ActiveX technologies are all built on top of the COM specification. In COM, an interface is a group of related functions that are implemented together. All interfaces are named beginning with I, such as IDataObject.
An ActiveX control must be a COM object. This means that an ActiveX control must support IUnknown, the interface supported by all COM objects. This allows for a great deal of latitude when deciding how a control is to be implemented;
Support for the IUnknown interface is provided automatically when you use MFC and ControlWizard to build your control.
Reducing the number of required interfaces allows ActiveX controls to be much smaller than the older OLE controls. It also makes it feasible to use ActiveX controls to implement functionality where the size of the control is an important factor. Web pages can be more intelligent when a control is downloaded and activated to your browser. For example, Microsoft's Internet Explorer has support for downloading ActiveX controls from a Web page. Although this opens a lot of exciting functionality, the size of the control to be downloaded must be kept as small as possible.
Interaction with an ActiveX component can be done properties, events, and methods.
- A property is an attribute associated with the control.
- An event is a notification message passed to the container by the control.
- A method is an exposed function that can be applied to the control via IDispatch.
Properties are exposed by ActiveX controls, as well as by the client where the control is located. There are four basic types of properties:
- Ambient properties are provided to the control by the container. The control uses these properties in order to "fit in" properly. Commonly used ambient properties include the container's background color, default font, and foreground color.
- Extended properties are implemented by the container but appear to be generated by the control. For example, the tab order of various controls in a container is an extended property.
- Stock properties are control properties implemented by the ActiveX control development kit. Examples of stock properties are the control font, the caption text, and the foreground and background colors.
- Custom properties are control properties that you implement.
An event is used to send a notification message to the control's container. Typically, events are used to notify the container when mouse clicks or other events take place. There are two basic types of events:
- Stock events are implemented by the ActiveX control development kit and are invoked just like a function call, such as FireError.
- Custom events are implemented by you, although the MFC class library and ClassWizard handle much of the work for you.
OLE Automation, now just Automation, is a way of enabling a COM object to be controlled by another party. Automation is provided through the IDispatch interface.
Automation was originally developed so that interpreted languages such as Visual Basic could control COM objects. Now most ActiveX controls use Automation to allow all sorts of programs--even those built using scripting languages such as JScript and VBScript--access to the methods of ActiveX controls.