Access element given structure UnsafeMutablePointer - swift

Question

In C, I have the following code to allocate a AudioBufferList with the appropriate size and then populate it with relevant data.
AudioObjectPropertyScope scope = mIsInput ? kAudioDevicePropertyScopeInput : kAudioDevicePropertyScopeOutput;
AudioObjectPropertyAddress address = { kAudioDevicePropertyStreamConfiguration, scope, 0 };
UInt32 propertySize;
__Verify_noErr(
AudioObjectGetPropertyDataSize(mID, &address, 0, NULL, &propertySize)
);
AudioBufferList *bufferList = (AudioBufferList *) malloc(propertySize);
__Verify_noErr(
AudioObjectGetPropertyData(mID, &address, 0, NULL, &propertySize, bufferList)
);
Then, I can access struct element :
UInt32 result { 0 };
for(UInt32 i = 0; i < bufferList->mNumberBuffers; ++i)
{
result += bufferList->mBuffers[i].mNumberChannels;
}
free(bufferList)
How can I replicate this behavior in Swift, given the fact that I use the same framework, i.e. AudioToolbox?
I have tried the following but I can't access the mNumberBuffers
let scope: AudioObjectPropertyScope = scope ? kAudioDevicePropertyScopeInput : kAudioDevicePropertyScopeOutput
var address: AudioObjectPropertyAddress = AudioObjectPropertyAddress(mSelector: kAudioDevicePropertyStreamConfiguration, mScope: scope, mElement: 0)
var size: UInt32 = 0
CheckError(
AudioObjectGetPropertyDataSize(mID, &address, 0, nil, &size),
"Couldn't get stream configuration data size."
)
var bufferList = UnsafeMutableRawPointer.allocate(bytes: Int(size), alignedTo: MemoryLayout<AudioBufferList>.alignment).assumingMemoryBound(to: AudioBufferList.self)
CheckError(
AudioObjectGetPropertyData(mID, &address, 0, nil, &size, bufferList),
"Couldn't get device's stream configuration"
)

Answer 1

You can create an AudioBufferList like this:
import AudioUnit
import AVFoundation
var myBufferList = AudioBufferList(
mNumberBuffers: 2,
mBuffers: AudioBuffer(
mNumberChannels: UInt32(2),
mDataByteSize: 2048,
mData: nil) )
When handed a bufferList with an unknown number of buffers, you can get the number of buffers and the sample data like this:
let myBufferListPtr = UnsafeMutableAudioBufferListPointer(myBufferList)
let numBuffers = myBufferListPtr.count
if (numBuffers > 0) {
let buffer : AudioBuffer = myBufferListPtr[0]
let bufferDataPointer = UnsafeMutableRawPointer(buffer.mData)
if let dataPtr = bufferDataPointer {
dataPtr.assumingMemoryBound(to: Float.self)[ i ] = x
...
The rest of my source code example is on GitHub: https://gist.github.com/hotpaw2/ba815fc23b5d642705f2b1dedfaf0107