Using Predicates on Array Objects in Swift returns Error

Question

When I filter an array of custom Swift classes using a Predicate I get the error:

*** NSForwarding: warning: object 0x78ed21a0 of class 'Plantivo1_6.Seed' does not implement methodSignatureForSelector: -- trouble ahead
Unrecognized selector -[Plantivo1_6.Seed valueForKey:]

If I remember correctly this would work in Objective-C. What's my mistake?

let names = ["Tom","Mike","Marc"]
println(names)
let searchPredicate = NSPredicate(format: "SELF CONTAINS[c] %@", "om")
let array = (names as NSArray).filteredArrayUsingPredicate(searchPredicate)
println(array)
println()

let mySeed1 = Seed()  // Seed is a class with a `culture` String property
let mySeed2 = Seed()
let mySeed3 = Seed()
mySeed1.culture = "Tom"
mySeed2.culture = "Mike"
mySeed3.culture = "Marc"

let mySeeds = [mySeed1,mySeed2,mySeed3]
println(mySeeds)
let searchPredicate1 = NSPredicate(format: "SELF.culture CONTAINS[c] %@", "om")
let array1 = (mySeeds as NSArray).filteredArrayUsingPredicate(searchPredicate1)
println(array1)

Answer 1

Does your Seed class inherit from NSObject?

If not, this is the message you will get.

Solution:

class Seed: NSObject {
   ...

Edit: stklieme is correct - to use NSPredicate, your object's class needs to implement -valueForKey as defined by the NSKeyValueCoding protocol. You can either define your own implementation of -valueForKey, or just make your class inherit from NSObject which takes care of that for you.

This is defined in the Apple docs for NSPredicate,

You can use predicates with any class of object, but the class must support key-value coding for the keys you want to use in a predicate.

Answer 2

In case if you don't want to inherit from NSObject, you can implement value(forKey key: String) -> Any? method by yourself:

extension Model {
    @objc func value(forKey key: String) -> Any? {
        switch key {
        case "id":
            return id
        // Other fields
        default:
            return nil
        }
    }
}

Note @objc prefix of the method: it's important, as it's allowing NSPredicate to see that the method is implemented. You'll still receive does not implement methodSignatureForSelector: crash without it.

Or, even better, make your objects to conform this protocol:

@objc protocol UsableInPredicate {
    @objc func value(forKey key: String) -> Any?
}