How can I cast an NSMutableArray to a Swift array of a specific type?

Question

I am migrating my iOS project to Swift. I am doing this class by class. When I call Objective C methods from Swift, a lot of Objective C types are converted to their Swift counterparts.

In my case an Objective C NSMutableArray gets converted to Swift's Array<AnyObject>. Now here comes my problem. Within my Swift class, I get such an array back from an Objective C object. Now that I am in the Swift world, I would like to cast this array to a specific type instead of AnyObject, because I know for sure what kind of objects exist in this array.

The compiler won't let me do that! Let me simplify my problem by saying I want to cast to an array containing strings. This is what I tried:

var strings = myObjcObject.getStrings() as [String] 

I get the following error from the compiler:

'String' is not identical to 'AnyObject'

I would have to agree with the compiler, since String is indeed not identical to AnyObject. But I don't see why that is a problem. I can downcast AnyObject to String if I want, right?

I also tried:

var strings = myObjcObject.getStrings() as? [String] 

This seems to be a step in the right direction, but getStrings() returns an NSMutableArray so I get the following error:

'NSArray' is not a subtype of 'NSMutableArray'

Is there any way to do what I am trying to do here?

Answer 1

You can make this work with a double downcast, first to NSArray, then to [String]:

var strings = myObjcObject.getStrings() as NSArray as [String] 

Tested in a Playground with:

import Foundation  var objCMutableArray = NSMutableArray(array: ["a", "b", "c"]) var swiftArray = objCMutableArray as NSArray as [String] 

Update:

In later versions of Swift (at least 1.2), the compiler will complain about as [String]. Instead you should use an if let with a conditional downcast as?:

import Foundation  var objCMutableArray = NSMutableArray(array: ["a", "b", "c"]) if let swiftArray = objCMutableArray as NSArray as? [String] {     // Use swiftArray here } 

If you are absolutely sure that your NSMutableArray can be cast to [String], then you can use as! instead (but you probably shouldn't use this in most cases):

import Foundation  var objCMutableArray = NSMutableArray(array: ["a", "b", "c"]) var swiftArray = objCMutableArray as NSArray as! [String] 

Answer 2

compactMap is your friend in Swift 4.1 and above, as well as in Swift 3.3-3.4 for that matter. This means that you don't have any double or forced casting.

let mutableArray = NSMutableArray(array: ["a", "b", "c"]) let swiftArray: [String] = mutableArray.compactMap { $0 as? String } 

---

In previous versions of Swift, 2.0-3.2 and 4.0, you'll want to use flatMap for this purpose. The usage is the same as compactMap:

let swiftArray: [String] = mutableArray.flatMap { $0 as? String }