AssociatedValue.swift

final class AssociatedValue<Value, Object: NSObject> {
    init(_ memoryManagement: AssociatedValueMemoryManagement, ofType valueType: Value.Type, on objectType: Object.Type) {
        self.memoryManagement = memoryManagement
    }
    
    let memoryManagement: AssociatedValueMemoryManagement
    
    var key: UnsafeRawPointer {
        return UnsafeRawPointer(Unmanaged.passUnretained(self).toOpaque())
    }
    
    subscript (object: Object) -> Value? {
        get {
            return objc_getAssociatedObject(object, key) as! Value?
        }
        set {
            objc_setAssociatedObject(object, key, newValue, memoryManagement.objcPolicy)
        }
    }
}

struct AssociatedValueMemoryManagement: OptionSet {
    var rawValue: Int = 0
    
    static var unsafeUnowned = AssociatedValueMemoryManagement(rawValue: 0)
    static var strong = AssociatedValueMemoryManagement(rawValue: 1)
    static var copied = AssociatedValueMemoryManagement(rawValue: 3)
    static var atomic = AssociatedValueMemoryManagement(rawValue: 8)
    
    init(rawValue: Int) {
        self.rawValue = rawValue
    }
    
    fileprivate var objcPolicy: objc_AssociationPolicy {
        switch self {
        case [.unsafeUnowned]:
            return .OBJC_ASSOCIATION_ASSIGN
        case [.strong]:
            return .OBJC_ASSOCIATION_RETAIN_NONATOMIC
        case [.copied]:
            return .OBJC_ASSOCIATION_COPY_NONATOMIC
        case [.strong, .atomic]:
            return .OBJC_ASSOCIATION_RETAIN
        case [.copied, .atomic]:
            return .OBJC_ASSOCIATION_COPY
        default:
            fatalError("Invalid AssociatedValueMemoryManagement")
        }
    }
}