C# EventHandler Example－天天向上

(1) 先自訂義事件資料類別ThresholdReachedEventArgs, 其繼承EventArgs

該類別存在兩個公開方法,分別記錄門檻值和達門檻值時間

// (1) 自訂義事件資料(event data)    public class ThresholdReachedEventArgs : EventArgs    {        public int Threshold { get; set; }        public DateTime TimeReached { get; set; }    }

(2) event EventHandler<ThresholdReachedEventArgs> ThresholdReached;

其中關鍵字 EventHandler是一種委派

public delegate void EventHandler(object sender, EventArgs e)

EventHandler後面接<事件參數型態>, 在此為自定義事件資料ThresholdReachedEventArgs(繼承自EventArgs)

(3) (Line25)protected virtual void OnThresholdReached(ThresholdReachedEventArgs e){…}

定義虛擬函式做為該事件負責的方法(method)樣版, 可以之後覆寫之…。

(4) 實體化ThresholdReachedEventArgs args = new ThresholdReachedEventArgs();

傳入args.Threshold和TimeReached

然後呼叫虛擬函式, 同時傳入該args至 OnThresholdReached(args)

最後來回頭看Counter這個類別就簡單多,

1. (Line 3,4) 私有成員分別為threshold(門檻值)和total(累加量)

2. (Line 5) 有傳參數創建子Counter(int passThreshold)，更新私有成員threshold(門檻值)

3. (Line 9) 公有方法Add(int x)，每次傳入一個數值進行累加至私有成員total

4. (Line 14) 一旦total超過門檻值，先實體化ThresholdReachEventArgs事件參數args，接著透過args兩個公開方法,分別記錄門檻值和達門檻值時間

5. (Line 21) 呼叫虛擬函式OnThresholdReached(args), 同時傳入該args至 OnThresholdReached(args)

   1: class Counter   2:   {   3:       private int threshold;   4:       private int total;   5:       public Counter(int passedThreshold )   6:       {   7:           threshold = passedThreshold;   8:       }   9:       public void Add(int x)  10:       {  11:           total += x;  12:           Console.WriteLine("The total is now equal to {0}", total);  13:    14:           if (total>=threshold)  15:           {  16:               //(4) 實體化ThresholdReachedEventArgs, 接著傳入門檻值和達到門檻值時間  17:               ThresholdReachedEventArgs args = new ThresholdReachedEventArgs();  18:               args.Threshold = threshold;  19:               args.TimeReached = DateTime.Now;  20:               //(5) 呼叫虛擬函式  21:               OnThresholdReached(args);  22:           }  23:       }  24:       //(3) 定義事件處理method  25:       protected virtual void OnThresholdReached(ThresholdReachedEventArgs e)  26:       {  27:           EventHandler<ThresholdReachedEventArgs> handler = ThresholdReached;  28:           if(handler !=null)  29:           {  30:               handler(this, e);  31:           }  32:       }  33:       //(2) 定義事件成員  34:       public event EventHandler<ThresholdReachedEventArgs> ThresholdReached;  35:   }

1. (Line 3)先亂數產生0~9亂數值，儲存至threshold

2. (Line 6)產生Counter實體c

3. 接著去定義c的ThresholdReached事件處理函式(Line 7)，在此由自訂函式c_ThresholdReach負責覆寫當初虛擬函式OnThresholdReached

4. 先看覆寫函式c_ThreshReached做了甚麼事情? 第一個參數sender事件來源(誰發出的), 第二參數事件參數(包含門檻值和達門檻時間)，在此僅印出(WriteLine)門檻值和達門檻時間

5. 回頭看上一段虛擬函式OnThresholdReached中(Line 25)，ThresholdReached如果非空(handler!=null, 因為handler即ThresholdReached)，則執行該委派handler(this, e)，其中this即Counter這個類別實體c本身，e則為傳入事件參數(包含門檻值和達門檻時間)。

6. 這一切成立條件為total>=threshold，實體化事件參數(args = new ThresholdReachEventArgs)，呼叫OnThresholdReached(args)

   1: static void Main(string[] args)   2:        {   3:            int threshold = new Random().Next(10);   4:            string info = string.Format("threshold = {0}", threshold);   5:            Console.WriteLine(info);   6:            Counter c = new Counter(threshold);   7:            c.ThresholdReached += c_ThresholdReached;   8:            Console.WriteLine("press 'a' key to increase total");   9:            while (Console.ReadKey(true).KeyChar == 'a')  10:            {  11:                Console.WriteLine("adding one");  12:                c.Add(1);  13:            }  14:        }  15:    16:        static void c_ThresholdReached(object sender, ThresholdReachedEventArgs e)  17:        {  18:            Console.WriteLine("The threshold of {0} was reached at {1}.", e.Threshold, e.TimeReached);  19:            Environment.Exit(0);  20:        }