ReadersWriterLock.swift

//
//  ReadersWriterLock.swift
//  Created by Christopher Fuller for Southern California Public Radio
//  Original version at https://github.com/SCPR/swift-experiments/blob/master/source/ReadersWriterLock.swift
//  Updated by Christopher Fuller for Swift 3 compatibility
//
//  Copyright (c) 2016 Southern California Public Radio
//
//  Permission is hereby granted, free of charge, to any person obtaining a copy
//  of this software and associated documentation files (the "Software"), to deal
//  in the Software without restriction, including without limitation the rights
//  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
//  copies of the Software, and to permit persons to whom the Software is
//  furnished to do so, subject to the following conditions:
//
//  The above copyright notice and this permission notice shall be included in all
//  copies or substantial portions of the Software.
//
//  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
//  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
//  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
//  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
//  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
//  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
//  SOFTWARE.
//

class ReadersWriterLock<T> {

    var value: T? {
        get {
            var value: T?
            queue.sync {
                value = synchronizedValue
            }
            return value
        }
        set {
            set(value: newValue)
        }
    }

    private var synchronizedValue: T?

    private let queue: DispatchQueue

    init(queueLabel label: String) {
        queue = DispatchQueue(label: label, attributes: .concurrent)
    }

    convenience init() {
        self.init(queueLabel: "ReadersWriterLock")
    }

    func set(value: T?) {
        queue.sync(flags: .barrier) {
            synchronizedValue = value
        }
    }

    func setAsync(value: T?, completion: ((T?) -> Void)? = nil) {
        queue.async(flags: .barrier) {
            [ weak self ] in
            guard let _self = self else { return }
            _self.synchronizedValue = value
            completion?(value)
        }
    }

}

Example Usage

let lock = ReadersWriterLock<String>() // String is just for example, can be any type.

Set value asynchronously:

lock.setAsync(value: "Hello World!")

Set value synchronously:

lock.value = "Hello World!" // equivalent to: lock.set(value: "Hello World!")

Read value (only synchronous obviously):

let _ = lock.value

See https://en.wikipedia.org/wiki/Readers–writer_lock for more information.