An example how to have a synchronised property in a protocol-oriented way

SynchronizedValueContaining.swift

public protocol SynchronizedValueContaining: class {
    typealias SynchronizedValueType
    var _valueAccessorSerialQueue: dispatch_queue_t { get }
    var _privateValue: SynchronizedValueType { get set }
}

public extension SynchronizedValueContaining {
    var synchronizedValue: SynchronizedValueType {
        get {
            var v: SynchronizedValueType!
            dispatch_sync(_valueAccessorSerialQueue) {
                v = self._privateValue
            }
            return v
        }
        set {
            dispatch_async(_valueAccessorSerialQueue) {
                self._privateValue = newValue
            }
        }
    }
}

public extension SynchronizedValueContaining where SynchronizedValueType == Bool {
    func synchronizedCheckAndEnableFlag() -> Bool {
        var enabled: Bool!
        dispatch_sync(_valueAccessorSerialQueue) {
            enabled = !self._privateValue
            if enabled! {
                self._privateValue = true
            }
        }
        return enabled
    }
}

public extension SynchronizedValueContaining where SynchronizedValueType: _ArrayType {
    func synchronizedAppendValue(value: SynchronizedValueType.Generator.Element) {
        dispatch_sync(_valueAccessorSerialQueue) {
            self._privateValue.append(value)
        }
    }
}