我可以在带有 nil 参数的 Swift 中使用 NSPredicate 吗? [英] Can I use an NSPredicate in Swift with a nil argument?

问题描述

我正在尝试将使用 Core Data 的项目从 Objective-C 转换为 Swift.

I'm trying to convert a project that uses Core Data from Objective-C to Swift.

数据模型的结构使我有一个包含其他文件夹的主文件夹 - 这些文件夹也可以通过父文件夹"关系包含其他文件夹.

The data model is structured so that I have one master folder which contains other folders - and those folders can also contain other folders, via a "parentFolder" relationship.

目前，我在 Objective-C 中执行此操作以检索主文件夹(它找到唯一没有parentFolder"的文件夹，并按预期工作):

Currently, I do this in Objective-C to retrieve the master folder (it finds the only folder without a "parentFolder", and works as expected):

NSFetchRequest *request = [NSFetchRequest fetchRequestWithEntityName:"Folder"];
request.predicate = [NSPredicate predicateWithFormat:@"parentFolder == %@", nil];

在转换为 Swift 时，我想做同样的事情:

In converting to Swift, I'd like to do the same thing:

let request = NSFetchRequest(entityName: "Folder")
request.predicate = NSPredicate(format: "parentFolder == %@", nil)

...但编译器抱怨在调用中缺少参数标签'argumentArray:'.(我似乎混淆了它认为我需要使用 NSPredicate(format: argumentArray:) 方法代替...)

... but the compiler complains with "Missing argument label 'argumentArray:' in call. (I seem to be confusing it into thinking I need to use the NSPredicate(format: argumentArray:) method instead...)

在 Swift 中是否有正确的方法来做到这一点?

Is there a correct way to do this in Swift?

推荐答案

看起来好像(使用当前的 Xcode 6 beta 3 版本)一般很难在变量参数列表中传递 nil.

It looks as if it is (with the current Xcode 6 beta 3 release) generally difficult to pass nil in a variable argument list.

这似乎有效:

let predicate = NSPredicate(format: "parentFolder == %@", 0)
print(predicate)
// Output: parentFolder == nil

但是最简单的解决方案是简单地将谓词写为

But the easiest solution would be to simply write the predicate as

NSPredicate(format: "parentFolder == nil")

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