NSLock
An object that coordinates the operation of multiple threads of execution within the same application.
Declaration
class NSLockOverview
An NSLock object can be used to mediate access to an application’s global data or to protect a critical section of code, allowing it to run atomically.
You should not use this class to implement a recursive lock. Calling the lock method twice on the same thread will lock up your thread permanently. Use the NSRecursiveLock class to implement recursive locks instead.
Unlocking a lock that is not locked is considered a programmer error and should be fixed in your code. The NSLock class reports such errors by printing an error message to the console when they occur.