Asynchronous Call-Backs and Events

See Also

When a client makes a method call, it’s blocked until the call returns. That is, the client can’t execute any code while it’s waiting. This is known as synchronous processing. By using asynchronous processing, you can free the client to do other things while it’s waiting.

In asynchronous processing, the method call that starts a task returns instantly, without supplying a result. The client goes about its business, while the component works on the task. When the task is complete, the component notifies the client that the result is ready.

Asynchronous processing is also useful when clients need to be notified of interesting occurrences — for example, changes in database values, or the arrival of messages. A client tells a component it wants to be notified when certain things happen, and the component sends notifications when those things occur.

Both of these scenarios depend on asynchronous notifications. The client application is minding its own business, when out of the blue comes a notification that an asynchronous request is complete, or that something of interest has occurred.

Visual Basic provides two mechanisms — events and call-back methods — for implementing asynchronous notifications, as discussed in the following topics.

Asynchronous Notifications Using Events The simplest notification technique is for the component to raise an event. The client handles the event, and takes some action in response.
Asynchronous Notifications Using Call-Back Methods The client implements an interface containing a call-back method, which the component calls when notifications are required.
When to Use Events or Call-Backs for Notifications Explains how the decision to use events or call-backs affects the character of your notifications, and provides criteria for deciding which to use.

For More Information "Creating an ActiveX EXE Component" contains step-by-step procedures for implementing asynchronous notifications using both events and call-back methods.