Mutable Swift array becomes NSArray when I initialize with Obj-C

Question

I'm creating my base models in Swift(2.0) and then controlling the views in Objective-C. I'm still new to Swift, so hopefully I'm just overlooking something simple, but here is the problem:

I’m making a mutable array in Swift, but when I initialize the array in my Objective-c portion of the program, it becomes an NSArray, more specifically it becomes: Swift._SwiftDeferredNSArray

Why is it becoming immutable when I initialize? Here’s my Swift code:

import Foundation

@objc public class Model : NSObject {

    var books:[Book]

    override init(){
        self.books = [Book]()
    }

}

And here’s my Obj-c Code;

Model *bookCollection = [[Model alloc]init];

I’m unable to add objects to my bookCollection.books array (because it has become an NSArray) and when I set a breakpoint and po it, I can see that it is a Swift._SwiftDeferredNSArray. bookCollection.books is supposed to be an NSMutableArray.

Any thoughts?

Try putting the dynamic keyword in front of the books declaration. — Qbyte
– Qbyte, Commented Sep 5, 2015 at 1:11
Just tried dynamic var books:[Book] , unfortunately it didn't change anything. — Alan Scarpa
– Alan Scarpa, Commented Sep 5, 2015 at 1:14

Shoaib · Accepted Answer · 2015-09-05 01:44:10Z

3

In swift, the difference between mutable and immutable array is;

var books:[Book] // is a mutable array
let books:[Book] = [book1, book2]; // is immutable array due to let

but I don't think, same rule is followed when bridging to ObjC.

Just for a fix, you may have mutableArray specifically.

import Foundation

@objc public class Model : NSObject {

    var books:NSMutableArray = NSMutableArray();

    override init(){
        super.init();
        // other code
    }

}

You will need to parse the values to Book Class when retrieving from the array.

edited Sep 5, 2015 at 1:44

answered Sep 5, 2015 at 1:37

Shoaib

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2 Comments

Alan Scarpa Over a year ago

Thank you! I'll mark this as the correct answer but is this the correct way to initialize my array in Swift in the ovverride init method: self.books = [] Or is there a more proper way?

Shoaib Over a year ago

There is an option to initialize it with it's definition as I did but I do not see any issue if you initialize the same in your init().

zneak · Accepted Answer · 2015-09-05 01:26:15Z

bookCollection.books is supposed to be an NSMutableArray.

No, it is not. Var does not mean that the bridged Objective-C object is to be mutable: it means that the property can be assigned to.

The Swift array type is a structure, not a class. This has important bridging implications. The reference itself cannot be shared without passing it as an inout value, and even then the references cannot be stored. If it bridged as a NSMutableArray, it would be possible to have undetectable mutating references, and Swift does not allow that.

You should be able to assign a new NSArray to your property from Objective-C code, though. For instance, this should work:

bookCollection.books = [bookCollection.books arrayByAddingObject:myNewBook];

Your other option, obviously, is to declare books as a NSMutableArray from the Swift side.

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