Is there any way to “wait here…” in code - just like an empty loop?

Question

Consider this code:

[self otherStuff];
// \"wait here...\" until something finishes
while(!self.someFlag){}
[self moreStuff];

Note that thi

Answer 1

This is a solution using a semaphore, be careful not to introduce a deadlock - you need some way of telling your application something has finished, you can either do that using the NSNotificationCentre like you suggested but using a block is much easier.

[self someOtherStuffWithCompletion:nil];

dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);

[self someOtherStuffWithCompletion:^{
  dispatch_semaphore_signal(semaphore);
}];

NSLog(@"waiting");
dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
NSLog(@"finished");

[self someOtherStuffWithCompletion:nil];

Answer 2

I would suggest to use a NSOperationQueue and to wait for all tasks until they are finished at specific points. Something like that:

self.queue = [[NSOperationQueue alloc] init];

// Ensure a single thread
self.queue.maxConcurrentOperationCount = 1;

// Add the first bunch of methods
[self.queue addOperation:[[NSInvocationOperation alloc] initWithTarget:self selector:@selector(method1) object:nil]];
[self.queue addOperation:[[NSInvocationOperation alloc] initWithTarget:self selector:@selector(method2) object:nil]];
[self.queue addOperation:[[NSInvocationOperation alloc] initWithTarget:self selector:@selector(method3) object:nil]];

// Wait here
[self.queue waitUntilAllOperationsAreFinished];

// Add next methods
[self.queue addOperation:[[NSInvocationOperation alloc] initWithTarget:self selector:@selector(method4) object:nil]];
[self.queue addOperation:[[NSInvocationOperation alloc] initWithTarget:self selector:@selector(method5) object:nil]];

// Wait here
[self.queue waitUntilAllOperationsAreFinished];

HTH