How to test for the class of a variable in Swift?

Question

I want to check if the elements of an Array are a subclass of UILabel in Swift:

import UIKit

var u1 = UILabel()
u1.text=\"hello\"
var u2 = UIView(frame: CGR         


        

Answer 1

Swift has the is operator to test the type of a value:

var onlyUILabels = myArray.filter { $0 is UILabel }

As a side note, this will still produce an Array<UIView>, not Array<UILabel>. As of the Swift 2 beta series, you can use flatMap for this:

var onlyUILabels = myArray.flatMap { $0 as? UILabel }

Previously (Swift 1), you could cast, which works but feels a bit ugly.

var onlyUILabels = myArray.filter { $0 is UILabel } as! Array<UILabel>

Or else you need some way to build a list of just the labels. I don't see anything standard, though. Maybe something like:

extension Array {
    func mapOptional<U>(f: (T -> U?)) -> Array<U> {
        var result = Array<U>()
        for original in self {
            let transformed: U? = f(original)
            if let transformed = transformed {
                result.append(transformed)
            }
        }
        return result
    }
}
var onlyUILabels = myArray.mapOptional { $0 as? UILabel }

Answer 2

You may compare classes using the following in swift:

 return object.dynamicType == otherObject.dynamicType

dynamicType will return an instance of Class which you may compare

Answer 3

In Swift you should do the is keyword if you are wondering about the according class. In the filter-closure you can use $0 to specify the first parameter.

Sample

var (a,b,c,d) = ("String", 42, 10.0, "secondString")
let myArray: Array<Any> = [a,b,c,d]
var onlyStrings = myArray.filter({ return $0 is String })
onlyStrings // ["String", "secondString"]

Answer 4

Without getting to the object.class( ) nirvana, in Swift, we can still use the Objective-C Runtime to get some useful information about the object´s class as follows (and not exactly bridging to Objective-C):

let objectClass: AnyClass = object_getClass(object) as AnyClass
let objectClassName =  NSStringFromClass(objectClass)
println("Class = \(objectClassName)")

Note that we get the "MyProject." or (sometimes) the "Swift." prefix, depending if you refer to your own classes or Swift classes:

UILabel // UILabel    
Swift._NSSwiftArrayImpl //Swift Array
MyProject.Customer_Customer_   //for a CoreData class named Customer