Swift:在变量不是可选的情况下处理意外的nil值 [英] Swift: handling an unexpected nil value, when variable is not optional

问题描述

我有一个UITableViewController通过NSFetchedResultsController从Core Data加载其条目.像这样:

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

let historyItem = fetchedResults.objectAtIndexPath(indexPath) as HistoryItem

historyItem具有如下定义的title属性:

historyItem has a title property defined like this:

@NSManaged var title: String

所以在cellForRowAtIndexPath中，代码说

cell?.textLabel?.text = historyItem.title

，那应该没问题. title不是可选的，不需要解包.

and that should all be fine. title is not an optional and does not need unwrapping.

但是，过去，存储的Core Data已经获取了一些对象，这些对象的title属性值为nil.它们存储在那里，等待引起错误.如果这些对象之一显示在一个单元格中，我将在上述Swift代码行上看到运行时地址异常，该地址为0.

However, in the past, the stored Core Data has acquired some objects where the value of the title property is nil. They are there stored, waiting to cause errors. If one of these objects is displayed in a cell, I will see a runtime address exception on the above Swift code line, where the address is 0.

出于鲁棒性，我需要编写代码以确保传递给我的程序的存储数据不会导致崩溃.但是，我不能写

For robustness, I need to write code to make sure the stored data delivered to my program does not cause crashes. However, I cannot write

if historyItem.title == nil { } // gives compiler error

因为title不是可选的，编译器也不允许我这样做.如果我写

because title is not an optional and the compiler will not let me. If I write

let optionalTitle:String? = historyItem.title

我仍然在该行上获得运行时EXC_BAD_ACCESS.

I still get a runtime EXC_BAD_ACCESS on that line.

如何检查title不是错误的nil?

How can I check that title is not erroneously nil?

谢谢！

推荐答案

@NSManaged var title: String应该为@NSManaged var title: String?.然后，使用可选绑定就不会出错:

@NSManaged var title: String should be @NSManaged var title: String? if there is possibility for a nil value. Then, you cannot go wrong with optional binding:

if let historyItemTitle = historyItem.title {
  cell?.textLabel?.text = historyItemTitle
} else {
  cell?.textLabel?.text = "Title missing"
}

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