Visual Studio EventHandler Delegate 和 EventArgs

EventHandler代理 用來表示處理一個沒有事件數據（event data）的事件（event）的 方法。

不管什麼時候事件發生時，事件代理就被調用來觸發之前事件驅動的其餘事件（監聽當前事件TCurrentEvent += TListenerEvent）。

public delegate void EventHandler(
	Object sender,    //事件發起者
	EventArgs e       //處理事件數據的對象
)

 

EventArgs 類是事件變量的基類，提供事件數據類型給某個事件。

System.Object 
  System.EventArgs
    More...

public class EventArgs  

 

The following example shows an event named ThresholdReached that is associated with an EventHandler delegate. The method assigned to the EventHandler delegate is called in the OnThresholdReached method.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace delegrate
{
    class Program
    {
        static void Main(string[] args)
        {
            Counter c = new Counter(new Random().Next(10));
            c.ThresholdReached += c_ThresholdReached;  //聲明一個事件監聽

            Console.WriteLine("press 'a' key to increase total");
            while (Console.ReadKey(true).KeyChar == 'a')
            {
                Console.WriteLine("adding one");  
                c.Add(1);    
            }
        }

        static void c_ThresholdReached(object sender, ThresholdReachedEventArgs e)
        {
            Console.WriteLine("The threshold of {0} was reached at {1}.", e.Threshold, e.TimeReached);
            Environment.Exit(0);
        }
    }

    class Counter
    {
        private int threshold;
        private int total;

        public Counter(int passedThreshold)
        {
            threshold = passedThreshold;
        }

        public void Add(int x)
        {
            total += x;
            if (total >= threshold)
            {
                ThresholdReachedEventArgs args = new ThresholdReachedEventArgs();
                args.Threshold = threshold;
                args.TimeReached = DateTime.Now;
                OnThresholdReached(args);          //傳輸一個 TEventArgs-->ThresholdReachedEventArgs
            }
        }

        protected virtual void OnThresholdReached(ThresholdReachedEventArgs e)
        {
            EventHandler<ThresholdReachedEventArgs> handler = ThresholdReached;  //handle an Event
            if (handler != null)
            {
                handler(this, e);   //觸發 c_ThresholdReached
            }
        }

        public event EventHandler<ThresholdReachedEventArgs> ThresholdReached;
    }
       
    public class ThresholdReachedEventArgs : EventArgs
    {
        public int Threshold { get; set; }
        public DateTime TimeReached { get; set; }
    }
}