Creating Events and Delegates
This article is excerpted from Chapter 4, "Object-Oriented Features," from Building Distributed Applications with VB.NET, by Dan Fox (ISBN 0672321300). It assumes the reader has some experience with VB, the Windows environment, event-based programming, basic HTML, and scripting. This material is based on the Beta2 release version of Microsoft's .NET technology.
The basic event handling syntax has not changed significantly from previous versions of VB although it has been extended (as you'll see shortly). As in previous versions, events can be declared within a class using the Event keyword. An event can be Public, Private, Protected, Friend, and Protected Friend to the class, can pass arguments either ByVal or ByRef, and cannot return values. Events can then be raised using the RaiseEvent keyword from within the class and passing the required arguments. A partial sample from a class called RegistrationWatch is shown in Listing 4.2.
Listing 4.2 Simple events. This class implements the simple event NewRegistrations using the Event keyword and fires it using RaiseEvent.
Public Class RegistrationWatch Public Event NewRegistrations(ByVal pStudents As DataSet) ' Other methods and properties Public Sub Look() Dim dsStuds As DataSet Dim flNew As Boolean ' Method that fires on a timer to look for new registrations ' since the last invocation of the method ' If one is found then create a DataSet with the new students ' and raise the event flNew = True dsStuds = New DataSet() If flNew Then RaiseEvent NewRegistrations(dsStuds) End If End Sub End Class
Notice that the class defines a NewRegistrations event as Public so that consumers of the class will be able to catch it. The event passes back to the consumer a variable containing an ADO.NET DataSet that stores information on the new registrations found. The event is raised in the Look method using the RaiseEvent statement.
To catch the event, a consumer can declare the RegistrationWatcher class using the WithEvents keyword (termed declarative event handling). Note that, as in VB 6.0, variables declared using WithEvents may be either Public or Private. However, in VB.NET, WithEvents can be declared at the module or class level, rather than only in classes and forms as in VB 6.0. The syntax for using WithEvents is as follows:
Private WithEvents mobjReg As RegistrationWatch
To set up the event handler (or event sink), you can then create a procedure that handles the event within the class or module, as shown in the following example:
Public Sub RegFound(ByVal pStudents As System.Data.DataSet) _ Handles mobjReg.NewRegistrations MsgBox("New Students Found!") End Sub
Note that VB.NET uses the new Handles keyword to indicate precisely which event the procedure handles rather than simply relying on the naming convention used by the event handler as in previous versions. Declarative event handling is certainly the most convenient way to handle events, although as mentioned, it requires the object variable to be scoped correctly and cannot be used to dynamically turn an event handler on or off.
Although it is true that declarative event handling does not allow you to turn events on and off, if the class raising the event is a custom class, you can implement your own EnableRaisingEvents property. The client can then use this property to stop the raising of events. You would then wrap your RaiseEvent statements in a check of this property.
When dealing with inheritance, keep in mind that if a class acting as a consumer does not implement the event handlers for a particular object, classes derived from it will not be able to implement them later. In addition, as with other methods, the event handlers can be specified with the Overridable, NotOverridable, and MustOverride keywords.
Events can also be marked as Shared, which allows all consumers of a class to receive an event notification when the RaiseEvent method is executed in any instance of the class. This might be useful for notifying several consumers when shared data changes, for example, in a service application that periodically queries a database and exposes the data through shared properties.
Dynamic Event Handling
VB.NET expands the ability to use events by implementing dynamic event handling in addition to the declarative approach discussed earlier. Dynamic event handling can be very useful when you want to turn event handling on or off (called hooking and unhooking an event) at a specific time or when the object variable that you want to hook does not reside at the module or class level. To hook and unhook events at run-time, use the AddHandler and RemoveHandler statements.
As an example, consider the RegistrationWatch class shown in Listing 4.3. In this example, we want to create client code in a class that hooks and unhooks the NewRegistrations event based on the setting of the public property. The client class that does this is shown in Listing 4.3.
Listing 4.3 Dynamic events. This class hooks and unhooks events dynamically using the AddHandler and RemoveHandler statements.
Public Class Registrations Private mRec As Boolean = False Private mRegWatch As RegistrationWatch Public Property ReceiveNotifications() As Boolean Get Return mRec End Get Set mRec = Value If mRec = True Then ' Add the event handler AddHandler mRegWatch.NewRegistrations, _ AddressOf Me.NewRegistrations Else ' Remove the event handler RemoveHandler mRegWatch.NewRegistrations, _ AddressOf Me.NewRegistrations End If End Set End Property Public Sub NewRegistrations(ByVal ds As DataSet) ' New Registrations found MsgBox("New registrations have been added!") End Sub Public Sub New() ' Instantiate the class level object mRegWatch = New RegistrationWatch() End Sub Public Sub TestNotify() ' Test method to simulate repeated queries for new registrations mRegWatch.Look() End Sub End Class
In Listing 4.3, the property ReceiveNotifications is used to determine whether the class is to receive notifications. The Set block of the property then reads the value and calls the AddHandler and RemoveHandler statements accordingly. Both of the statements accept two parameters:
A reference to the event to be hooked (in this case, the NewRegistrations event of the module level mRegWatch variable)
The address of the method to use as the event handler
Note that the AddressOf and Me keywords are used to create a pointer to the method and specify a method internal to the class, respectively.
From the client's perspective, the code differs only in that the ReceiveNotification property can be set to True when notifications are desired (because the mRec class level variable defaults to False), as shown in the following example:
Dim objReg As New Registrations() objReg.TestNotify() ' No notification objReg.ReceiveNotifications = True objReg.TestNotify() ' Notification received
Mapping Events to Delegates
As mentioned in Chapter 1, the infrastructure for events is based on the concept of delegates, and so it is not surprising that the event keywords (such as Event, RaiseEvent, AddHandler, and RemoveHandler) simply abstract the creation and processing of delegates. However, VB.NET developers can also access delegates directly through the Delegate keyword. To understand delegates, let's first explore how delegates are used to implement events in VB.NET.
Remember first and foremost that delegates are simply types that hold references to functions in other objects (type-safe function pointers). There are two basic types of delegates: single-cast and multi-cast. The former allows a single function pointer to be stored and the latter creates a linked-list of pointers (events are implemented as multi-cast delegates). In addition to the function pointer, the delegate stores the arguments that the function will accept. As a result, from the developer's view, the delegate can be thought of simply as a method signature.
The basic idea behind using a delegate is that a program (called A for this example) creates a delegate that points to one of its own functions and then passes the delegate to some other program (B). At some time in the future, B executes A's function, making sure to push the appropriate arguments on the stack, by running the code at the address provided by the delegate. Of course, this is exactly the model used when dealing with events.
In the example in Listing 4.3, what happens when the Event keyword and RaiseEvent statements are added to the class is this. Behind the scenes, the VB compiler creates a Delegate with the same signature as the event and stores it in a field of the class as follows:
Delegate Sub NewRegistrations(ByVal pStudents As DataSet)
The compiler also creates add and remove methods for the delegate that take as an argument a reference to a delegate defined (in this case, NewRegistration). In addition, the RaiseEvent is replaced with a call to the delegate's Invoke method, which accepts the single argument as defined in the Delegate.
The consumer then uses the WithEvents keyword and implements the event handler with the Handles statement. At run-time, the delegate is instantiated (it is actually a class derived from System.Delegate) and a reference to the event handler is passed to the add method in RegistrationWatch. When the delegate is invoked, the function pointer to the event handler is used to call the method. This simple mapping of delegates to events should also explain why it is easy for VB.NET to support the AddHandler and RemoveHandler statements. They simply call the add and remove methods implemented by the compiler at specified times rather than upon instantiation and deallocation.
To make this a little clearer, examine the code in Listing 4.4 that shows the RegistrationWatcher class rewritten with a delegate in place of the event.
Listing 4.4 Simple delegate. This class uses a delegate in place of an event to perform simple notification.
Public Class RegistrationWatch Delegate Sub NewRegistrations(ByVal pStudents As DataSet) ' Other methods and properties Private mfound as NewRegistrations Public Sub RegisterClient(ByVal found As NewRegistrations) Mfound = found End Sub Public Sub Look() Dim dsStuds As DataSet Dim flNew As Boolean ' Method that fires on a timer to look for new registration ' If one is found then create a DataSet with the new students ' and raise the event flNew = True dsStuds = New DataSet() If flNew Then mfound.Invoke(dsStuds) 'invoke the delegate End If End Sub End Class
The differences between Listing 4.4 and Listing 4.3 can be summarized as follows:
The Event statement has been replaced with Delegate.
The RaiseEvent statement has been replaced with Delegate.Invoke.
The RegisterClient method is now used to pass in the reference to the delegate stored in a private class variable, whereas the Look method simply invokes the delegate at the appropriate time.
Notice also you don't have to explicitly create the add and remove methods, even when specifying the delegate yourself; the VB.NET compiler will add these automatically.
The client code also changes in order to instantiate the delegate as it is being passed to the Look method. Note that the address of the event handler is passed as the only argument in the constructor of the delegate, as shown in the following example:
Private mobjReg As RegistrationWatch ' in a class or module mobjReg = New RegistrationWatch() mobjReg.RegisterClient(New RegistrationWatch.NewRegistrations( _ AddressOf NewRegistrations)) mobjReg.Look() Private Sub NewRegistrations(ByVal ds As DataSet) End Sub
Events can also work with delegates as a kind of shortcut for declaring the event signature. For example, rather than declaring an event as
Event NewRegistrations(ByVal ds As DataSet)
you could make the declarations
Delegate Sub NewRegistrations(ByVal ds As DataSet) Event NewReg as NewRegistrations
This allows you to reuse the definition of the delegate inside the event. This can be useful when you have many events that require the same arguments.
Obviously, using delegates in place of events might be construed as overkill because events work quite nicely for scenarios where an event model is required. However, because delegates are function pointers, they also provide other capabilities that you can take advantage of, such as function substitution and asynchronous operation.