Events基本概念----Beginning Visual C#

一个事件的发生需要有触发条件，如定时发生，鼠标点击等，以及即将发生的事件本身，因此，events相关的最简单的示例如下：

// 时间触发器
class TimerTrigger
{
    private Timer myTimer;

    public TimerTrigger() 
    {
        myTimer = new Timer(100);
    }
    
    public TimerTrigger(int timeInterval)
    {
        myTimer = new Timer(timeInterval);
    }

    // 向时间触发器添加具事件的handler
    public void AddEventHander(ElapsedEventHandler source)
    {
        myTimer.Elapsed += source;
    }

    public void Start()
    {
        myTimer.Start();
    }

    public void Stop()
    {
        myTimer.Stop();
    }
}

class Program
{
    static int counter = 0;
    static string displayString = 
        "This string will appear one letter at a time. ";

    // 即将被添加到时间触发器的事件
    static void WriteChar(object source, ElapsedEventArgs e)
    {
        Console.Write(displayString[counter++ % displayString.Length]);
    }

    public static void Main()
    {
        TimerTrigger timerTrigger = new TimerTrigger(100);
        timerTrigger.AddEventHander(new ElapsedEventHandler(WriteChar));
        timerTrigger.Start();
        Console.ReadLine();
    }
}

在上面的例子的main函数中，直接使用了c#自带的ElapsedEventHandler delegated类型，那么如何自定义事件处理函数呢？具体如下：

// 自定义时间处理代表
public delegate void MessageHandler(string message);

class TimerTrigger
{
    private Timer myTimer;
    private event MessageHandler MessageArrived;

    public TimerTrigger(int timeInterval)
    {
        myTimer = new Timer(timeInterval);
        myTimer.Elapsed += new ElapsedEventHandler(CheckForMessage);
    }

    public void AddEventHandler(MessageHandler source)
    {
        MessageArrived += source; 
    }

    public void Start()
    {
        myTimer.Start();
    }

    public void Stop()
    {
        myTimer.Stop();
    }

    private void CheckForMessage(object source, ElapsedEventArgs e)
    {
        Console.WriteLine("Checking for new messages.");
        if (MessageArrived != null)
        {
            MessageArrived("hello world");
        }

    }
}

class Program
{
    static int counter = 0;

    static void WriteFirstMessage(string message)
    {
        Console.WriteLine("The First {0} number: {1}", message, counter++);
    }

    static void WriteSecondMessage(string message)
    {
        Console.WriteLine("The Second {0} number: {1}", message, counter++);
    }

    public static void Main()
    {
        TimerTrigger timerTrigger = new TimerTrigger(100);
        timerTrigger.AddEventHandler(new MessageHandler(WriteFirstMessage));
        timerTrigger.AddEventHandler(new MessageHandler(WriteSecondMessage));
        timerTrigger.Start();
        Console.ReadLine();
    }
}

在上述的例子中，将消息传递给处理事件所用的函数为WriteFirstMessage(string message)，其传递的参数为string类型的。再考虑道<Timer>.ElapsedEventHandler所代表的函数的传递参数为：(object source, ElapsedEventArgs e)两个参数，那么我们能否实现类似的形式呢？这是可以的，如下：

public class MessageArrivedEventArgs : EventArgs
{
    private string message;
    public string Message
    {
        get
        {
            return message;
        }
    }
    public MessageArrivedEventArgs()
    {
        message = "No message sent.";
    }
    public MessageArrivedEventArgs(string newMessage)
    {
        message = newMessage;
    }
}
         
public delegate void MessageHandler(TimerTrigger source,
    MessageArrivedEventArgs e);

public class TimerTrigger
{
    private Timer myTimer;
    private event MessageHandler MessageArrived;

    public TimerTrigger(int timeInterval)
    {
        myTimer = new Timer(timeInterval);
        myTimer.Elapsed += new ElapsedEventHandler(CheckForMessage);
    }

    public void AddEventHandler(MessageHandler source)
    {
        MessageArrived += source; 
    }

    public void Start()
    {
        myTimer.Start();
    }

    public void Stop()
    {
        myTimer.Stop();
    }

    private void CheckForMessage(object source, ElapsedEventArgs e)
    {
        Console.WriteLine("Checking for new messages.");
        if (MessageArrived != null)
        {
            MessageArrived(this, new MessageArrivedEventArgs("hello world"));
        }

    }
}

class Program
{
    static int counter = 0;

    static void WriteFirstMessage(TimerTrigger source, MessageArrivedEventArgs e)
    {
        Console.WriteLine("The First {0} number: {1}", e.Message, counter++);
    }

    static void WriteSecondMessage(TimerTrigger source, MessageArrivedEventArgs e)
    {
        Console.WriteLine("The Second {0} number: {1}", e.Message, counter++);
    }

    public static void Main()
    {
        TimerTrigger timerTrigger = new TimerTrigger(100);
        timerTrigger.AddEventHandler(new MessageHandler(WriteFirstMessage));
        timerTrigger.AddEventHandler(new MessageHandler(WriteSecondMessage));
        timerTrigger.Start();
        Console.ReadLine();
    }
}