• 多个线程访问同一块资源时候，容易引发数据错乱与数据安全问题
• 使用GCD中的串行队列，是实现线程同步的一个常规方案，并且效率也不低

资源竞争的案例

    // 起初有50块钱
    private var money = 50
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        /// 存钱取钱测试代码
        moneyTest()
    }
    
    
    /// 多线程竞争money这个资源
    func moneyTest() {
        DispatchQueue.global().async {
            for _ in 0...10 {
                self.saveMoney()
            }
        }
        
        DispatchQueue.global().async {
            for _ in 0...10 {
                self.drawMoney()
            }
        }
    }

    
    /// 存钱
    private func saveMoney() {
        var oldMoney = self.money
        sleep(1)
        oldMoney = oldMoney + 50
        self.money = oldMoney
        print("存50，还剩余\(oldMoney) - \(Thread.current)")
    }
    
    /// 取钱
    private func drawMoney() {
        var oldMoney = self.money
        sleep(1)
        oldMoney = oldMoney - 20
        self.money = oldMoney
        print("取20，还剩余\(oldMoney) - \(Thread.current)")
    }

竞争导致的后果

取20，还剩余0 - <NSThread: 0x60000300b400>{number = 6, name = (null)}
存50，还剩余100 - <NSThread: 0x6000030461c0>{number = 7, name = (null)}
取20，还剩余50 - <NSThread: 0x60000300b400>{number = 6, name = (null)}
存50，还剩余150 - <NSThread: 0x6000030461c0>{number = 7, name = (null)}
取20，还剩余0 - <NSThread: 0x60000300b400>{number = 6, name = (null)}
存50，还剩余200 - <NSThread: 0x6000030461c0>{number = 7, name = (null)}
取20，还剩余-50 - <NSThread: 0x60000300b400>{number = 6, name = (null)}
存50，还剩余250 - <NSThread: 0x6000030461c0>{number = 7, name = (null)}
取20，还剩余-100 - <NSThread: 0x60000300b400>{number = 6, name = (null)}
存50，还剩余300 - <NSThread: 0x6000030461c0>{number = 7, name = (null)}
取20，还剩余-150 - <NSThread: 0x60000300b400>{number = 6, name = (null)}
存50，还剩余350 - <NSThread: 0x6000030461c0>{number = 7, name = (null)}
取20，还剩余-200 - <NSThread: 0x60000300b400>{number = 6, name = (null)}
存50，还剩余400 - <NSThread: 0x6000030461c0>{number = 7, name = (null)}
取20，还剩余-250 - <NSThread: 0x60000300b400>{number = 6, name = (null)}
存50，还剩余450 - <NSThread: 0x6000030461c0>{number = 7, name = (null)}
取20，还剩余-300 - <NSThread: 0x60000300b400>{number = 6, name = (null)}
存50，还剩余500 - <NSThread: 0x6000030461c0>{number = 7, name = (null)}
取20，还剩余-350 - <NSThread: 0x60000300b400>{number = 6, name = (null)}
存50，还剩余550 - <NSThread: 0x6000030461c0>{number = 7, name = (null)}
取20，还剩余-400 - <NSThread: 0x60000300b400>{number = 6, name = (null)}
存50，还剩余600 - <NSThread: 0x6000030461c0>{number = 7, name = (null)}

如何解决该问题

    // 起初有50块钱
    private var money = 50
    
    // 创建一个串行队列
    private let queue = DispatchQueue(label: "com.nsl.serialQueue")
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        /// 存钱取钱测试代码
        moneyTest()
    }
    
    /// 多线程竞争money这个资源
    func moneyTest() {
        DispatchQueue.global().async {
            for _ in 0...10 {
                self.saveMoney()
            }
        }
        
        DispatchQueue.global().async {
            for _ in 0...10 {
                self.drawMoney()
            }
        }
    }

    /// 存钱
    private func saveMoney() {
        // 串行 + 同步
        self.queue.sync {
            var oldMoney = self.money
            sleep(1)
            oldMoney = oldMoney + 50
            self.money = oldMoney
            print("存50，还剩余\(oldMoney) - \(Thread.current)")
        }
  
    }
    
    /// 取钱
    private func drawMoney() {
        // 串行 + 同步
        self.queue.sync {
            var oldMoney = self.money
            sleep(1)
            oldMoney = oldMoney - 20
            self.money = oldMoney
            print("取20，还剩余\(oldMoney) - \(Thread.current)")
        }
    }

解决问题后的输出

取20，还剩余30 - <NSThread: 0x600001222740>{number = 6, name = (null)}
存50，还剩余80 - <NSThread: 0x60000122e780>{number = 7, name = (null)}
取20，还剩余60 - <NSThread: 0x600001222740>{number = 6, name = (null)}
存50，还剩余110 - <NSThread: 0x60000122e780>{number = 7, name = (null)}
取20，还剩余90 - <NSThread: 0x600001222740>{number = 6, name = (null)}
存50，还剩余140 - <NSThread: 0x60000122e780>{number = 7, name = (null)}
取20，还剩余120 - <NSThread: 0x600001222740>{number = 6, name = (null)}
存50，还剩余170 - <NSThread: 0x60000122e780>{number = 7, name = (null)}
取20，还剩余150 - <NSThread: 0x600001222740>{number = 6, name = (null)}
存50，还剩余200 - <NSThread: 0x60000122e780>{number = 7, name = (null)}
取20，还剩余180 - <NSThread: 0x600001222740>{number = 6, name = (null)}
存50，还剩余230 - <NSThread: 0x60000122e780>{number = 7, name = (null)}
取20，还剩余210 - <NSThread: 0x600001222740>{number = 6, name = (null)}
存50，还剩余260 - <NSThread: 0x60000122e780>{number = 7, name = (null)}
取20，还剩余240 - <NSThread: 0x600001222740>{number = 6, name = (null)}
存50，还剩余290 - <NSThread: 0x60000122e780>{number = 7, name = (null)}
取20，还剩余270 - <NSThread: 0x600001222740>{number = 6, name = (null)}
存50，还剩余320 - <NSThread: 0x60000122e780>{number = 7, name = (null)}
取20，还剩余300 - <NSThread: 0x600001222740>{number = 6, name = (null)}
存50，还剩余350 - <NSThread: 0x60000122e780>{number = 7, name = (null)}
取20，还剩余330 - <NSThread: 0x600001222740>{number = 6, name = (null)}
存50，还剩余380 - <NSThread: 0x60000122e780>{number = 7, name = (null)}