Observer 观察者模式

项目：气象监测系统

需求分析：：此系统中三个部分是气象站（获取实际气象数据的物理装置）、WeatherData对象（追踪气象站的数据，并更新布告板）和布告板（显示目前天气状况给用户看）。

WeatherData对象知道如何跟物理气象站联系，以取得更新的数据。WeatherData对象会随机更新三个布告板的显示：目前状况（温度、湿度、气压）、气象统计和天气预报。

当WeatherData对象发生变化时，即刻更新数据并推送给三个布告板。因此WeatherData需要获得布告板的引用，在数据更新时发送给布告板。用代码说话：

首先设计2个接口：

1. Subject,包含3个方法：Register, Remove, NotifyObservers

2.Observer， 包含1个方法：Update

设计WeatherData，实现Subject。除此之外还需要获得布告板引用，添加字段observerList集合，以及温度、湿度、气压字段。还有设置天气属性的的方法SetMeasurements,

MeasurementsChanged方法，当设置天气属性时即调用改方法，然后通知注册的用户。

namespace ObserverPattern
{
    public  interface Subject
    {
        void RegisterObserver(Observer o);
        void RemoveObserver(Observer o);
        void NotifyObservers();
    }
}

namespace ObserverPattern
{
    public interface Observer
    {
        void Update(float temp, float humidity, float pressure);
    }
}

using System.Collections.Generic;

namespace ObserverPattern
{
    public class WeatherData : Subject
    {
        private List<Observer> observerList = new List<Observer>();
        private float temperature;
        private float humidity;
        private float pressure;

        public void NotifyObservers()
        {
            foreach (Observer o in observerList)
            {
                o.Update(temperature, humidity, pressure);
            }
        }

        public void RegisterObserver(Observer o)
        {
            observerList.Add(o);
        }

        public void RemoveObserver(Observer o)
        {
            if(observerList.Contains(o))
            {
                observerList.Remove(o);
            }
          
        }
        /// <summary>
        /// 设置温度、湿度、气压
        /// </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)
        {
            this.temperature = temperature;
            this.humidity = humidity;
            this.pressure = pressure;
            MeasurementsChanged();
        }
        private void MeasurementsChanged( )
        {
            NotifyObservers();
        }
    }
}

接下来时设计3个布告板类，直接上代码：

using System;

namespace ObserverPattern
{
    /// <summary>
    /// 此布告板根据WeatherData对象显示当前观测值
    /// </summary>
    public class CurrentConditionsDisplay : Observer, DisplayElement
    {
        private float temperature;
        private float humidity;
      
        private Subject weatherData;

        public CurrentConditionsDisplay()
        {

        }
        public CurrentConditionsDisplay(Subject w)
        {
            weatherData = w;
            weatherData.RegisterObserver(this);
        }
        /// <summary>
        /// 把最近的温度和湿度显示出来
        /// </summary>
        public void Display()
        {
            Console.WriteLine("Current conditions:{0} F degress and {1} % humidity", temperature, humidity);
        }

     
        public void Update(float temp, float humidity, float pressure)
        {
            this.temperature = temp;
            this.humidity = humidity;
            Display();
        }
    }
}

using System;

namespace ObserverPattern
{
    /// <summary>
    /// 此布告板根据气压计显示天气预报
    /// </summary>
    public class ForecastDisplay : Observer, DisplayElement
    {
        private WeatherData weatherData;
        private float currentPressure = 29.92f;
        private float lastPressure;
        public ForecastDisplay()
        {

        }
        public ForecastDisplay(WeatherData w)
        {
            weatherData = w;
            w.RegisterObserver(this);
        }
        public void Display()
        {
            Console.Write("Forecast: ");
            if(currentPressure > lastPressure)
            {
                Console.WriteLine("天气状况转良!");
            }
            else if(currentPressure == lastPressure)
            {
                Console.WriteLine("气压没有变化");
            }
            else
            {
                Console.WriteLine("注意天气变凉，雨水天气");
            }
        }

      

        public void Update(float temp, float humidity, float pressure)
        {
            lastPressure = currentPressure;
            currentPressure = pressure;
            Display();
        }
    }
}

using System;

namespace ObserverPattern
{
    /// <summary>
    /// 此布告板跟踪最小、平均、最大的观测值，并显示它们
    /// </summary>
    public class StatisticsDisplay : Observer, DisplayElement
    {
        private WeatherData weatherData;
        private float maxTemp = 0f;
        private float minTemp = 200f;
        private float tempSum = 0f;
        private static int numReadings = 0;
        public StatisticsDisplay(WeatherData w)
        {
            weatherData = w;
            weatherData.RegisterObserver(this);
        }
        public StatisticsDisplay()
        {
                
        }
        public void Display()
        {
            Console.WriteLine("Avg/Max/Min temperature = " + (tempSum / numReadings)
            + "/" + maxTemp + "/" + minTemp);
        }

       

        public void Update(float temp, float humidity, float pressure)
        {
            tempSum += temp;
            numReadings++;
            if(temp > maxTemp)
            {
                maxTemp = temp;
            }
            if(temp < minTemp)
            {
                minTemp = temp;
            }
            Display();
        }
    }
}

                                     

using System;

namespace ObserverPattern
{
    class WeatherStation
    {
        static void Main(string[] args)
        {
            WeatherData weatherData = new WeatherData();
            CurrentConditionsDisplay currentDisplay = new CurrentConditionsDisplay(weatherData);
            ForecastDisplay forecastDisplay = new ForecastDisplay(weatherData);
            StatisticsDisplay statisticsDisplay = new StatisticsDisplay(weatherData);
            //模拟气象测量
            weatherData.SetMeasurements(80f, 65f, 30.4f);
            weatherData.SetMeasurements(100f, 62f, 28.0f);
            weatherData.SetMeasurements(86f, 72f, 26.3f);
            Console.ReadKey();
        }
    }
}

运行结果：