Swift - self.navigationController 在转换后变为 nil

Question

我的应用程序遇到了一个非常奇怪的错误，在两个视图之间，self.navigationController 变成了nil

我有几个视图控制器：MainViewController、SecondViewControllerPastSessionsViewController、JournalViewController。我使用JournalViewController 有两个目的，将新条目保存到CoreData 或编辑旧条目。详细信息与此错误无关。

当我尝试将JournalViewController 从堆栈中弹出并返回到MainViewController 时会发生错误，但仅在JournalViewController 处于“编辑”模式时，而不是在它处于“编辑”模式时“保存新的进入模式”

知道 1) 为什么会发生这种情况，以及 2) 如何正确解决它，以便在编辑模式下从 JournalViewController 返回时返回 PastSessionsViewController？

这里有一些代码可以让事情变得具体。

在AppDelegate.swift 中（didFinishLaunchingWithOptions 内部）：

navController = UINavigationController()

navController!.navigationBarHidden = true
var viewController = MainViewController()
navController!.pushViewController(viewController, animated: false)

window = UIWindow(frame: UIScreen.mainScreen().bounds)
window?.backgroundColor = UIColor.whiteColor()
window?.rootViewController = navController
window?.makeKeyAndVisible()

在MainViewController:

func goToPastSessions(sender: UIButton) {
    let pastSessionsVC = PastSessionsViewController()
    self.navigationController?.pushViewController(pastSessionsVC, animated: true)
}

func goToWriteJournalEntry(sender: UIButton) {
    let journalVC = JournalViewController(label: "Record your thoughts")
    self.navigationController?.pushViewController(journalVC, animated: true)
}

在PastSessionsViewController:

func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) {
    let editJournalVC = JournalViewController(label: "Edit your thoughts")

    let indexPath = tableView.indexPathForSelectedRow()
    let location = indexPath?.row
    let currentCell = tableView.cellForRowAtIndexPath(indexPath!) as! EntryCell

    if let objects = pastSessionsDataSource.coreDataReturn {
        if let location = location {
            editJournalVC.journalEntryCoreDataLocation = location
            editJournalVC.editEntry = true
            editJournalVC.journalEntryToEdit = objects[location].journalEntry
        }
    }

    self.navigationController?.presentViewController(editJournalVC, animated: true, completion: nil)
}

最后，在JournalViewController：

func doneJournalEntry(sender: UIButton) {
    journalEntryTextArea?.resignFirstResponder()
    var entry = journalEntryTextArea?.text

    if let entry = entry {
        let appDelegate = (UIApplication.sharedApplication().delegate as! AppDelegate)
        let managedObjectContext = appDelegate.managedObjectContext!
        let request = NSFetchRequest(entityName: "Session")
        var error: NSError?

        // new entry
        if journalEntryText == "Record your thoughts" {
            let result = managedObjectContext.executeFetchRequest(request, error: &error)

            if let objects = result as? [Session] {
                if let lastTime = objects.last {
                    lastTime.journalEntry = entry
                }
            }
        } else {
            // existing entry
            let sortDescriptor = NSSortDescriptor(key: "date", ascending: false)
            request.sortDescriptors = [sortDescriptor]

            let result = managedObjectContext.executeFetchRequest(request, error: &error)
            if let objects = result as? [Session] {
                var location = journalEntryCoreDataLocation

                var object = objects[location!]
                object.journalEntry = entry
            }
        }

        if !managedObjectContext.save(&error) {
            println("save failed: \(error?.localizedDescription)")
        }
    }

   // in "edit" mode, self.navigationController is `nil` and this fails
   // in "record a new entry" mode, it's not nil and works fine
    self.navigationController?.popViewControllerAnimated(true)
}

func cancelEntryOrEditAndReturn(sender: UIButton) {
    self.journalEntryTextArea?.resignFirstResponder()

   // in "edit" mode, self.navigationController is `nil` and this fails
   // in "record a new entry" mode, it's not nil and works fine
    self.navigationController?.popViewControllerAnimated(true)
}

感谢观看

Answer 1

如果您希望它在同一个导航堆栈中，您应该推送 editJournalVC 而不是呈现。因为当您呈现控制器时，它不再在同一个导航堆栈中。此外，如果您正在展示它，您应该将其关闭而不是弹出。所以如果你想展示控制器，你应该使用

self.dismissViewControllerAnimated(true, completion: nil)

改为

self.navigationController?.popViewControllerAnimated(true)