NSCoding in Swift

Archivable.swift

import Foundation
import Mapper

protocol Archivable: Mappable {
    func encode(archive: Archiver)
}

final class Archiver {
    private var mapping: [String: AnyObject] = [:]
    required init(_ object: Archivable) {
        object.encode(self)
    }

    func encode(key: String, value: AnyObject?) {
        mapping[key] = value
    }

    func encode<T: Archivable>(key: String, value: T?) {
        if let value = value {
            mapping[key] = Archiver(value).mapping
        }
    }

    func encode<T: Archivable>(key: String, value: [T]?) {
        mapping[key] = value?.map { Archiver($0).mapping }
    }
}

private extension Mappable {
    @warn_unused_result
    static func from(JSON: [String: AnyObject]) throws -> Self {
        return try self.init(map: Mapper(JSON: JSON))
    }

    @warn_unused_result
    static func from(JSON: [[String: AnyObject]]) throws -> [Self] {
        return try JSON.map { try self.from($0) }
    }
}

enum ArchiverError: ErrorType {
    case ConvertibleError(value: AnyObject?, type: Any.Type)
}

extension Archivable {
    @warn_unused_result
    static func decode(data: NSData) throws -> Self {
        var unarchived: AnyObject?
        do {
            try INSObjC.catchException {
                unarchived = NSKeyedUnarchiver.unarchiveObjectWithData(data)
            }
        } catch {
            throw error
        }
        guard let dict = unarchived as? [String: AnyObject] else {
            throw ArchiverError.ConvertibleError(value: unarchived, type: [String: AnyObject].self)
        }

        return try Self.from(dict)
    }

    @warn_unused_result
    static func decodeArray(data: NSData) throws -> [Self] {
        var unarchived: AnyObject?
        do {
            try INSObjC.catchException {
                unarchived = NSKeyedUnarchiver.unarchiveObjectWithData(data)
            }
        } catch {
            throw error
        }
        guard let array = unarchived as? [[String: AnyObject]] else {
            throw ArchiverError.ConvertibleError(value: unarchived,
                                                 type: [[String: AnyObject]].self)
        }

        return try Self.from(array)
    }

    @warn_unused_result
    func encode() throws -> NSData {
        var data: NSData?
        do {
            try INSObjC.catchException {
                data = NSKeyedArchiver.archivedDataWithRootObject(Archiver(self).mapping)
            }
        } catch {
            throw error
        }

        if let data = data {
            return data
        } else {
            fatalError() // Impossible
        }
    }
}

extension SequenceType where Generator.Element: Archivable {
    @warn_unused_result
    func encode() throws -> NSData {
        let mappings = map { Archiver($0).mapping }
        var data: NSData?
        do {
            try INSObjC.catchException {
                data = NSKeyedArchiver.archivedDataWithRootObject(mappings)
            }
        } catch {
            throw error
        }

        if let data = data {
            return data
        } else {
            fatalError() // Impossible
        }
    }
}

Would be great to embed example usage as code comments for posterity.