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// Declare a IOOperationHandler block to set on the IOUserAudioDevice.
// The block will be called from a real time context when a i/o operation
// occurs on the IOUserAudioStream buffers for the device.
io_operation = ^kern_return_t(IOUserAudioObjectID in_device,
                              IOUserAudioIOOperation in_io_operation,
                              uint32_t in_io_buffer_frame_size,
                              uint64_t in_sample_time,
                              uint64_t in_host_time)
{
    // Add custom code to make modifications to the buffers as necessary
    if (in_io_operation == IOUserAudioIOOperationWriteEnd) {
        ...
    } else if (in_io_operation == IOUserAudioIOOperationBeginRead) {
        ...
    }
    return kIOReturnSuccess;
};
this->SetIOOperationHandler(io_operation);