There are many cases where a feature similar to Qt signals and slots is needed, especially in the Engine class.

For example, to run a function when a redraw is requested from the engine, you need to set a callback using Engine::setRedrawHandler(). The callback will then be called internally by the engine.
Another example could be Engine::setAddMonitorHandler() which is used to set a callback for when a stage monitor is added.
This approach makes the API difficult to use and maintain.