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I have a generic function that fetches from DB objects based on a given type and filter:

    class Employee: Object {
        @objc dynamic var id: String = UUID().uuidString
        @objc dynamic var hardWorking = false
    }

    class Student: Object {
        @objc dynamic var id: String = UUID().uuidString
        @objc dynamic var regular = false
    }

    enum Filter {
        case all
        case regularStudents
        case hardWorkingEmployees
    }

    func fetch<T: Object>(_ type: T.Type, filter: Filter) -> [T] {
        var predicate: NSPredicate?
        switch filter {
        case .regularStudents: predicate = NSPredicate(format: "regular = %@", NSNumber(true))
        case .hardWorkingEmployees: predicate = NSPredicate(format: "hardWorking = %@", NSNumber(true))
        default: break
        }
        guard let filterPredicate = predicate else {
            return realm!.objects(type).toArray()
        }
        return realm!.objects(type).filter(filterPredicate).toArray()
    }

I want to throw a compile error when this function is call and the given type mismatch the given filter. E.g.: You can not apply .regularStudents filter on Employees. 

    let students = fetch(Employee.self, filter: .regularStudents) // a compile error should be throw

I was thinking of using #if and #error() to throw a compile error but I could not find a solution to declare custom compiler directives in swift. In Objective C I would do something like:

#define CHECK(param1, param2) param1 == param2

Is there any way to do this custom #define in swift?

Update

For persitance I use Realm which gives me the posibility the fech objects from a given table based on a type. The .all filter works on all types since it fetch all objects for the given type. The problem appear on the last two filters that depends on a specific type. For example .regularStudents works only for Student type, for any other type will crash since regular property is only part of the student object.

Iosif
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  • could you please describe the case you want to throw a compiler error? what does it mean "the given type mismatch the given filter"? – emrcftci May 29 '20 at 09:45
  • Your `fetch` method seems like it should be redesigned. Can you tell me more about how `fetch` works? Would `fetch(Employee.self, filter: .all)` be ok? Can I pass in any other types to the first parameter other than `Employee.self` and `Student.self`? – Sweeper May 29 '20 at 09:46

0 Answers0