QuickTime 4 Reference

Processing data

Data handlers do not know anything about the content of the data they process. It is the responsibility of the client (that is, a media handler component or a channel component) to process the data. In the case of a movie's video data during movie playback, for example, the media handler takes the data from a data handler and uses the facilities of the Image Compression Manager to display the movie data on the computer screen. See Inside Macintosh: QuickTime Components for more information about media handlers.

While data handlers do not work with the content of the data they process, they must be aware of all of the details involved in storing and retrieving data from the storage medium they support. Apple provides several data handlers and a selection mechanism for choosing an appropriate handler. For example, one supports data access from HFS volumes and another supports the memory-based data handler, which allows QuickTime to retrieve movies from memory handles. These two data handler components use very different mechanisms to store and retrieve movie data.

You might need to write your own data handler when you are accessing a storage medium for which there is no Apple-supplied data handler or when playing movies from a multimedia server, as you will need to use a data handler that understands the network protocols and data formats necessary to communicate with that server.