定義事件

如果您不熟悉事件的委派 (Delegate) 模型，請參閱處理和引發事件。

事件功能由下列項目提供。

• 儲存事件資料的類別 (例如，EventArgs、ImageClickEventArgs)。

• 事件委派 (例如，EventHandler、ImageClickEventHandler)。

  注意 前面兩個類別通常在您的控制項外部定義。下列兩個成員定義於您的控制項中。

• 定義於您控制項中的事件成員。這將由 event 關鍵字來辨識。

• 您的控制項中叫用委派 (例如，OnClick、OnTextChanged) 的方法。

下列範例定義自訂控制項 MyButton 中的 Click 事件。

// If the event does not generate data, you do not have
// to define a class for the event data or define an event delegate. 
// Use System.EventArgs for event data
// and System.EventHandler as the event delegate.
// MyButton uses EventHandler and EventArgs.

using System;
using System.Web.UI;

namespace CustomControls 
{  
   public class MyButton: Control, IPostBackEventHandler 
   {     
      // Defines the Click event.
      public event EventHandler Click;

      // OnClick dispatches the event to delegates that
      // are registered with the Click event.
      // Controls that derive from MyButton can handle the
      // Click event by overriding OnClick
      // instead of attaching a delegate. The event data
      // is passed as an argument to this method.
      protected virtual void OnClick(EventArgs e) 
      {     
         if (Click != null) 
         {
            Click(this, e);
         }  
      }
      
      // Method of IPostBackEventHandler that raises change events.
      public void RaisePostBackEvent(string eventArgument)
      {     
         OnClick(EventArgs.Empty);
      }
      
      protected override void Render(HtmlTextWriter output) 
      {     
         output.Write("<INPUT TYPE = submit name = " + this.UniqueID + 
            " Value = 'Click Me' />"); 
      }
   }    
}
[Visual Basic]
' If the event does not generate data, you do not have
' to define a class for the event data or define an event delegate. 
' Use System.EventArgs for event data
' and System.EventHandler as the event delegate.
' MyButton uses EventHandler and EventArgs.
Option Explicit
Option Strict

Imports System
Imports System.Web.UI

Namespace CustomControls
   Public Class MyButton
      Inherits Control
      Implements IPostBackEventHandler
      
      ' Defines the Click event.
      Public Event Click As EventHandler
      
      ' OnClick dispatches the event to delegates that
      ' are registered with the Click event.
      ' Controls that derive from MyButton can handle the
      ' Click event by overriding OnClick
      ' instead of attaching a delegate. The event data
      ' is passed as an argument to this method.
      Protected Overridable Sub OnClick(e As EventArgs)
         RaiseEvent Click(Me, e)
      End Sub
      
      ' Method of IPostBackEventHandler that raises change events.
      Public Sub RaisePostBackEvent(eventArgument As String) Implements IPostBackEventHandler.RaisePostBackEvent
         OnClick(EventArgs.Empty)
      End Sub 'RaisePostBackEvent
      
      Protected Overrides Sub Render(output As HtmlTextWriter)
         output.Write(("<INPUT TYPE = submit name = " & Me.UniqueID & " Value = 'Click Me' />"))
      End Sub
   End Class
End Namespace

除了定義事件外，控制項開發人員也必需決定如何引發事件 (要叫用 OnEventName 方法的地方)。例如，MyButton 從其 RaisePostBackEvent 方法 (為 IPostBackEventHandler 合約的一部分) 引發 Click 事件。如需這個範例的詳細資訊，請參閱擷取回傳事件。

最佳化事件實作

前面說明的事件實作就效能而言並非是最佳化的。它按每個委派執行個體產生一個欄位，這在有許多事件定義於控制項時會增加儲存成本。基底類別 System.Web.UI.Control 提供更有效率的資料結構 (透過其 Events 屬性) 來儲存和擷取事件委派。Events 屬性的型別為 EventHandlerList，這是針對事件委派的有效率儲存和擷取而設計的資料結構。下列範例示範使用 Events 屬性的事件實作。這個 C# 範例與稍早定義於這個主題的 MyButton 範例只在其 Click 事件的實作 (Implementation) 中有差異。實作這個事件的程式碼以粗體字型強調。

注意 這個版本中的 Visual Basic .NET 並不支援事件屬性。

using System;
using System.Web.UI;

namespace CustomControls 
{
   public class OptimizedEventButton: Control, IPostBackEventHandler 
   {
      // Defines a key for storing the delegate for the Click event
      // in the Events list.
      private static readonly object ClickEvent = new object();
      
      // Defines the Click event using the event property syntax.
      // The Events property stores all the event delegates of
      // a control as name/value pairs. 
      public event EventHandler Click 
      {
         // When a user attaches an event handler to the Click event 
         // (Click += myHandler;), the Add method 
         // adds the handler to the 
         // delegate for the Click event (keyed by ClickEvent 
         // in the Events list).
         add          {            Events.AddHandler(ClickEvent, value);         }
         // When a user removes an event handler from the Click event 
         // (Click -= myHandler;), the Remove method 
         // removes the handler from the 
         // delegate for the Click event (keyed by ClickEvent 
         // in the Events list).
         remove          {            Events.RemoveHandler(ClickEvent, value);         }
      }
      
      // Invokes delegates registered with the Click event.
      //
      protected virtual void OnClick(EventArgs e) 
      {
         // Retrieves the event delegate for the Click event
         // from the Events property (which stores
         // the control's event delegates). You must
         // cast the retrieved delegate to the type of your 
         // event delegate.
         EventHandler clickEventDelegate = (EventHandler)Events[ClickEvent];         if (clickEventDelegate != null) {            clickEventDelegate(this, e);         }  
      }
      
      // Method of IPostBackEventHandler that raises change events.
      //
      public void RaisePostBackEvent(string eventArgument)
      {
         
         OnClick(new EventArgs());
      }
      
      protected override void Render(HtmlTextWriter output) 
      {
         
         output.Write("<INPUT TYPE = submit name = " + this.UniqueID + 
            " Value = 'Click Me' />"); 
      }
   }    
}

注意 為簡單起見，文件中的一些其他範例使用事件欄位來定義事件。然而在您的控制項中，您應該使用這裡討論的最佳化實作。

請參閱

處理和引發事件 | 反昇事件 | 處理回傳資料 | 擷取回傳事件 | 產生回傳的用戶端指令碼