GUI routines

Allegro contains an object-oriented dialog manager, which was originally based on the Atari GEM system (form_do(), objc_draw(), etc: old ST programmers will know what we are talking about :-) You can use the GUI as-is to knock out simple interfaces for things like the test program and grabber utility, or you can use it as a basis for more complicated systems of your own. Allegro lets you define your own object types by writing new dialog procedures, so you can take complete control over the visual aspects of the interface while still using Allegro to handle input from the mouse, keyboard, joystick, etc.

A GUI dialog is stored as an array of DIALOG objects, read chapter "Structures and types defined by Allegro" for an internal description of the DIALOG structure. The array should end with an object which has the proc pointer set to NULL. Each object has a flags field which may contain any combination of the bit flags:
   D_EXIT          - this object should close the dialog when it is
   D_SELECTED      - this object is selected
   D_GOTFOCUS      - this object has got the input focus
   D_GOTMOUSE      - the mouse is currently on top of this object
   D_HIDDEN        - this object is hidden and inactive
   D_DISABLED      - this object is greyed-out and inactive
   D_DIRTY         - this object needs to be redrawn
   D_INTERNAL      - don't use this! It is for internal use by the
   D_USER          - any powers of two above this are free for your
		     own use
Each object is controlled by a dialog procedure, which is stored in the proc pointer. This will be called by the dialog manager whenever any action concerning the object is required, or you can call it directly with the object_message() function. The dialog procedure should follow the form:
   int foo(int msg, DIALOG *d, int c);
It will be passed a flag (msg) indicating what action it should perform, a pointer to the object concerned (d), and if msg is MSG_CHAR or MSG_XCHAR, the key that was pressed (c). Note that d is a pointer to a specific object, and not to the entire dialog.

The dialog procedure should return one of the values:
   D_O_K          - normal return status
   D_CLOSE        - tells the dialog manager to close the dialog
   D_REDRAW       - tells the dialog manager to redraw the entire
   D_REDRAWME     - tells the dialog manager to redraw the current
   D_WANTFOCUS    - requests that the input focus be given to this
   D_USED_CHAR    - MSG_CHAR and MSG_XCHAR return this if they used
		    the key
Dialog procedures may be called with any of the messages:

Tells the object to initialise itself. The dialog manager sends this to all the objects in a dialog just before it displays the dialog.

Sent to all objects when closing a dialog, allowing them to perform whatever cleanup operations they require.

Tells the object to draw itself onto the screen. The mouse pointer will be turned off when this message is sent, so the drawing code does not need to worry about it.

Informs the object that a mouse button has been clicked while the mouse was on top of the object. Typically an object will perform its own mouse tracking as long as the button is held down, and only return from this message handler when it is released.

If you process this message, use the functions gui_mouse_*() to read the state of the mouse.

Sent when the user double-clicks on an object. A MSG_CLICK will be sent when the button is first pressed, then MSG_DCLICK if it is released and pressed again within a short space of time.

If you process this message, use the functions gui_mouse_*() to read the state of the mouse.

Sent when the keyboard shortcut for the object is pressed, or if enter, space, or a joystick button is pressed while it has the input focus.

When a key is pressed, this message is sent to the object that has the input focus, with a readkey() format character code (ASCII value in the low byte, scancode in the high byte) as the c parameter. If the object deals with the keypress it should return D_USED_CHAR, otherwise it should return D_O_K to allow the default keyboard interface to operate. If you need to access Unicode character input, you should use MSG_UCHAR instead of MSG_CHAR.

If an object ignores the MSG_CHAR input, this message will be sent immediately after it, passed the full Unicode key value as the c parameter. This enables you to read character codes greater than 255, but cannot tell you anything about the scancode: if you need to know that, use MSG_CHAR instead. This handler should return D_USED_CHAR if it processed the input, or D_O_K otherwise.

When a key is pressed, Allegro will send a MSG_CHAR and MSG_UCHAR to the object with the input focus. If this object doesn't process the key (ie. it returns D_O_K rather than D_USED_CHAR), the dialog manager will look for an object with a matching keyboard shortcut in the key field, and send it a MSG_KEY. If this fails, it broadcasts a MSG_XCHAR to all objects in the dialog, allowing them to respond to special keypresses even when they don't have the input focus. Normally you should ignore this message (return D_O_K rather than D_USED_CHAR), in which case Allegro will perform default actions such as moving the focus in response to the arrow keys and closing the dialog if ESC is pressed.

Queries whether an object is willing to accept the input focus. It should return D_WANTFOCUS if it does, or D_O_K if it isn't interested in getting user input.

Sent whenever an object gains or loses the input focus. These messages will always be followed by a MSG_DRAW, to let objects display themselves differently when they have the input focus. If you return D_WANTFOCUS in response to a MSG_LOSTFOCUS event, this will prevent your object from losing the focus when the mouse moves off it onto the screen background or some inert object, so it will only lose the input focus when some other object is ready to take over the focus (this trick is used by the d_edit_proc() object).

Sent when the mouse moves on top of or away from an object. Unlike the focus messages, these are not followed by a MSG_DRAW, so if the object is displayed differently when the mouse is on top of it, it is responsible for redrawing itself in response to these messages.

Sent whenever the dialog manager has nothing better to do.

Sent by radio button objects to deselect other buttons in the same group when they are clicked. The group number is passed in the c message parameter.

Sent to the focused object whenever the mouse wheel moves. The c message parameter contains the number of clicks.

Sent when the corresponding mouse button is pressed.

Sent when the corresponding mouse button is released.

The first free message value. Any numbers from here on (MSG_USER, MSG_USER+1, MSG_USER+2, ... MSG_USER+n) are free to use for whatever you like.

Allegro provides several standard dialog procedures. You can use these as they are to provide simple user interface objects, or you can call them from within your own dialog procedures, resulting in a kind of OOP inheritance. For instance, you could make an object which calls d_button_proc to draw itself, but handles the click message in a different way, or an object which calls d_button_proc for everything except drawing itself, so it would behave like a normal button but could look completely different.

Since the release of Allegro version 3.9.33 (CVS), some GUI objects and menus are being drawn differently unlike in previous Allegro versions. The changes are the following:

Menus had been forgotten during the changes for 3.9.33 (CVS), so they were still drawn too large until version 4.1.0.

GUI variables

The behaviour of the dialog manager can be controlled by the following global variables.

GUI font

You can change the global 'font' pointer to make the GUI objects use something other than the standard 8x8 font. The standard dialog procedures, menus, and alert boxes, will work with fonts of any size, but the gfx_mode_select() dialog will look wrong with anything other than 8x8 fonts.

GUI menus

Popup or pulldown menus are created as an array of MENU structures. Read chapter "Structures and types defined by Allegro" for an internal description of the MENU structure.

Each menu item contains a text string. This can use the '&' character to indicate keyboard shortcuts, or can be an zero-length string to display the item as a non-selectable splitter bar. If the string contains a "\t" tab character, any text after this will be right-justified, eg. for displaying keyboard shortcut information. The proc pointer is a function which will be called when the menu item is selected, and child points to another menu, allowing you to create nested menus. Both proc and child may be NULL. The proc function returns an integer which is ignored if the menu was brought up by calling do_menu(), but which is passed back to the dialog manager if it was created by a d_menu_proc() object. The array of menu items is terminated by an entry with a NULL text pointer.

Menu items can be disabled (greyed-out) by setting the D_DISABLED bit in the flags field, and a check mark can be displayed next to them by setting the D_SELECTED bit. With the default alignment and font this will usually overlap the menu text, so if you are going to use checked menu items it would be a good idea to prefix all your options with a space or two, to ensure there is room for the check.