Generally speaking, if your widget can reasonably have a @property T value(); or @property bool checked(); method, it should probably emit this event when that value changes to inform its parents that they can now read a new value. Whether you emit it on each keystroke or other intermediate values or only when a value is committed (e.g. when the user leaves the field) is up to the widget. You might even make that a togglable property depending on your needs (emitting events can get expensive).

The delegate you pass to the constructor ought to be a handle to your getter property. If your widget has @property string value() for example, you emit ChangeEvent!string(&value);

Since it is emitted after the value has already changed, preventDefault is unlikely to do anything.