Swift：在运行时检测非可选的意外nil值：转换为可选失败 [英] Swift: detecting an unexpected nil value in a non-optional at runtime: casting as optional fails

问题描述

我有一个 UITableViewController 通过 NSFetchedResultsController 从Core Data加载它的条目。像这样：

  let historyItem = fetchedResults.objectAtIndexPath（indexPath）as HistoryItem 
  

historyItem 有一个 title this：

  @NSManaged var title：String 
  

但是不知为什么核心数据在一些条目中具有 title 的nil值，导致EXC_BAD_ACCESS，因为 title 不是 String？。此问题已在和我以前的重复问题 Swift：处理意外的nil值，当变量不是可选的
但它不适用于我。我使用Xcode 6.2和iOS8。

我误解了什么吗？

$ p

解决方案

我认为如果核心数据可以返回nil的值， p>

  @NSManaged var title：String？ 
  

无需强制转换即可测试

  if let possibleTitle = historyItem.title {
 NSLog（possibleTitle is set OK）
} else {
 NSLog（possibleTitle is nil）
} 
  

I have a UITableViewController loading its entries from Core Data via a NSFetchedResultsController. Like this:

let historyItem = fetchedResults.objectAtIndexPath(indexPath) as HistoryItem

historyItem has a title property defined like this:

@NSManaged var title: String

but somehow the core data has a nil value for title in some entries which causes EXC_BAD_ACCESS because title is not String?. This problem has been addressed at Check if property is set in Core Data? and the high-voted answer there suggests something like this:

    if let possibleTitle = historyItem.title as String? {
        NSLog("possibleTitle was set OK")
    } else {
        NSLog("possibleTitle was nil")
    }

but I just tried that and it still gave me EXC_BAD_ACCESS:

That same problem and solution is also mentioned at Swift - casting a nil core data string as an optional value and my earlier duplicate question Swift: handling an unexpected nil value, when variable is not optional but it doesn't work for me. I'm using Xcode 6.2 and iOS8.

Am I misunderstanding something, please? Should this approach work?

解决方案

I think you should make your title an optional if core data can return nil value for title

@NSManaged var title: String?

And test it without the cast

if let possibleTitle = historyItem.title{
    NSLog("possibleTitle was set OK")
} else {
    NSLog("possibleTitle was nil")
}

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