A callback is an action that is executed when a widget receives valid input. MFlow has 'waction's (see this example)
They execute a sequence of pages when invoked. When the flow action is finished, the execution return to the original page, to execute further actions or to return the result of the page form. A waction takes complete control of the navigation.
wcallback does that:
it does change the rendering of a particular widget by other rendering when the widget is validated.
In this example (see code below) wcallback call the same counter recursively when one of the two links are pressed, with the parameter increased or decreased.
This seaside widget counter is replicated five times in the same page:
The multicounter example in this site reuses this very same widget.
unlike the multicounter example, this example does not use autoRefresh, therefore the whole page is refreshed. Note that the counter state is reflected in the URL. Each counter state has its own REST URL.