This document explain how script works inside the engine.

1. Anatomy of a script call

In this section, we will see a scenario of a script call. At the beginning of the adventure, the hero walks in a zone that sends an IntroDialog message. Here is what happens in the code.

  1. In the main function (in, the world is updated with WorldProcessor::update() (in

  2. The position of the hero is updated in the physics engine and the physics engine is updated with a call to b2World::update()

  3. The physics engine detects a collision between the hero and the sensor, and triggers a call to PhysicsListener::BeginContact() (in

  4. The listener checks that the requirements for the message are fulfilled, and then call Script::onMessageDefered() (in

  5. Back in WorldProcessor::update(), just after the update of the physics engine, Script::handleDeferedMessages() is called (in

  6. The message that was defered is handled at this time and passed to Script::onMessage() (in

  7. A call to the Wren script is prepared with the name of the message, and then Advendture::onMessage() is called (in adventure.wren)

  8. The handler in the script retrieves the callback and, in this case, calls an anonymous function that was registered in Chapter1::initialize() (in chapter1.wren)

  9. A call to World::startDialog() is made, this Wren call is foreign (in world.wren) and actually call Script::startDialog() (in

  10. Finally the startDialog() function sets the current operation to Talk and adds a dialog to the world state.

The messages are defered because the message handler could change the physical properties of the hero or characters. Changing the physical properties during b2World::update() is forbidden, the world is locked. So the message handler is called when b2World::update() is finished.