设计模式之观察者模式C#实现

说明：主要参考《Head First设计模式(中文版)》，使用C#代码实现。

代码：Github

1、观察者模式UML图

2、气象监测类图

3、气象监测代码(书中C#版)

3.1 Observer

public interface IObserver
{
    void Update(float temperature, float humidity, float pressure);
}

public interface IDisplayElement
{
    void Display();
}

public class CurrentConditionDisplay : IObserver, IDisplayElement
{
    private readonly ISubject _weatherData;

    private float _temperature;
    private float _humidity;

    public CurrentConditionDisplay(ISubject weatherData)
    {
        _weatherData = weatherData;
        _weatherData?.RegisterObserver(this);
    }

    public void Update(float temperature, float humidity, float pressure)
    {
        _temperature = temperature;
        _humidity = humidity;
        Display();
    }

    public void Display()
    {
        Console.WriteLine($"Current Conditions: {_temperature}F degress and {_humidity}% humidity");
    }
}

3.2 Subject

public interface ISubject
{
    void RegisterObserver(IObserver o);
    void RemoveObserver(IObserver o);
    void NotifyObservers();
}

public class WeatherData : ISubject
{
    //观察者列表
    private readonly List<IObserver> _observerList;

    //天气数据
    private float _temperature;
    private float _humidity;
    private float _pressure;

    public WeatherData()
    {
        _observerList = new List<IObserver>();
    }

    /// <summary>
    /// 订阅观察者
    /// </summary>
    /// <param name="o">观察者对象</param>
    public void RegisterObserver(IObserver o)
    {
        _observerList.Add(o);
    }

    /// <summary>
    /// 移除观察者
    /// </summary>
    /// <param name="o">观察者对象</param>
    public void RemoveObserver(IObserver o)
    {
        if (_observerList.Contains(o))
        {
            _observerList.Remove(o);
        }
    }

    /// <summary>
    /// 发布数据
    /// </summary>
    public void NotifyObservers()
    {
        foreach (var observer in _observerList)
        {
            observer.Update(_temperature, _humidity, _pressure);
        }
    }

    /// <summary>
    /// 数据发生改变
    /// </summary>
    private void MeasurementChanged()
    {
        NotifyObservers();
    }

    /// <summary>
    /// 更新数据
    /// </summary>
    /// <param name="temperature">温度</param>
    /// <param name="humidity">湿度</param>
    /// <param name="pressure">气压</param>
    public void SetMeasurements(float temperature, float humidity, float pressure)
    {
        _temperature = temperature;
        _humidity = humidity;
        _pressure = pressure;
        MeasurementChanged();
    }
}

3.3 测试代码

private static void TestWeatherData()
{
    var weatherData = new WeatherData();
    var currentConditionDisplay = new CurrentConditionDisplay(weatherData);

    weatherData.SetMeasurements(80, 65, 30.4f);
    weatherData.SetMeasurements(82, 70, 29.2f);
    weatherData.SetMeasurements(78, 90, 29.2f);
}

4、使用C#中IObservable接口实现气象监测

4.1 辅助类/结构体

public struct WeatherMessage
{
    public float Temperature { get; set; }
    public float Humidity { get; set; }
    public float Pressure { get; set; }
}

public class MessageUnknownException : Exception
{
    internal MessageUnknownException()
    {

    }
}

4.2 IObserver具体实现

class CurrentConditionDisplayX : IObserver<WeatherMessage>, IDisplayElement
{
    private IDisposable _unsubscribe;

    private float _temperature;
    private float _humidity;

    public void Subscribe(IObservable<WeatherMessage> provider)
    {
        if (provider != null)
        {
            _unsubscribe = provider.Subscribe(this);
        }
    }

    public void Unsubscribe()
    {
        _unsubscribe?.Dispose();
        _unsubscribe = null;
    }

    public void OnCompleted()
    {
        Console.WriteLine("CurrentConditionDisplayX: OnCompleted");
        Unsubscribe();
    }

    public void OnError(Exception error)
    {
        Console.WriteLine("CurrentConditionDisplayX: OnError");
    }

    public void OnNext(WeatherMessage value)
    {
        _temperature = value.Temperature;
        _humidity = value.Humidity;
        Display();
    }

    public void Display()
    {
        Console.WriteLine($"Current Conditions: {_temperature}F degress and {_humidity}% humidity");
    }
}

4.3 IObservable具体实现

public class WeatherDataX : IObservable<WeatherMessage>
{
    private readonly List<IObserver<WeatherMessage>> _observerList;

    public WeatherDataX()
    {
        _observerList = new List<IObserver<WeatherMessage>>();
    }

    /// <summary>
    /// 通知提供程序：某观察程序将要接收通知。
    /// </summary>
    /// <param name="observer">要接收通知的对象。</param>
    /// <returns>使资源释放的观察程序的接口。</returns>
    public IDisposable Subscribe(IObserver<WeatherMessage> observer)
    {
        if(!_observerList.Contains(observer))
        {
            _observerList.Add(observer);
        }
        return new Unsubcriber(_observerList, observer);
    }

    public void SetMeasurements(Nullable<WeatherMessage> message)
    {
        foreach (var observer in _observerList)
        {
            if (!message.HasValue)
            {
                observer.OnError(new MessageUnknownException());
            }
            else
            {
                observer.OnNext(message.Value);
            }
        }
    }

    public void EndTransmission()
    {
        foreach (var observer in _observerList.ToArray())
        {
            if (_observerList.Contains(observer))
            {
                observer.OnCompleted();
            }
        }
        _observerList.Clear();
    }

    private class Unsubcriber : IDisposable
    {
        private List<IObserver<WeatherMessage>> _observerList;
        private IObserver<WeatherMessage> _observer;

        public Unsubcriber(List<IObserver<WeatherMessage>> observerList, IObserver<WeatherMessage> observer)
        {
            _observerList = observerList;
            _observer = observer;
        }

        public void Dispose()
        {
            if (_observerList != null && _observerList.Contains(_observer))
            {
                _observerList.Remove(_observer);
            }
        }
    }
}

4.4 测试代码

private static void TestWeatherDataX()
{
    var weatherData = new WeatherDataX();
    var currentConditionDisplay = new CurrentConditionDisplayX();

    currentConditionDisplay.Subscribe(weatherData);

    weatherData.SetMeasurements(new WeatherMessage()
    {
        Temperature = 80,
        Humidity = 65,
        Pressure = 30.4f
    });
    weatherData.SetMeasurements(new WeatherMessage()
    {
        Temperature = 82,
        Humidity = 70,
        Pressure = 29.2f
    });
    weatherData.SetMeasurements(new WeatherMessage()
    {
        Temperature = 78,
        Humidity = 90,
        Pressure = 29.2f
    });
    weatherData.EndTransmission();
}