观察者模式 The Observer Pattern

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        /// <summary>
        /// 主题对象
        /// </summary>
            public interface ISubject
            {
                List<IObserver> Observers
                {
                    get;
                    set;
                }
                void RegisterObserver(IObserver observer);
                void RemoveObserver(IObserver observer);
                void NotifyObservers(string command);
            }
        /// <summary>
        /// 观察者
        /// </summary>
            public interface IObserver
            {
                void Update(string command);
            }
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