How do I encode [Character : Int] property using NSCoder in Swift 3?

Question

import UIKit

class Foo: NSObject, NSCoding {
    var cx: [Character : Int]

    init(cx: [Character : Int]) {
        self.cx = cx
    }

    // MARK: - <NSCoding>

    required convenience init(coder aDecoder: NSCoder) {
        let cx = aDecoder.decodeObject(forKey: "cxKey") as! [Character : Int]
        self.init(cx: cx)
    }

    func encode(with aCoder: NSCoder) {
        aCoder.encode(cx, forKey: "cxKey")
    }
}

calling:

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.

        var foo = Foo(cx: ["C": 5, "X": 6])

        let encodedData = NSKeyedArchiver.archivedData(withRootObject: foo)
        print("encodedData: \(encodedData))")

        if let foo1 = NSKeyedUnarchiver.unarchiveObject(with: encodedData) as? Foo {
            print("cx = ", foo1.cx)
        } else{
            print("There is an issue")
        }
    }
}

Xcode throws an error: *** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[_SwiftValue encodeWithCoder:]: unrecognized selector sent to instance

Answer 1

Reason

That is because the Character-typed keys in cx will be boxed as _SwiftValue objects which will be sent encodeWithCoder: which leads to the unrecognized selector exception.

See the comment at the top of SwiftValue.h:

This implements the Objective-C class that is used to carry Swift values that have been bridged to Objective-C objects without special handling. The class is opaque to user code, but is NSObject- and NSCopying- conforming and is understood by the Swift runtime for dynamic casting back to the contained type.

Solution

If you can change the type of cx to [String : Int], everything will work out of the box (no pun intended).

Otherwise you will have to convert cx in Foo.encode(with:) to something that can be encoded (like [String : Int], for instance) and vice versa in the decoding initializer.

See How do I encode Character using NSCoder in swift? and How do I encode enum using NSCoder in swift? for some code.