ios - layoutIfNeeded() 上的模糊崩溃

Question

出现崩溃:

navigationController.view.layoutIfNeeded()

log : fatal error: unexpectedly found nil while unwrapping an Optional value

第一次启动应用程序(在 xcode 构建之后)以及当我的用户从他的帐户断开连接并重新连接时，会出现崩溃。如果我在用户已连接的情况下重新启动应用程序，那就没问题了。

完整功能代码:

func prepareControllers(){
    homeTabs = [homeView, teamView, rankView, generalView]

    for view in homeTabs{
        view.addGestureRecognizer(UITapGestureRecognizer(target: self, action: #selector(ABMainViewController.didTapTab(_:))))
    }

    mainControllers = [
        UIStoryboard(name: "Main", bundle: Bundle.main).instantiateViewController(withIdentifier: "home") as! ABHomeViewController,
        UIStoryboard(name: "Main", bundle: Bundle.main).instantiateViewController(withIdentifier: "teamController") as! ABTeamViewController,
        UIStoryboard(name: "Main", bundle: Bundle.main).instantiateViewController(withIdentifier: "leaderboardController") as! ABLeaderboardViewController,
        ABInformationViewController(nibName: "ABInformationViewController", bundle: Bundle.main) 
    ]

    var previousController: UINavigationController? = nil

    for viewController in mainControllers{
        viewController.delegate = self;
        let navigationController = UINavigationController(rootViewController: viewController)
        navigationControllers.append(navigationController)
        navigationController.view.frame = scrollView.bounds
        navigationController.setNavigationBarHidden(true, animated: false)
        navigationController.view.translatesAutoresizingMaskIntoConstraints = false

        scrollView.addSubview(navigationController.view)

        scrollView.addConstraint(NSLayoutConstraint(item: navigationController.view, attribute: .top, relatedBy: .equal, toItem: scrollView, attribute: .top, multiplier: 1, constant: 0))
        scrollView.addConstraint(NSLayoutConstraint(item: navigationController.view, attribute: .height, relatedBy: .equal, toItem: scrollView, attribute: .height, multiplier: 1, constant: 0))
        scrollView.addConstraint(NSLayoutConstraint(item: navigationController.view, attribute: .width, relatedBy: .equal, toItem: scrollView, attribute: .width, multiplier: 1, constant: 0))

        if(previousController != nil){
            scrollView.addConstraint(NSLayoutConstraint(item: navigationController.view, attribute: .left, relatedBy: .equal, toItem: previousController!.view, attribute: .right, multiplier: 1, constant: 0))
        }else{
            scrollView.addConstraint(NSLayoutConstraint(item: navigationController.view, attribute: .leading, relatedBy: .equal, toItem: scrollView, attribute: .leading, multiplier: 1, constant: 0))
        }

        navigationController.view.layoutIfNeeded()  // CRASH HERE
        previousController = navigationController
    }

    scrollView.addConstraint(NSLayoutConstraint(item: scrollView, attribute: .trailing, relatedBy: .equal, toItem: previousController!.view, attribute: .trailing, multiplier: 1, constant: 0))
}

在调试导航器中:

    0x102b691f8 <+116>: bl     0x102a5cb80               ; function signature    specialization <preserving fragile attribute, Arg[1] = [Closure Propagated : reabstraction thunk helper from @callee_owned (@unowned Swift.UnsafeBufferPointer<Swift.UInt8>) -> () to @callee_owned (@unowned Swift.UnsafeBufferPointer<Swift.UInt8>) -> (@out ()), Argument Types : [@callee_owned (@unowned Swift.UnsafeBufferPointer<Swift.UInt8>) -> ()]> of generic specialization <preserving fragile attribute, ()> of Swift.StaticString.withUTF8Buffer <A> ((Swift.UnsafeBufferPointer<Swift.UInt8>) -> A) -> A
 ->  0x102b691fc <+120>: brk    #0x1    //HERE

有没有办法找到这个展开值在哪里？如果有人有想法，我该如何解决它或者如何更清楚地调试它。谢谢!

Answer 1

正如 Eperrin95 所说，也许您正在尝试布局尚不存在的东西。布局方法应该在 viewDidAppear 之后调用，或者至少在 viewWillAppear 之后调用。