on method
Allows to create a custom HTML event, for example, if you can't find the desired one.
Implementation
WidgetElement on(String eventName, Function(Element, Event) fun) {
// The element event can only be activated
// after the element has been inserted.
onMount((Element element) => element.on[eventName].listen((Event event) {
fun.call(element, event);
}));
return this;
}