There are two kinds of occurrence you might need to provide events for: user interactions and state changes. User-interaction events are nearly always triggered by a message from Windows, indicating that the user did something your component may need to respond to. State-change events may also be related to messages from Windows (focus changes or enabling, for example), but they can also occur through changes in properties or other code. 

You have total control over the triggering of the events you define. Define the events with care so that developers are able to understand and use them.