细节

Xcode 8.3.1, Swift 3.1

Task

从不同的线程读写值(异步)。

Code

class AsyncObject<T>:CustomStringConvertible {
    private var _value: T
    public private(set) var dispatchQueueName: String
   
    let dispatchQueue: DispatchQueue
    
    init (value: T, dispatchQueueName: String) {
        _value = value
        self.dispatchQueueName = dispatchQueueName
        dispatchQueue = DispatchQueue(label: dispatchQueueName)
    }
    
    func setValue(with closure: @escaping (_ currentValue: T)->(T) ) {
        dispatchQueue.sync { [weak self] in
            if let _self = self {
                _self._value = closure(_self._value)
            }
        }
    }
    
    func getValue(with closure: @escaping (_ currentValue: T)->() ) {
        dispatchQueue.sync { [weak self] in
            if let _self = self {
                closure(_self._value)
            }
        }
    }
    
    
    var value: T {
        get {
            return dispatchQueue.sync { _value }
        }
        
        set (newValue) {
            dispatchQueue.sync { _value = newValue }
        }
    }

    var description: String {
        return "\(_value)"
    }
}

使用

print("Single read/write action")
// Use it when when you need to make single action
let obj = AsyncObject<Int>(value: 0, dispatchQueueName: "Dispatch0")
obj.value = 100
let x = obj.value
print(x)

print("Write action in block")
// Use it when when you need to make many action
obj.setValue{ (current) -> (Int) in
    let newValue = current*2
    print("previous: \(current), new: \(newValue)")
    return newValue
}

完整的样品

扩展DispatchGroup

extension DispatchGroup {
    
    class func loop(repeatNumber: Int, action: @escaping (_ index: Int)->(), completion: @escaping ()->()) {
        let group = DispatchGroup()
        for index in 0...repeatNumber {
            group.enter()
            DispatchQueue.global(qos: .utility).async {
                action(index)
                group.leave()
            }
        }
        
        group.notify(queue: DispatchQueue.global(qos: .userInitiated)) {
            completion()
        }
    }
}

类ViewController

import UIKit

class ViewController: UIViewController {

    override func viewDidLoad() {
        super.viewDidLoad()

        //sample1()
        sample2()
    }
    
    func sample1() {
        print("=================================================\nsample with variable")
        
        let obj = AsyncObject<Int>(value: 0, dispatchQueueName: "Dispatch1")
        
        DispatchGroup.loop(repeatNumber: 5, action: { index in
            obj.value = index
        }) {
            print("\(obj.value)")
        }
    }
    
    func sample2() {
        print("\n=================================================\nsample with array")
        let arr = AsyncObject<[Int]>(value: [], dispatchQueueName: "Dispatch2")
        DispatchGroup.loop(repeatNumber: 15, action: { index in
            arr.setValue{ (current) -> ([Int]) in
                var array = current
                array.append(index*index)
                print("index: \(index), value \(array[array.count-1])")
                return array
            }
        }) {
            print("\(arr.value)")
        }
    }
}

类似于Objective-C中的@synchronized指令，在Swift中可以有任意的返回类型和漂亮的重抛出行为。

// Swift 3
func synchronized<T>(_ lock: AnyObject, _ body: () throws -> T) rethrows -> T {
    objc_sync_enter(lock)
    defer { objc_sync_exit(lock) }
    return try body()
}

使用defer语句可以直接返回一个值，而不需要引入临时变量。

在Swift 2中，为闭包添加@noescape属性可以进行更多的优化:

// Swift 2
func synchronized<T>(lock: AnyObject, @noescape _ body: () throws -> T) rethrows -> T {
    objc_sync_enter(lock)
    defer { objc_sync_exit(lock) }
    return try body()
}

基于GNewc[1](我喜欢任意返回类型)和Tod Cunningham[2](我喜欢延迟)的答案。

你可以使用GCD。它比@synchronized更详细一点，但可以作为替代品:

let serialQueue = DispatchQueue(label: "com.test.mySerialQueue")
serialQueue.sync {
    // code
}

你可以把语句夹在objc_sync_enter(obj: AnyObject?)和objc_sync_exit(obj: AnyObject?)之间。@synchronized关键字在幕后使用这些方法。即。

objc_sync_enter(self)
... synchronized code ...
objc_sync_exit(self)

另一种方法是创建一个超类，然后继承它。这样你可以更直接地使用GCD

class Lockable {
    let lockableQ:dispatch_queue_t

    init() {
        lockableQ = dispatch_queue_create("com.blah.blah.\(self.dynamicType)", DISPATCH_QUEUE_SERIAL)
    }

    func lock(closure: () -> ()) {
        dispatch_sync(lockableQ, closure)
    }
}


class Foo: Lockable {

    func boo() {
        lock {
            ....... do something
        }
    }

在2018年WWDC的“理解崩溃和崩溃日志”会议414中，他们展示了以下使用DispatchQueues与sync的方法。

在swift 4中应该像下面这样:

class ImageCache {
    private let queue = DispatchQueue(label: "sync queue")
    private var storage: [String: UIImage] = [:]
    public subscript(key: String) -> UIImage? {
        get {
          return queue.sync {
            return storage[key]
          }
        }
        set {
          queue.sync {
            storage[key] = newValue
          }
        }
    }
}

无论如何，您也可以使用带屏障的并发队列使读取更快。同步和异步读取同时执行，写入新值等待前一个操作完成。

class ImageCache {
    private let queue = DispatchQueue(label: "with barriers", attributes: .concurrent)
    private var storage: [String: UIImage] = [:]

    func get(_ key: String) -> UIImage? {
        return queue.sync { [weak self] in
            guard let self = self else { return nil }
            return self.storage[key]
        }
    }

    func set(_ image: UIImage, for key: String) {
        queue.async(flags: .barrier) { [weak self] in
            guard let self = self else { return }
            self.storage[key] = image
        }
    }
}