什麼是Event?
簡單來說，Event其實就是通知(Notifications)，如果某個方法(例如按按鈕)跟某個事件(另外一個方法)綁在一起，那做了這件事情之後就會觸發事件，也就是做了某個方法會觸發另一個方法。
Event的特點
- 綁定Event的物件不需要清楚知道是哪個物件在處理Event的
- Event透過EventArgs來傳遞資料
Event實作方式
如果要自己定義Event，要使用event關鍵字:
// Delegate
public delegate void WorkPerformedHandler(int hours, WorkType workType);
public enum WorkType
{
GoToMeetings,
Golf,
GenerateReports
}
public event WorkPerformedHandler WorkPerformed;
// WorkPerformedHandler -> 是Delegate!!
呼叫Event
當作方法呼叫即可:
if(WorkPerformed != null)
{
WorkPerformed(8, WorkType.GenerateReports);
}
因為WorkPerformed其實是delegate，所以我們也可以明確轉型成delegate來使用:
WorkPerformedHandler del = WorkPerformed as WorkPerformedHandler;
if(del != null)
{
del(8, WorkType.GenerateReports);
}
以下是範例:
public delegate void WorkPerformedHandler(int hours, WorkType workType);
public class Worker
{
public event WorkPerformedHandler WorkPerformed; // Event的定義
public virtual void DoWork(int hours, WorkType workType)
{
// 做一些事,然後通知用戶方法已被呼叫
OnWorkPerformed(hours, workType);
}
protected virtual void OnWorkPerformed(int hour, WorkType workType)
{
if(WorkPerformed != null)
{
WorkPerformed(hours, workType);
}
}
// ------ 或是 ------
protected virtual void OnWorkPerformed(int hour, WorkType workType)
{
WorkPerformedHandler del = WorkPerformed as WorkPerformedHandler;
if(del != null)
{
del(hours, workType); // 呼叫Event
}
}
}
使用EventArgs
一般比較建議的做法是讓EventHandler有以下的格式:
public delegate void WorkPerformedHandler(object sender, EventArgs e);
sender代表啟動Event的方法，而EventArgs則是Event自帶的資料。
如果要自己定義這個EventArgs，我們可以自己寫一個類別，然後來繼承至System.EventArgs:
public class WorkPerformedEventArgs : System.EventArgs
{
public int Hours {get; set; }
public WorkType WorkType {get; set; }
}
使用EventHandler來註冊及觸發Event
如果沒看過EventHandler請參照: C# EventHandler 介紹
首先我們可以將delegate的WorkPerformedHandler改寫成以下的程式碼:
public event EventHandler<WorkPerformedEventArgs> WorkPerformed;
我們可以透過以下的方式來註冊EventHandler到Worker類別:
var worker = new Worker();
worker.WorkPerformed +=
new EventHandler<WorkPerformedEventArgs>(worker_WorkPerformed);
void worker_WorkPerformed(object sender, WorkPerformedEventArgs e)
{
Console.WriteLine(e.Hours.ToString());
}
完整範例
接下來來改寫一下上面的Worker類別，然後做一個簡單的Demo:
//public delegate void WorkPerformedHandler(int hours, WorkType workType);
public class Worker
{
public event EventHandler<WorkPerformedEventArgs> WorkPerformed; // Event的定義
public event EventHandler WorkCompleted;
public virtual void DoWork(int hours, WorkType workType)
{
for(int i = 0;i < hours; i++)
{
// 每小時通知一次
OnWorkPerformed(i + 1, workType);
}
// 結束時再通知一次
OnWorkCompleted();
}
protected virtual void OnWorkPerformed(int hours, WorkType workType)
{
var del = WorkPerformed as EventHandler<WorkPerformedEventArgs>;
if(del != null)
{
del(this, new WorkPerformedEventArgs(hours, workType))
}
}
protected virtual void OnWorkCompleted()
{
var del = WorkCompleted as EventHandler;
if(del != null)
{
del(this, EventArgs.Empty); // 如果不打算帶資料可以使用EventArgs.Empty
}
}
}
然後寫一個簡單的Console App來測試一下結果:
public class Program
{
public static void Main()
{
var worker = new Worker();
worker.WorkPerformed += new EventHandler<WorkPerformedEventArgs>(Worker_WorkPerformed);
worker.WorkCompleted += new EventHandler(Worker_WorkCompleted);
worker.DoWork(8, WorkType.GenerateReports);
Console.Read();
}
public static void Worker_WorkPerformed(object sender, WorkPerformedEventArgs e)
{
Console.WriteLine(e.Hours + " " + e.WorkType);
}
public static void Worker_WorkCompleted(object sender, EventArgs e)
{
Console.WriteLine("Work Completed!");
}
}
測試結果如下:
1 GenerateReports
2 GenerateReports
3 GenerateReports
4 GenerateReports
5 GenerateReports
6 GenerateReports
7 GenerateReports
8 GenerateReports
Work Completed!