Jumpy/inconsistent parameter value changes when using ParameterAttachment on VST3

Audio Plugins
8

i experienced this issue with ableton in an uncommon situation (a VST3 wrapper for an AUv3 plugin) and i had to do something like this in the VST3 to get it working (ymmv, this was essentially proxying changes from host ↔ vst3 ↔ auv3):

// in processor
juce::ThreadLocalValue<bool> inAudioProcessorParameterChangedCallback;

void parameterChanged(const juce::String &parameterID, float newValue) {
  // we only want to listen for changes that come from the host so we can pass
  // them on. however - it's possible to end up here in the
  // audioProcessorParameterChanged callback when we call
  // setValueNotifyingHost(). the inAudioProcessorParameterChangedCallback flag
  // is an attempt to avoid this loop (change from slider in our editor ->
  // audioProcessorParameterChanged -> setValueNotifyingHost -> parameterChanged
  // -> setValue on on processor -> audioProcessorParameterChanged) see also the
  // similar "inParameterChangedCallback" code in the au wrappers
  if (inAudioProcessorParameterChangedCallback.get()) {
    inAudioProcessorParameterChangedCallback = false;
    return;
  }
  // otherwise this handles changes from host -> processor
}

void audioProcessorParameterChanged(
    juce::AudioProcessor *, int parameterIndex, float newValue) {
  if (getParameters()[parameterIndex] != nullptr) {
    inAudioProcessorParameterChangedCallback = true;
    getParameters()[parameterIndex]->setValueNotifyingHost(newValue);
  }
}

there is code that does something like this already but it may not be doing the right thing?

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