Callbacks

To do work in a GUI, it is necessary to know when certain actions occur. Ewl handles notification of actions using a common technique called callbacks. When the end programmer wants to know when a specific event occurs to a widget, they can add a callback to it using ewl_callback_append() or one of the similar functions. One of the arguments to these functions is a pointer to a function. This function will get called when Ewl receives the specified event on that widget. You can attach callbacks to any widget, and with containers you can even mark them to intercept events to their child widgets.

One feature of Ewl that is different from other toolkits is that it makes extensive use of internal callbacks. In fact, almost all appearance changes for widgets are actually callbacks, and most of the ewl_widget calls actually do very little work, but trigger specific callbacks. This feature allows for overriding specific actions on a widget, or for ordering user specified callbacks relative to internal ones.