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getting AU kernels to react to stream format changes

Something that I've noticed before, but hasn't actually affected me until something I'm working on now, is that there's no easy way using AUEffectBase and AUKernelBase to have your kernels react to a change in sample rate or max frames per slice. Right now I'm working on an AU that uses kernels and each one allocates a buffer that is dependent on the sample rate. So I had to add my own technique for letting each kernel know about this, but it seems to me that it would make a lot of sense to have something like this in AUEffectBase as I'm sure that it must come up for others, too.

I was thinking that maybe there could be a SampleRateChanged() method and a MaxFramesPerSliceChanged() method added to AUKernelBase, and then AUEffectBase could call these when appropriate. I was thinking that the checking and notifying could occur in AUEffectBase::MaintainKernels(), perhaps. Here's a possible implementation:


void AUEffectBase::MaintainKernels()
{
UInt32 nChannels = GetNumberOfChannels();
UInt32 prevNumChannels = mKernelList.size();

if (mKernelList.size() < nChannels) {
mKernelList.reserve(nChannels);
for (UInt32 i = mKernelList.size(); i < nChannels; ++i)
mKernelList.push_back(NewKernel());
} else
while (mKernelList.size() > nChannels) {
AUKernelBase *kernel = mKernelList.back();
delete kernel;
mKernelList.pop_back();
}

for(unsigned int i = 0; i < nChannels; i++ )
{
if(mKernelList[i]) {
mKernelList[i]->SetLastKernel(i == nChannels-1 );
mKernelList[i]->SetChannelNum (i);

if (i < prevNumChannels)
{
if (GetSampleRate() != mPrevSampleRate)
mKernelList[i]->SampleRateChanged(GetSampleRate());
if (GetMaxFramesPerSlice() != mPrevMaxFramesPerSlice)
mKernelList[i]->MaxFramesPerSliceChanged(GetMaxFramesPerSlice());
}
}
}

mPrevSampleRate = GetSampleRate();
mPrevMaxFramesPerSlice = GetMaxFramesPerSlice();
}


In that example, mPrevSampleRate and mPrevMaxFramesPerSlice would also have to be added as members of AUEffectBase (and initialized to something in the constructor).

What do others think?


thanks,
Marc

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