 } 
else {
     // IF VC is nil
}

Answer 6

根据您的代码，在循环中，如果导航堆栈包含相应的视图控制器，则会弹出相应的页面。但问题是如果同一个视图控制器出现两次，将导致同时执行循环。这可能会导致崩溃。所以在poptoviewcontroller之后添加一个休息时间可以避免这个问题。请检查以下代码，将为您提供帮助。

 if (VC.isKind(of: HomeViewController.self)) {

                bScreen = true
                self.navigationController?.popToViewController(VC, animated: true)
                break
            }

iOS Swift3检查ViewController对象的nil值

6 个答案: