This event occurs before the header display

The third event generated in the query occurs immediately before the application's header display, during the loading of the page's display HTML elements.

The onHeader is only obtained when the option to display the header is enabled in the application's layout settings.

In this event, it is possible to perform queries in the database, allowing you to read information about the application and the values of the Query's fields.

It is used, among other things, to change the application's CSS, javascript codes and display information linked to fields, such as a sales total or a legend for a line graph.