CODEWITHSUNDEEP
core-dataswiftnspredicate
Jim Rhoades
Jim Rhoades

Reputation: 3320

Can I use an NSPredicate in Swift with a nil argument?

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?

Upvotes: 28

Views: 19160

Answers (2)

simonnickel
simonnickel

Reputation: 557

The easiest solution and fitting for you case would be to simply write the predicate as:

NSPredicate(format: "parentFolder == nil")

But if it is necessary to insert nil as an argument you could use NSNull. This might be useful if you are using an Optional

let folderName: String? = nil
// ... assign folderName if available
NSPredicate(format: "parentFolder == %@", folderName ?? NSNull())

This will result in all folder with the same parent, if the parent is nil, it will fetch all folders without parent.

Upvotes: 9

Martin R
Martin R

Reputation: 539745

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")

Upvotes: 53

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