deadlock.swift

class Account: NSObject {
    var balance: Double
    var id: Int
    
    override init(id: Int, balance: Double) {
        self.id = id
        self.balance = balance
    }
    
    func withdraw(amount: Double) {
        balance -= amount
    }
    
    func deposit(amount: Double) {
        balance += amount
    }
}

let a = Account(id: 1, balance: 1000)
let b = Account(id: 2, balance: 300)

DispatchQueue.global(qos: .background).async {
    transfer(from: a, to: b, amount: 200)
}

DispatchQueue.global(qos: .background).async {
    transfer(from: b, to: a, amount: 200)
}

func transfer(from: Account, to: Account, amount: Double) {
    from.synchronized(lockObj: self) { () -> T in
        to.synchronized(lockObj: self) { () -> T in
            from.withdraw(amount: amount)
            to.deposit(amount: amount)
        }
    }
}

extension NSObject {
    func synchronized<T>(lockObj: AnyObject!, closure: () throws -> T) rethrows ->  T
    {
        objc_sync_enter(lockObj)
        defer {
            objc_sync_exit(lockObj)
        }
        
        return try closure()
    }
}