我可以在nift参数中使用Swift中的NSPredicate吗？ [英] Can I use an NSPredicate in Swift with a nil argument?

问题描述

我试图将使用Core Data的项目从Objective-C转换为Swift。

数据模型的结构使我有一个主文件夹包含其他文件夹 - 这些文件夹也可以通过parentFolder关系包含其他文件夹。

目前，我在Objective-C中检索主文件夹它找到唯一没有parentFolder的文件夹，并按预期工作）：

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

在转换为Swift时，我想做同样的事情：

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


...但是编译器在调用中抱怨Missing argument label'argumentArray：我似乎很困惑它认为我需要使用NSPredicate（format：argumentArray :)方法代替...）

有正确的方法来做到这一点Swift？

解决方案

它看起来像是（与当前Xcode 6 beta 3版本）一般很难

工作方式： p>

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

但最简单的解决方案可以简单地写谓语为

  NSPredicate（format：parentFolder == nil）
  

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

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.

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];

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

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

... 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...)

Is there a correct way to do this in Swift?

解决方案

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

This seems to work:

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

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

NSPredicate(format: "parentFolder == nil")

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