struct TaskRunner<'a> {
    cb: Option<&'a TaskFinished>,
}

impl<'a> TaskRunner<'a> {
    fn new() -> Self { TaskRunner { cb: None } }

    fn start_task(&mut self, cb: &'a TaskFinished) {
        self.cb = Some(cb);
    }

    fn finish_task(&mut self) {
        if let Some(cb) = self.cb.take() {
            cb.task_finished();
        }
    }
}

trait TaskFinished {
    fn task_finished(&self);
}

struct Manager<'a> {
    task_runner: TaskRunner<'a>,
}

impl<'a> Manager<'a> {
    fn new() -> Self {
        Manager {
            task_runner: TaskRunner::new(),
        }
    }

    fn run_task(&'a mut self) {
        self.task_runner.start_task(self);
        self.task_runner.finish_task();
    }
}

impl<'a> TaskFinished for Manager<'a> {
    fn task_finished(&self) {
        println!("Task finished! Going to do the next thing!");
    }
}

fn main() {
    let mut manager = Manager::new();
    manager.run_task();
}