When you are programming a client application and want to use the callback functionality that
an Interface provides, you must take the following steps outlined in Step by Step 12.6 to use a callback
object in your application.
STEP BY STEP
12.6 Implementing a Callback Object in the Client
Add a class to the client application and name it appropriately.
Make sure that the Instancing property of the new class is Private. (If the client project
is not an ActiveX EXE or ActiveX DLL, the Class Instancing property will not be available
to you, and Instancing will be Private by default.)
Implement the server's callback Interface in this class (see Listing 12.22). Put appropriate
notification code in the callback's notification method.
Instantiate copies of the callback object (should be Public variables) and call the
appropriate methods of the server class objects that will receive instances of the callback
object as parameters (see Listing 12.22).
Listing 12.22 gives the code in the class module that you would add to the client. As mentioned
in step 3, this Client class implements the server's Interface class, INotification, which was
discussed in the section titled "Providing an Interface for the Callback Object."
Because INotification has only one member, the Notify
method, you need only to write code to implement that method. The Notify
method will be called from inside the server to send a notification back to this client. Because
this client implements the Notify method, it can therefore specify the exact way
in which it will be notified.
Recall from the preceding section that the Interface for INotification specifies
that the Notify method will accept one String parameter, so you must
implement the String parameter here.
IMPLEMENTING THE INTERFACE FOR A CALLBACK IN A Client CLASS
'Class Name = CallBack
Private Sub Inotificatiion_Notify(msg As String)
'or do something else with msg
In Listing 12.23, the client code instantiates a server class object, instantiates an object
from the callback class that you defined in Listing 12.22, and then passes the resulting callback
object to the server object's LongProcess method.
PASSING A Callback OBJECT TO A SERVER OBJECT
Public objServer As New MyServer.MyClass
Public cbCurrent As CallBack
Set cbCurrent = New CallBack
The server's LongProcess method is specifically
designed to accept an object that's been instantiated from a class that implements
the INotification Interface (see the section titled "Manipulating
the Callback Object in the Server"). After the server has received the object,
it may at some point call the object's Notify method—
which you have implemented in the client to provide a client-side notification.