ActionChange is called automatically when the action object specified by the Action property, or any of its properties, changes. 

In TCustomComboBoxEx, ActionChange checks whether the action is a TActionList object, and if so, updates its Images and ItemIndex properties, as appropriate. It then calls the inherited method, which updates the Caption, Enabled, Hint, and Visible properties and the OnClick event handler. 

The Sender parameter is the action object that changed. The CheckDefaults parameter indicates whether the control's properties should all be updated or only those that have not been explicitly changed from their default values. When CheckDefaults is true, only those properties and event handlers that have not been changed from the default values are updated. 

OnChange 

Action 

TActionList