How to set primary key in Swift for Realm model

As of Realm Swift v10.10.0, you declare a primary key with @Persisted(primaryKey: true):

class Foo: Object {
    @Persisted(primaryKey: true) var id = 0
    @Persisted var title = ""
}

Older versions:

primaryKey needs to be a class function which returns the name of the property which is the primary key, not an instance method which returns the value of the primary key.

@objcMembers class Foo: RLMObject {
    dynamic var id = 0
    dynamic var title = ""

    override class func primaryKey() -> String? {
        return "id"
    }
}