@synchronized()

Objective-C supports multithreading in applications. Therefore, two threads can try to modify the same object at the same time, a situation that can cause serious problems in a program. To protect sections of code from being executed by more than one thread at a time, Objective-C provides the @synchronized() directive.

The @synchronized()directive locks a section of code for use by a single thread. Other threads are blocked until the thread exits the protected code—that is, when execution continues past the last statement in the @synchronized()block.

The @synchronized() directive takes as its only argument any Objective-C object, including self. This object is known as a mutual exclusion semaphore or mutex. It allows a thread to lock a section of code to prevent its use by other threads. You should use separate semaphores to protect different critical sections of a program. It’s safest to create all the mutual exclusion objects before the application becomes multithreaded, to avoid race conditions.

Locking a method using self

- (void)criticalMethod

{

@synchronized(self) {

// Critical code.

...

}

}

Locking a method using a custom semaphore

Account *account = [Account accountFromString:[accountField stringValue]];

// Get the semaphore.

id accountSemaphore = [Account semaphore];

@synchronized(accountSemaphore) {

// Critical code.

...

}