如何同步对具有 didSet 的属性的访问？

如何同步使用 didSet 的属性的访问（使用 GCD 或 objc_sync_enter）？

我有一处房产，有一个房产观察员。如何使用私有队列来同步属性的获取/设置

var state: State = .disconnected {
  // Q: How to sync get/set access here?
  didSet {
    // do something
  }
}

最简单的方法是使用串行队列

import Dispatch

struct S {
    private var i: Int = 0 {
        didSet {
            print("someone did set new value:", i)
        }
    }
    private let queue = DispatchQueue(label: "private", qos: .userInteractive) // high priority
    var value: Int {
        get {
            return queue.sync {
                return i
            }
        }
        set {
            if newValue == value {
                return
            }
            queue.sync {
                i = newValue
            }
        }
    }
}

更高级的示例使用并发读取和同步屏障进行写入

import Dispatch

struct S {
    private var i: Int = 0 {
        didSet {
            print("someone did set new value:", i)
        }
    }
    private let queue = DispatchQueue(label: "private", qos: .userInteractive, attributes: .concurrent) // high priority
    var value: Int {
        get {
            return queue.sync {
                return i
            }
        }
        set {
            if newValue == value {
                return
            }
            queue.sync(flags: .barrier) {
                i = newValue
            }
        }
    }
}

对于类属性，您可以使用并发私有队列并同时从不同线程读取并使用屏障异步分派写入

import Dispatch

class S {
    private var i: Int = 0 {
        didSet {
            print("someone did set new value:", i)
        }
    }
    private let queue = DispatchQueue(label: "private", qos: .userInteractive, attributes: .concurrent) // high priority
    var value: Int {
        get {
            return queue.sync {
                return i
            }
        }
        set {
            if newValue == value {
                return
            }
            queue.async(flags: .barrier) { [unowned self] in
                self.i = newValue
            }
        }
    }
}