IUP - Portable User Interface

Events and Callbacks Guide


Even though callbacks have different purposes from attributes, they are actually associated to an element by means of an attribute.

The OLD method to associate a function to a callback, the application must employ the IupSetAttribute function, linking the action to a name (passed as a string). From this point on, this name will refer to a callback. By means of function IupSetFunction, the user connects this name to the callback.

For example:

int myButton_action(Ihandle* self);
IupSetAttribute(myButton, "ACTION", "my_button_action");
IupSetFunction("my_button_action", (Icallback)myButton_action);

In LED, callback are only assigned by their names. It will be still necessary to associate the name with the corresponding function in C using IupSetFunction. For example:

bt = button("Title", my_button_action)  # In LED, is equivalent to the IupSetAttribute command in the previous example.

In the NEW method, the application does not needs a global name, it directly sets the callback using the attribute name using IupSetCallback. For example:

int myButton_action(Ihandle* self);
IupSetCallback(myButton, "ACTION", (Icallback)myButton_action);

The new method is more efficient and more secure, because there is no risk of a name conflict. If the application uses LED, just ignore the name in the LED, and replace IupSetFunction by IupSetCallback.

Therefore, callbacks also have some of the attributes' functionalities. The most important one is inheritance. Though many callbacks are specific to a given element, a callback can be set to a composition element, such as a vbox, which contains other elements, and while the composition element does not call that callback all other elements contained in it will call the same callback, unless the callback is redefined in the element.

All callbacks receive at least the element which activated the action as a parameter (self).

The callbacks implemented in C by the application must return one of the following values:

  • IUP_DEFAULT: Proceeds normally with user interaction. In case other return values do not apply, the callback should return this value.
  • IUP_CLOSE: Makes the IupMainLoop function return the control to the application. Depending on the state of the application it will close all windows.
  • IUP_IGNORE: Makes the native system ignore that callback action. Applies only to some actions. Please refer to specific action documentation to know whether it applies or not.
  • IUP_CONTINUE: Makes the element to ignore the callback and pass the treatment of the execution to the parent element.

An important detail when using callbacks is that they are only called when the user actually executes an action over an element. A callback is not called when the programmer sets a value via IupSetAttribute. For instance: when the programmer changes a selected item on a list, no callback is called.

Inside a callback if one of the parameters is a string, this string may be modified during the callback if another IUP function (such as IupGetAttribute) is called.

To help the definition of callbacks in C, the header "iupcbs.h" can be used, there are typedefs for all the callbacks.

Main Loop

IUP is an event-oriented interface system, so it will keep a loop �waiting� for the user to interact with the application. For this loop to occur, the application must call the IupMainLoop function, which is generally used right before IupClose.

When the application is closed by returning IUP_CLOSE in a callback or by user closing the last dialog, the function IupMainLoop will return.

The IupLoopStep and the IupFlush functions force the processing of incoming events while inside an application callback.


Callbacks in Lua have the same names and receive the same parameters as callbacks in C, in the same order. In Lua the callbacks they can either return a value or not, the IupLua binding will automatically return IUP_DEFAULT if no value is returned. In Lua callbacks can be Lua functions or strings with Lua code.

The callbacks can also be implemented as methods, using the language�s resources for object orientation. Thus, the element is implicitly passed as the self parameter.

The following example shows the definition of an action for a button.

function myButton:action ()
  local aux = self.fgcolor
  self.fgcolor = self.bgcolor
  self.bgcolor = aux

Or you can do

function myButton_action(self)
myButton.action = myButton_action

Or also

myButton.action = function (self)

Or, as a string

myButton.action = "local aux = self.fgcolor; self.fgcolor = self.bgcolor; self.bgcolor = aux"

Altough some callbacks exists only in specific controls, all the callbacks can be set for all the controls. This is usefull to set a callback for a box, so it will be inherited by all the elements inside that box which implements that callback.