static async Task Main(string[] args)
{
IObserver consoleObserver = new ConsoleObserver();
IObserver fileObserver = new FileObserver();
Subject subject = new Subject();
subject.Subscribe(consoleObserver);
subject.Subscribe(fileObserver);
await subject.DoImportantWorkAsync();
subject.Unsubscribe(fileObserver);
await subject.DoImportantWorkAsync();
}
