CallOncePerRunloopHelper.m

/// Performs `block` immediately and ignores subsequent calls during the same runloop.
#define pspdf_ensureCalledOnlyOncePerRunloop(block) do { \
static const char __onceKey; _pspdf_ensureCalledOnlyOncePerRunloop(self, &__onceKey, block); } while(0)
extern void _pspdf_ensureCalledOnlyOncePerRunloop(id self, const void *key, dispatch_block_t block);

void _pspdf_ensureCalledOnlyOncePerRunloop(id self, const void *key, dispatch_block_t block) {
    NSCParameterAssert(block);
    NSCParameterAssert(self);
    PSPDFAssertOnMainThread(); // run loop needs the main thread.

    BOOL hasBeenCalled = [objc_getAssociatedObject(self, key) boolValue];
    if (!hasBeenCalled) {
        objc_setAssociatedObject(self, key, @YES, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
        block();

        // clean object in the next runloop
        dispatch_async(dispatch_get_main_queue(), ^{
            objc_setAssociatedObject(self, key, nil, OBJC_ASSOCIATION_ASSIGN);
        });
    }
}

// Usage:


- (void)willRotateToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation duration:(NSTimeInterval)duration {
    [super willRotateToInterfaceOrientation:toInterfaceOrientation duration:duration];

    pspdf_ensureCalledOnlyOncePerRunloop(^{
        NSLog(@"once.");
    });
}