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Q: How does persistence work?

An instantiated plug-in often has state information that must be saved in order to properly re-instantiate that plug-in at a later time. A VST 3 plug-in has two states which are saved and reloaded: its component state and its controller state.

The sequence of actions for saving is:

  • component->getState (compState)
  • controller->getState (ctrlState)

The sequence of actions for loading is:

  • component->setState (compState)
  • controller->setComponentState (compState)
  • controller->setState (ctrlState)

In the latter sequence you can see that the controller part will receive the component state. This allows the 2 parts to synchronize their states.

Q: What's the difference between IEditController::setComponentState and IEditController::setState?

After a preset is loaded, the host calls Vst::IEditController::setComponentState and Vst::IComponent::setState. Both delivering the same information. Vst::IEditController::setState is called by the host so that the plug-in is able to update its controller dependent parameters, e.g. position of scroll bars. Prior to this, there should have been a call by the host to Vst::IEditController::getState, where the plug-in writes these various parameters into the stream.

See also Q: How does persistence work?.