Swift 变量是原子的吗？

Question

在 Objective-C 中，您可以区分原子属性和非原子属性：

@property (nonatomic, strong) NSObject *nonatomicObject;
@property (atomic, strong) NSObject *atomicObject;

据我了解，您可以安全地从多个线程读取和写入定义为原子的属性，而同时从多个线程写入和访问非原子属性或 ivars 可能会导致未定义的行为，包括错误的访问错误。

所以如果你在 Swift 中有这样的变量：

var object: NSObject

我可以安全地并行读写这个变量吗？ （不考虑这样做的实际意义）。

Answer 1

现在假设没有可用的低级文档还为时过早，但您可以从汇编中学习。 Hopper Disassembler 是一个很棒的工具。

@interface ObjectiveCar : NSObject
@property (nonatomic, strong) id engine;
@property (atomic, strong) id driver;
@end

将objc_storeStrong 和objc_setProperty_atomic 分别用于非原子和原子，其中

class SwiftCar {
    var engine : AnyObject?    
    init() {
    }
}

使用来自libswift_stdlib_core 的swift_retain，并且显然没有内置线程安全。

我们可以推测以后可能会引入额外的关键字（类似于@lazy）。

2015 年 7 月 20 日更新：根据这个 blogpost on singletons swift 环境可以使某些情况对您来说是线程安全的，即：

class Car {
    static let sharedCar: Car = Car() // will be called inside of dispatch_once
}

private let sharedCar: Car2 = Car2() // same here
class Car2 {

}

2016 年 5 月 25 日更新：留意 swift evolution 提议 https://github.com/apple/swift-evolution/blob/master/proposals/0030-property-behavior-decls.md - 看起来有可能由您自己实现 @atomic 行为。 p>

Answer 2

Swift 没有围绕线程安全的语言结构。假设您将使用提供的库来进行自己的线程安全管理。 在实现线程安全方面有很多选择，包括 pthread 互斥锁、NSLock 和 dispatch_sync 作为互斥锁机制。请参阅 Mike Ash 最近关于该主题的帖子：https://mikeash.com/pyblog/friday-qa-2015-02-06-locks-thread-safety-and-swift.html 所以直接回答你的问题“我可以安全地并行读写这个变量吗？”是没有。

Answer 3

现在回答这个问题可能为时过早。目前 swift 缺少访问修饰符，因此没有明显的方法来添加代码来管理围绕属性 getter/setter 的并发性。此外，Swift 语言似乎还没有任何关于并发的信息！ （它也缺少 KVO 等...）

我认为这个问题的答案将在未来的版本中变得清晰。

Answer 4

详情

• Xcode 9.1，Swift 4
• Xcode 10.2.1 (10E1001)、Swift 5

链接

• apple.developer.com Dispatch
• Grand Central Dispatch (GCD) and Dispatch Queues in Swift 3
• Creating Thread-Safe Arrays in Swift
• Mutexes and closure capture in Swift

实现的类型

• QueueSafeValue - 一个通用包装器，提供对值的安全多线程访问
• AtomicArray
• AtomicInteger
• AtomicValue

主要思想

class Example {
    
    private lazy var semaphore = DispatchSemaphore(value: 1)
    
    func executeThreadSafeFunc1() {
        // Lock access. Only first thread can execute code below.
        // Other threads will wait until semaphore.signal() will execute
        semaphore.wait()
        // your code
        semaphore.signal()         // Unlock access
    }
    
    func executeThreadSafeFunc2() {
        // Lock access. Only first thread can execute code below.
        // Other threads will wait until semaphore.signal() will execute
        semaphore.wait()
        DispatchQueue.global(qos: .background).async {
            // your code
            self.semaphore.signal()         // Unlock access
        }
    }
}

原子访问示例

class Atomic {
    
    let dispatchGroup = DispatchGroup()
    private var variable = 0
    
    // Usage of semaphores
    
    func semaphoreSample() {
        
        // value: 1 - number of threads that have simultaneous access to the variable
        let atomicSemaphore = DispatchSemaphore(value: 1)
        variable = 0
        
        runInSeveralQueues { dispatchQueue  in
            // Only (value) queqes can run operations betwen atomicSemaphore.wait() and atomicSemaphore.signal()
            // Others queues await their turn
            atomicSemaphore.wait()            // Lock access until atomicSemaphore.signal()
            self.variable += 1
            print("\(dispatchQueue), value: \(self.variable)")
            atomicSemaphore.signal()          // Unlock access
        }
        
        notifyWhenDone {
            atomicSemaphore.wait()           // Lock access until atomicSemaphore.signal()
            print("variable = \(self.variable)")
            atomicSemaphore.signal()         // Unlock access
        }
    }
    
    // Usage of sync of DispatchQueue
    
    func dispatchQueueSync() {
        let atomicQueue = DispatchQueue(label: "dispatchQueueSync")
        variable = 0
        
        runInSeveralQueues { dispatchQueue  in
            
            // Only queqe can run this closure (atomicQueue.sync {...})
            // Others queues await their turn
            atomicQueue.sync {
                self.variable += 1
                print("\(dispatchQueue), value: \(self.variable)")
            }
        }
        
        notifyWhenDone {
            atomicQueue.sync {
                print("variable = \(self.variable)")
            }
        }
    }
    
    // Usage of objc_sync_enter/objc_sync_exit
    
    func objcSync() {
        variable = 0
        
        runInSeveralQueues { dispatchQueue  in
            
            // Only one queqe can run operations betwen objc_sync_enter(self) and objc_sync_exit(self)
            // Others queues await their turn
            objc_sync_enter(self)                   // Lock access until objc_sync_exit(self).
            self.variable += 1
            print("\(dispatchQueue), value: \(self.variable)")
            objc_sync_exit(self)                    // Unlock access
        }
        
        notifyWhenDone {
            objc_sync_enter(self)                   // Lock access until objc_sync_exit(self)
            print("variable = \(self.variable)")
            objc_sync_exit(self)                    // Unlock access
        }
    }
}

// Helpers

extension Atomic {

    fileprivate func notifyWhenDone(closure: @escaping ()->()) {
        dispatchGroup.notify(queue: .global(qos: .utility)) {
            closure()
            print("All work done")
        }
    }
    
    fileprivate func runInSeveralQueues(closure: @escaping (DispatchQueue)->()) {
        
        async(dispatch: .main, closure: closure)
        async(dispatch: .global(qos: .userInitiated), closure: closure)
        async(dispatch: .global(qos: .utility), closure: closure)
        async(dispatch: .global(qos: .default), closure: closure)
        async(dispatch: .global(qos: .userInteractive), closure: closure)
    }
    
    private func async(dispatch: DispatchQueue, closure: @escaping (DispatchQueue)->()) {
        
        for _ in 0 ..< 100 {
            dispatchGroup.enter()
            dispatch.async {
                let usec = Int(arc4random()) % 100_000
                usleep(useconds_t(usec))
                closure(dispatch)
                self.dispatchGroup.leave()
            }
        }
    }
}

用法

Atomic().semaphoreSample()
//Atomic().dispatchQueueSync()
//Atomic().objcSync()

结果

Answer 5

从 Swift 5.1 开始，您可以使用 property wrappers 为您的属性创建特定逻辑。这是原子包装器实现：

@propertyWrapper
struct atomic<T> {
    private var value: T
    private let lock = NSLock()

    init(wrappedValue value: T) {
        self.value = value
    }

    var wrappedValue: T {
      get { getValue() }
      set { setValue(newValue: newValue) }
    }

    func getValue() -> T {
        lock.lock()
        defer { lock.unlock() }

        return value
    }

    mutating func setValue(newValue: T) {
        lock.lock()
        defer { lock.unlock() }

        value = newValue
    }
}

使用方法：

class Shared {
    @atomic var value: Int
...
}

Answer 6

这是我广泛使用的原子属性包装器。我把实际的锁定机制做成了一个协议，所以我可以体验不同的机制。我尝试了信号量DispatchQueues 和pthread_rwlock_t。选择 pthread_rwlock_t 是因为它的开销似乎最低，并且优先级反转的可能性较低。

/// Defines a basic signature that all locks will conform to. Provides the basis for atomic access to stuff.
protocol Lock {
    init()
    /// Lock a resource for writing. So only one thing can write, and nothing else can read or write.
    func writeLock()
    /// Lock a resource for reading. Other things can also lock for reading at the same time, but nothing else can write at that time.
    func readLock()
    /// Unlock a resource
    func unlock()
}

final class PThreadRWLock: Lock {
    private var rwLock = pthread_rwlock_t()

    init() {
        guard pthread_rwlock_init(&rwLock, nil) == 0 else {
            preconditionFailure("Unable to initialize the lock")
        }
    }

    deinit {
        pthread_rwlock_destroy(&rwLock)
    }

    func writeLock() {
        pthread_rwlock_wrlock(&rwLock)
    }

    func readLock() {
        pthread_rwlock_rdlock(&rwLock)
    }

    func unlock() {
        pthread_rwlock_unlock(&rwLock)
    }
}

/// A property wrapper that ensures atomic access to a value. IE only one thing can write at a time.
/// Multiple things can potentially read at the same time, just not during a write.
/// By using `pthread` to do the locking, this safer then using a `DispatchQueue/barrier` as there isn't a chance
/// of priority inversion.
@propertyWrapper
public final class Atomic<Value> {

    private var value: Value
    private let lock: Lock = PThreadRWLock()

    public init(wrappedValue value: Value) {
        self.value = value
    }

    public var wrappedValue: Value {
        get {
            self.lock.readLock()
            defer { self.lock.unlock() }
            return self.value
        }
        set {
            self.lock.writeLock()
            self.value = newValue
            self.lock.unlock()
        }
    }

    /// Provides a closure that will be called synchronously. This closure will be passed in the current value
    /// and it is free to modify it. Any modifications will be saved back to the original value.
    /// No other reads/writes will be allowed between when the closure is called and it returns.
    public func mutate(_ closure: (inout Value) -> Void) {
        self.lock.writeLock()
        closure(&value)
        self.lock.unlock()
    }
}