C#学习笔记9

public class Thermostat
{
    private float currentTemperature;

    public float CurrentTemperature
    {
        get { return currentTemperature; }
        set
        {
            if (currentTemperature != value)
            {
                currentTemperature = value;
                /* 1.在这里并没有一开始就检查空值，而是首先将OnTemperatureChange赋值给另一个委托变量localOnChange，这个简单的修改可以确保在检查空值与发送通知之间，假如
                 * 所有的OnTemperatureChange订阅者都被移除（由一个不同的线程），那么不会触发NullReferenceException异常。
                 * 2.
                */
                var localOnChange = OnTemperatureChange;
                if (localOnChange == null)
                {
                    return;
                }
                //此处的遍历与错误处理，是防止在多播委托执行时，若在委托链中有一个委托方法出现错误，后续的委托方法执行就会中断。
                foreach (EventHandler<TemperatureArgs> item in localOnChange.GetInvocationList())
                {
                    try
                    {
                        item(this, new TemperatureArgs(value));
                    }
                    catch (Exception ex)
                    {
                        Console.WriteLine(ex.Message);
                    }
                }
            }
        }
    }
    //OnTemperatureChange = delegate { };赋值一个空委托，代表零个侦听者构成的集合，就可以引发侦听而不必检查是否有任何侦听者。
    public event EventHandler<TemperatureArgs> OnTemperatureChange;

    /// <summary>
    /// 观察者模式示例
    /// </summary>
    public static void Observer()
    {
        Adjust heater = new Adjust("heater", 50f, (original, newValue) => original > newValue);
        Adjust cooler = new Adjust("cooler", 70f, (original, newValue) => original < newValue);
        Thermostat thermostat = new Thermostat();
        thermostat.OnTemperatureChange += heater.OnTemperatureChange;
        thermostat.OnTemperatureChange += (sender, eArgs) => { throw new ApplicationException(); };
        thermostat.OnTemperatureChange += cooler.OnTemperatureChange;
        thermostat.CurrentTemperature = 40f;
        thermostat.OnTemperatureChange(null, null);
    }
}

public class TemperatureArgs : EventArgs
{
    public TemperatureArgs(float newTemprature)
    {
        NewTemprature = newTemprature;
    }

    public float NewTemprature { get; set; }
}

//C#编译器一旦遇到event关键字，就会生成与Thermostat2类中代码的等价代码的CIL代码。
public class Thermostat2
{
    /*private EventHandler<TemperatureArgs> onTemperatureChange;

    public void add_OnTemperatureChange(EventHandler<TemperatureArgs> handler)
    {
        Delegate.Combine(onTemperatureChange, handler);
    }

    public void remove_OnTemperatureChange(EventHandler<TemperatureArgs> handler)
    {
        Delegate.Remove(onTemperatureChange, handler);
    }

    public event EventHandler<TemperatureArgs> OnTemperatureChange
    {
        add { add_OnTemperatureChange(value); }
        remove { remove_OnTemperatureChange(value); }
    }*/
}

//自定义事件的实现
public class Thermostat3
{
    protected EventHandler<TemperatureArgs> onTemperatureChange;
    public event EventHandler<TemperatureArgs> OnTemperatureChange
    {
        add
        {
            //最后注册的订阅者，在通知时被第一个通知
            Delegate.Combine(value, onTemperatureChange);
        }
        remove { Delegate.Remove(onTemperatureChange, value); }
    }
}

/// <summary>
/// 调整器，用于控制加热器与制冷器的开关
/// </summary>
public class Adjust
{
    public Adjust(string controllerName, float temperature, Func<float, float, bool> predicate)
    {
        ControllerName = controllerName;
        Temperature = temperature;
        predicateSwitch = predicate;
    }
    /// <summary>
    /// 控制器名称，可以是制冷器、或加热器
    /// </summary>
    public string ControllerName { get; set; }
    public float Temperature { get; set; }
    public Func<float, float, bool> predicateSwitch { get; private set; }
    public void OnTemperatureChange(object sender, TemperatureArgs eArgs)
    {
        Console.WriteLine("{0} : {1}", ControllerName, predicateSwitch(Temperature, eArgs.NewTemprature) ? "On" : "Off");
    }
}