OC96 and Windowless Controls

The OC96 (or OLE Controls '96) specification is not new, but full support is only now appearing in mainstream applications such as Office 97 forms and Internet Explorer 4.0. Therefore, this specification deserves further attention. It includes many performance enhancements because one of the primary driving factors with this specification was to improve the performance of controls. One of the key features is support for windowless controls. Not only does this improve performance, but it also allows a control to have transparency and irregular shapes and thus partake in overlapping 2D layout with specific z-order. All the enhancements are designed to be compatible with existing OLE Controls technology, so a backward-compatible control should be easy to implement.

Key features of OC96 include:

• Delayed activation. Delaying the activation of a control means that a page can instantiate and be ready for interaction with the user faster, particularly if there are multiple controls on the page. A new interface, IPointerInactive, allows a control to remain inactive but still interact minimally with the mouse and drag and drop. Although the control still activates, it only does so as the result of some interaction with the user.
• Windowless. By not having its own window, a control can make efficient use of its container's window. Performance advantages thus are brought about by fewer resources being consumed and faster activation/deactivation. New interfaces are introduced to support windowless controls. The main new interfaces are IOleInPlaceObjectWindowless and IOleInPlaceSiteWindowless, which derive from their namesakes without the Windowless extension. When drawing itself, a control asks the IOleInPlaceSiteWindowless interface for a device context, and returns the device context after drawing. Although windowless support of a control is often seen in terms of providing transparency for interesting visual effects, supporting windowless modes of operation will further improve performance.
• Hit detection. To complete the visual benefits of transparency that a windowless control can provide, hit detection is available so that a control can have an irregular shape. If the container detects that the rectangular extent occupied by a windowless control has been hit, it calls IViewObjectEx::QueryHitPoint (another new interface for windowless controls). The control then determines if it has been hit and returns an indication to the container.
• Quick activation. To further speed activation of a control, a new interface, IQuickActivate is introduced and can optionally be implemented by the control. A method on this interface is called by the container to pass a structure to the control, and to receive a structure back from the control. These structures contain interface pointers frequently needed for controls and ambient properties. Using these structures to pass often-needed information during activation can cut own on the handshaking negotiation that frequently takes place when instantiating a control.