simple block-based task queue with cascade and async trail processing

gistfile1.txt

dispatch_queue_t trail_queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t trail_group = dispatch_group_create();

NSMutableArray *tasks = [[NSMutableArray alloc] init];
void (^nextTask)() = ^() {
    [tasks removeObjectAtIndex: 0];
    if ([tasks count] > 0) {
        void (^task)() = [tasks objectAtIndex: 0];
        task();
    } else {
        dispatch_group_notify(trail_group, trail_queue, ^{
            NSLog(@"finish");
        }); 
        dispatch_group_wait(trail_group, DISPATCH_TIME_FOREVER);
        dispatch_release(trail_group);
    }
};

[tasks addObject: ^() {
    NSLog(@"task1");
    dispatch_group_async(trail_group, trail_queue, ^{
        NSLog(@"task1 trail");
    });
    nextTask();
}];

[tasks addObject: ^() {
    NSLog(@"task2");
    dispatch_group_async(trail_group, trail_queue, ^{
        NSLog(@"task2 trail");
    });
    nextTask();
}];

void (^task)() = [tasks objectAtIndex: 0];
task();