2019-01-23

  dispatch_async(dispatch_get_main_queue(), ^{
        NSLog(@"A");
    });
    NSLog(@"B");//DISPATCH_QUEUE_PRIORITY_BACKGROUND
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0);
    dispatch_sync(queue, ^{
        NSLog(@"C");
    });
    dispatch_async(queue, ^{
        NSLog(@"D");
    });
    dispatch_async(dispatch_get_main_queue(), ^{
        NSLog(@"E");
    });
    [self performSelector:@selector(aa) withObject:nil afterDelay:0.0];
    NSLog(@"G");
}
-(void)aa{
    NSLog(@"F");
}

DISPATCH_QUEUE_PRIORITY_BACKGROUND
 BCGAEDF
 BCGDAEF
 BCGAEFD