@synchronized - skip instead of wait

Question

I've got an Objective-c app with several blocks of code where I only want one thread to be able to access it at a time. Using a @synchronized(self) block works fine for that.

However, I've got one block where I want it to skip the block, if another thread is in one of the @synchronized blocks, rather than wait. Is there a way to simply test if self (or whatever my lock object is) is being held by another synchronized block?

// block 1 - wait until lock is available
@synchronized(self) {
    ...
}
...
// block 2 - wait until lock is available
@synchronized(self) {
    ...
}
...
// block 3 - wait until lock is available
@synchronized(self) {
    ...
}
...
// block 4 - skip if lock is not immediately available - no waiting!
howDoISkipIfLockIsNotAvailable(self) {
    ...
}

Answer 1

What you want to achieve is possible when using NSLock or NSRecursiveLock instead of the @synchronized syntax sugar. They key feature it offers is the tryLock method:

NSRecursiveLock *lock = [[NSRecursiveLock alloc] init];

[lock lock];
@try {
    // ... do synchronized stuff
}
@finally {
    [lock unlock];
}

if ([lock tryLock]) {
    @try {
        // do synchronized stuff if possible
    }
    @finally {
        [lock unlock];
    }
}