How can I get an NSCoder to encode/decode a Swift array of structs?

Question

I have an object that must conform to NSCoding and that holds an array of UInt64 values. How can I encode/decode it with an NSCoder at all? Bonus question: how can I encode it most compactly? (It has to go into saved Game Center state data, whose size is limited.)

Ideally, I just want to write an Int which is the size n of the array, and then write n times the 64 bits of a UInt64, and read it similarly. Can I do this?

coder.encodeObject(values, forKey: "v") doesn't work.

class MyObject: NSCoding {

    private var values: [UInt64]

    // …

    // MARK: - NSCoding

    required init(coder decoder: NSCoder) {
        // ???
    }

    func encodeWithCoder(coder: NSCoder) {
        // ???
    }


}

Community · Accepted Answer · 2017-05-23 12:15:03Z

Here is a possible solution that encodes the UInt64 array as an array of bytes. It is inspired by the answers to How to serialize C array with NSCoding?.

class MyObject: NSObject, NSCoding {

    var values: [UInt64] = []

    init(values : [UInt64]) {
        self.values = values
    }

    // MARK: - NSCoding
    required init(coder decoder: NSCoder) {
        super.init()
        var count = 0
        // decodeBytesForKey() returns an UnsafePointer<UInt8>, pointing to immutable data.
        let ptr = decoder.decodeBytesForKey("values", returnedLength: &count)
        // If we convert it to a buffer pointer of the appropriate type and count ...
        let buf = UnsafeBufferPointer<UInt64>(start: UnsafePointer(ptr), count: count/sizeof(UInt64))
        // ... then the Array creation becomes easy.
        values = Array(buf)
    }

    func encodeWithCoder(coder: NSCoder) {
        // This encodes both the number of bytes and the data itself.
        coder.encodeBytes(UnsafePointer(values), length: values.count * sizeof(UInt64), forKey: "values")
    }
}

Test:

let obj = MyObject(values: [1, 2, 3, UInt64.max])
let data = NSKeyedArchiver.archivedDataWithRootObject(obj)

let dec = NSKeyedUnarchiver.unarchiveObjectWithData(data) as! MyObject
print(dec.values) // [1, 2, 3, 18446744073709551615]

Update for Swift 3 (Xcode 8):

class MyObject: NSObject, NSCoding {

    var values: [UInt64] = []

    init(values : [UInt64]) {
        self.values = values
    }

    // MARK: - NSCoding
    required init(coder decoder: NSCoder) {
        super.init()
        var count = 0
        // decodeBytesForKey() returns an UnsafePointer<UInt8>?, pointing to immutable data.
        if let ptr = decoder.decodeBytes(forKey: "values", returnedLength: &count) {
            // If we convert it to a buffer pointer of the appropriate type and count ...
            let numValues = count / MemoryLayout<UInt64>.stride
            ptr.withMemoryRebound(to: UInt64.self, capacity: numValues) {
                let buf = UnsafeBufferPointer<UInt64>(start: UnsafePointer($0), count: numValues)
                // ... then the Array creation becomes easy.
                values = Array(buf)
            }
        }
    }

    public func encode(with coder: NSCoder) {
        // This encodes both the number of bytes and the data itself.
        let numBytes = values.count * MemoryLayout<UInt64>.stride
        values.withUnsafeBufferPointer {
            $0.baseAddress!.withMemoryRebound(to: UInt8.self, capacity: numBytes) {
                coder.encodeBytes($0, length: numBytes, forKey: "values")
            }
        }
    }
}


let obj = MyObject(values: [1, 2, 3, UInt64.max])
let data = NSKeyedArchiver.archivedData(withRootObject: obj)

let dec = NSKeyedUnarchiver.unarchiveObject(with: data) as! MyObject
print(dec.values) // [1, 2, 3, 18446744073709551615]

Thanks a lot. Is there a way to further optimise the output NSData for size? Or would I have to write my own subclass of NSCoder for that (or maybe use this one)?
@Jean-PhilippePellet: I inspected the archived data and noticed that the UInt64 array is really stored as a length byte plus 4*8 bytes, so unless you "compress" the data somehow you cannot reduce it further. Archiving the object without the array already results in 226 bytes. I have no experience with alternative coders.

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