Inside Macintosh: QuickTime Reference

Not Instantiating Media at Import Time

The SMIL importer does not instantiate the media elements at import time in order to get their spatial and temporal characteristics. The reasons for this are enumerated as follows:

Time. Opening every media element will be slow, particularly over a network. And for streaming content, opening the media element isn't sufficient. The duration and dimensions aren't available until media data is actually flowing, which requires actually starting to play the content.
Counting. Some of the anticipated users of SMIL may wish to track views of particular pieces of content. If the SMIL importer were to instantiate the media at import time, the hit counters would be incremented before the media had actually been viewed. In a composition of any duration, this means that it would be likely that some media would be counted even though it was never viewed.
Changes in duration or spatial characteristics. The media element could change in duration or spatial characteristics between the time that the SMIL importer has finished its work and the media is actually viewed. In a longer composition this is more likely, but it can also occur if the user opens a composition but doesn't play it immediately. Having the ability to compensate for these changes at runtime regardless of why they occur creates a more robust system.

Inside Macintosh: QuickTime Reference