dispatch_once_on_main_thread

gistfile1.txt

void dispatch_once_on_main_thread(dispatch_once_t *predicate,
                                  dispatch_block_t block) {
    if ([NSThread isMainThread]) {
        dispatch_once(predicate, block);
    } else {
        if (DISPATCH_EXPECT(*predicate == 0L, NO)) {
            dispatch_sync(dispatch_get_main_queue(), ^{
                dispatch_once(predicate, block);
            });
        }
    }
}