Custom Loading

Specialized applications might create their own object types that encapsulate data from a file or resource. It can be convenient to have the DirectMusic loader handle the loading of such objects. This is especially true if the custom object is referenced by other objects or contains references to other objects.

To implement a loading mechanism that takes advantage of the DirectMusic loader, you must take the following steps:

• Register the object class so that it can be found by IDirectMusicLoader::GetObject.
• Implement the IDirectMusicObject interface on the object, so that the loader can get the information it needs for finding and caching it.
• Implement the IPersistStream interface on the object, with full functionality in the IPersistStream::Load method. This is where you parse the data, which you obtain through calls on the IStream interface passed by the DirectMusicLoader.
• In the implementation of Load, ensure that references to other objects are dealt with by querying the IStream for the IDirectMusicGetLoader interface, then calling IDirectMusicGetLoader::GetLoader to obtain a pointer to the DirectMusicLoader that created the stream. Once you have this pointer, you can call IDirectMusicLoader::GetObject to load the new object.

There are circumstances where an application may need to manage file input itself — for example, because all objects are stored in a special compressed resource file. The application can create its own loader by creating an object that supports the IDirectMusicLoader interface, with the IDirectMusicLoader::GetObject method implemented. All other methods are optional. This implementation of the loader must also create its own stream object that has both the IStream and the IDirectMusicGetLoader interfaces.