Q: I'm trying to use NewCallBack with callBackAtExtremes and triggerAtStop to detect when a Movie has stopped playing, but the QuickTime documentation doesn't say where to specifically set these flags. Depending on what flags I do use, NewCallBack sometimes returns NULL. How do you set up a callback function using callBackAtExtremes and triggerAtStop?
A: triggerAtStop is a flag passed to the CallMeWhen API as the param1 parameter when callBackAtExtremes was used as the cbType flag to NewCallBack. Listing 1 demonstrates how to set this up.
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Listing 1.
| // A generic callback function, installed by CallMeWhen.
// pascal void MyQTCallBackProc(QTCallBack cb, long refCon);
pascal void MyDoSomethingCallBack(QTCallBack cb, long refCon)
{
MyAppDataPtr myStuff;
if (NULL == refCon) return;
myStuff = (MyAppDataPtr)refCon;
myStuff->didSomething = true;
return;
}
// Setting up a CallBack to fire using the callBackAtExtremes and
// triggerAtStop flags.
// MyDoSomethingCallBack function will be called when the time
// base has stopped.
OSErr SetUpTriggerAtStopCallBack(MyAppDataPtr inAppData)
{
QTCallBack theQTCallBack;
QTCallBackUPP theCallBackUPP;
OSErr err = paramErr;
theQTCallBack = NewCallBack(GetMovieTimeBase(inAppData->myMovie),
callBackAtExtremes);
if (theQTCallBack) {
theCallBackUPP = NewQTCallBackUPP(MyDoSomethingCallBack);
err = CallMeWhen(theQTCallBack, theCallBackUPP,
inAppData, triggerAtStop, 0, 0);
}
// save these so we can toss them later
inAppData->myQTCallBack = theQTCallBack;
inAppData->myCallBackUPP = theCallBackUPP;
return err;
}
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When done with the QTCallBack and QTCallBackUPP, remember to call DisposeCallBack and DisposeQTCallBackUPP.
References:
[Aug 12, 2003]
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