Inside Macintosh: Sound
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The Sound Manager currently allows you to play sounds asynchronously only if you allocate sound channels yourself, using techniques described in "Managing Sound Channels" . But if you use such a technique, your application will need to dispose of a sound channel whenever the application finishes playing a sound. In addition, your application might need to release a sound resource that you played on a sound channel.
To avoid the problem of not knowing when to dispose of a sound channel playing a sound asynchronously, your application could simply allocate a single sound channel when it starts up (or receives a resume event) and dispose of the channel when the user quits (or the application receives a suspend event). However, this solution will not work if you need to release a resource when a sound finishes playing. Also, you might not want to keep a sound channel allocated when you are not using it. For instance, you might want to use the memory taken up by a sound channel for other tasks when no sound is playing.
Your application could call the SndChannelStatus function once each time through its main event loop to determine if a channel is still making sound. When the scBusy field of the sound channel status record becomes FALSE , your application could then dispose of the channel. This technique is easy, but calling SndChannelStatus frequently uses up processing time unnecessarily.
The Sound Manager provides other mechanisms that allow your application to find out when a sound finishes playing, so that your application can arrange to dispose of sound channels no longer being used and of other data (such as a sound resource) that you no longer need after disposing of a channel. If you are using the SndPlay function or low-level commands to play sound in a channel, then you can use callback procedures. If you are using the SndStartFilePlay function to play sound in a channel, then you can use completion routines. The following sections illustrate how to use callback procedures and completion routines.
Callback procedures are a form of completion routine. However, for clarity, this section uses the terminology "completion routine" only for the routines associated with the SndStartFilePlay function.
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