## Radio Buttons and Radio Groups

### OBJECTS

      typedef void (*ControlFunc)  (Control *c);

### FUNCTIONS

      Control *new_radio_button(Window *w, Rect r, char *text,
                                    ControlFunc fn);
      Control *add_radio_button(Control *c, Rect r, char *text,
                                    ControlFunc fn);

      Control *new_radio_group(Window *w);
      Control *add_radio_group(Control *c);

      int      is_checked(Control *c);  /* is it checked? */
      void     check(Control *c);       /* check the radio button */
      void     uncheck(Control *c);     /* uncheck the button */

### NOTES

The new_radio_button function creates a radio button control, which is similar to a check box except that it has a circle which is filled in with a large dot when the user clicks with the mouse.

The add_radio_button function works in the same way as new_radio_button, except that it attaches the radio button to a control rather than directly to a window.

Whenever the user changes the checked-state of a radio button, the action function `fn` is called after the change. Often this parameter can be left as NULL, which signifies that no function need be called in response to a change of state; instead the state can be determined at some later stage.

One difference between check boxes and radio buttons is that a check box can be freely switched on and off by the user, while radio buttons automatically belong to a mutually exclusive set of radio buttons, known as a 'radio group'. Activating one radio button will therefore switch off the previously active radio button.

Initially, no radio buttons in a radio group will be checked. If a 'default' option needs to be specified, use the check function to check one radio button after creating it.

To begin a new radio group, the new_radio_group function can be used. After calling this function, all radio buttons subsequently created attached to the same window will belong to a new mutually exclusive set of buttons. Similarly add_radio_group starts a new radio group within a control. Radio groups can exist only within one window or control, so each new window has its own initial radio group, as does each separate control.

The function is_checked can be used to determine if a radio button is currently checked. The function check can be used to activate a radio button while uncheck can be used to remove the dot from a radio button.