Wijmo events are similar to .NET events. Any class may define events by declaring them as fields. Any class may subscribe to events using the event's addHandler method and unsubscribe using the removeHandler method.

Wijmo event handlers take two parameters: sender and args. The first is the object that raised the event, and the second is an object that contains the event parameters.

Classes that define events follow the .NET pattern where for every event there is an on[EVENTNAME] method that raises the event. This pattern allows derived classes to override the on[EVENTNAME] method and handle the event before and/or after the base class raises the event. Derived classes may even suppress the event by not calling the base class implementation.

For example, the TypeScript code below overrides the onValueChanged event for a control to perform some processing before and after the valueChanged event fires:

// override base class
onValueChanged(e: EventArgs) {
  // execute some code before the event fires
  console.log('about to fire valueChanged');
  // optionally, call base class to fire the event
  super.onValueChanged(e);
  // execute some code after the event fired
  console.log('valueChanged event just fired');
}