Question

My problem is that I’m using dispatch_async(dispatch_get_main_queue(), ^(void) { ... }); to call a method asynchronously, in this method depending on some conditions i set a boolean to YES. When this boolean is read in this method, it’s always read by it’s old value which is NO.

The weird thing is that when i made a breakpoint on the line where the bool is checked, everything went fine and as intended !

EDIT:
Here is the code where the threads are spawned

dispatch_async(dispatch_get_main_queue(), ^(void) {
    [self drawFaceBoxesForFeatures:features forVideoBox:claporientation:curDeviceOrientation image:img];
});

The method itself

- (void)drawFaceBoxesForFeatures:(NSArray *)features forVideoBox:(CGRect)clap orientation:    (UIDeviceOrientation)orientation image:(UIImage *)image;
{
    if (![self getSendingRequestStatus]) {
        NSLog(@"Sending req");
        // send async request

         dispatch_async(dispatch_get_main_queue(),^ {
             sendingRequest = YES;
         } );
      }
}

Answer 1

It looks like you are modifying an ivar that was created outside of a block inside of the block. In order to do this and have the ivar hold the correct value, you are going to need to use the __block keyword like so:

@interface MyCoolClass : NSObject {
    @private
    __block int sendingRequest_;
}

As Jack Lawrence said in the commend above, “[the runtime] takes a snapshot of all of the relevant objects/variables at that point in time”. The __block identifier will tell the runtime that it should not copy that ivar to the heap and will allow you to assign values to sendingRequest_ inside of a block, even if that block is simply being run on the main thread.

A lot of good information to start with (including the above) can be found in the Blocks Programming Guide.