Minor additions and fixes

[~andy/gtk] / docs / widget_system.txt

Notes about the inner workings of the widget system of GTK+
===========================================================

This file contains some notes as to how the widget system does
and should work. It consists of three parts:

 I) A description of the meaning of the various flags

 II) A list of invariants about the states of the widgets. 
    (Throughout this document, we refer to the states of the
     widgets by referring to the flags for GtkWidget)

 III) Some notes about the ways that a widget changes states

 IV) A list of responsibilities of various widget signals when
    the states change.

Any action necessary to maintain the invariants in II which is not
explicitly mentioned in IV), is the responsibility of the core GTK
code, which is roughly defined as:

  gtkobject.c
  gtkwidget.c
  gtkcontainer.c
  gtkmain.c
  gtksignal.c

Section II is mostly of interest to those maintaining GTK, the
other sections may also be interesting to people writing
new widgets.

Main outline:
        - Owen Taylor <owt1@cornell.edu>
          1998/02/03

Flag descriptions:
        - Tim Janik <timj@gimp.org>
          1998/02/04

I. Flags
--------

GtkObject:

GTK_DESTROYED:
        This flagged is set for a GtkObject right before its
        destruction code is executed. Its main use is the
        prevention of multiple destruction invokations.
        
GTK_FLOATING:
        This flag reflects the fact that the holder of the
        initial reference count is unknown. Refer to refcounting.txt
        for further details.

GTK_RESERVED_1:
GTK_RESERVED_2:
        Reserved flags.


GtkWidget, public flags:

GTK_TOPLEVEL:
        Widgets without a real parent, as there are GtkWindows and
        GtkMenus have this flag set throughout their lifetime.
        Toplevel widgets always contain their own GdkWindow.
        
GTK_NO_WINDOW:
        This flag is indicative for a widget that does not provide
        its own GdkWindow. Visible action (e.g. drawing) is performed
        on the parent's GdkWindow.

GTK_REALIZED:
        Set by gtk_widget_realize, unset by gtk_widget_unrealize.
        Relies on ((widget->parent && widget->parent->window)
                   || GTK_WIDGET_TOPLEVEL (widget));
        Means: widget has an associated GdkWindow (XWindow).

GTK_MAPPED:
        Set by gtk_widget_map, unset by gtk_widget_unmap.
        May only be set if GTK_WIDGET_REALIZED (widget).
        Means: gdk_window_show() has been called on the widgets window(s).

GTK_VISIBLE:
        Set by gtk_widget_show.
        Implies that a widget will be flagged GTK_MAPPED as soon as its
        parent is mapped.
!GTK_VISIBLE:
        Set by gtk_widget_hide.
        Implies that a widget is not onscreen, therefore !GTK_MAPPED.

GTK_SENSITIVE:
        Set and unset by gtk_widget_set_sensitive.
        The sensitivity of a widget determines whether it will receive
        certain events (e.g. button or key presses). One premise for
        the widgets sensitivity is to have GTK_SENSITIVE set.

GTK_PARENT_SENSITIVE:
        Set and unset by gtk_widget_set_sensitive operations on the
        parents of the widget.
        This is the second premise for the widgets sensitivity. Once
        it has GTK_SENSITIVE and GTK_PARENT_SENSITIVE set, its state is
        effectively sensitive. This is expressed (and can be examined) by
        the GTK_WIDGET_IS_SENSITIVE macro.

GTK_CAN_FOCUS:
        There are no directly corresponding functions for setting/unsetting
        this flag, but it can be affected by the GtkWidget::has_focus argument
        via gtk_widget_set_arg.
        This flag determines whether a widget is able to handle focus grabs.

GTK_HAS_FOCUS:
        This flag will be set by gtk_widget_grab_focus for widgets that also
        have GTK_CAN_FOCUS set. The flag will be unset once another widget
        grabs the focus.
        
GTK_CAN_DEFAULT:
GTK_HAS_DEFAULT:
        These two flags are mostly equal in functionality to their *_FOCUS
        counterparts, but for the default widget.

GTK_HAS_GRAB:
        Set by gtk_grab_add, unset by gtk_grab_remove.
        Means: widget is in the grab_widgets stack, and will be the preferred
        one for receiving events other than ones of cosmetic value.

GTK_BASIC:
        The GTK_BASIC flag is an attempt at making a distinction
        between widgets that handle user input e.g. key/button presses
        and those that don't. Subsequent parent<->child relation ships
        of non `basic' widgets should be avoided. The checking for
        this is currently not properly enforced in the code. For
        example GtkButton is a non `basic' widget, that will therefore
        disallow to act as a container for another GtkButton. Now the
        gnit is, one can add a GtkHBox (which is a `basic' widget) to
        the first button, and put the second into the box.

GTK_RESERVED_3:

GTK_RC_STYLE:
        This flag indicates that its style has been looked up through
        the rc mechanism. It does not imply that the widget actually
        had a style defined through the rc mechanism.


GtkWidget, private flags:

GTK_USER_STYLE:
        A widget is flagged to have a user style, once gtk_widget_set_style
        has been invoked for it. The use of this flag is to tell widgets
        wich share a global user style from the ones which got a certain
        style assign from outside the toolkit.
        
GTK_REDRAW_PENDING:
        Relies on GTK_WIDGET_MAPPED (widget).
        [FIXME: this is not really enforced throughout the code, but should
         be. it only requires a few checks for GTK_WIDGET_MAPPED and
         minor changes to gtk_widget_unmap, we can then remove the check
         in gtk_widget_real_destroy]
        Means: there is an idle handler waiting for the widget, that
        will cause a full redraw (gtk_widget_draw (widget, NULL)).

GTK_RESIZE_PENDING:
        First, this is only valid for GtkContainers.
        [some of the code should move to gtkcontainer.c therefore]
        Relies on GTK_WIDGET_REALIZED(widget)
        [this is not really enforced throughout the code, but should
         be. it only requires a few checks for GTK_WIDGET_RELIZED and
         minor changes to gtk_widget_unrealize, we can then remove the check
         in gtk_widget_real_destroy]
        Means: there is an idle handler waiting for the container to
        resize it.

GTK_RESIZE_NEEDED:
        Relies on GTK_WIDGET_REALIZED(widget)
        [this is not really enforced throughout the code, but should
         be. once this is done special checking in gtk_widget_real_destroy
         can be avoided]
        Means: a widget has been added to the resize_widgets list of
        its _toplevel_ container (keep this in mind for GtkViewport).
        Remark: this flag is also used internaly by gtkwindow.c during
        the evaluation of resizing worthy widgets.

GTK_LEAVE_PENDING:
        A widget is flagged as such if there is a leave_notify event
        pending for it. It will receive this event regardless of a grab
        through another widget or its current sensitivity.
        [this should be made relying on GTK_REALIZED]

GTK_HAS_SHAPE_MASK:
        Set by gtk_widget_shape_combine_mask if a widget got a shape mask
        assigned (making use of the X11 shaped window extension).

GTK_IN_REPARENT:
        During the act of reparentation widgets which are already
        realized and will be added to an already realized parent need
        to have this flag set to prevent natural unrealization on the
        process of getting unparented.

Related Macros:

GTK_WIDGET_DRAWABLE:
        This macro examines whether a widget is flagged as GTK_WIDGET_VISIBLE
        and GTK_WIDGET_MAPPED.
        Means: it _makes sense_ to draw in a widgets window.

GTK_WIDGET_IS_SENSITIVE:
        This macro tells the real sensitivity state of a widget. It returns
        whether both the widget and all its parents are in sensitive state.


II. Invariants:
---------------

This section describes various constraints on the states of 
the widget:

In the following

  A => B     means  if A is true, than B is true
  A <=> B    means  A is true, if and only if B is true
                    (equivalent to A => B and A <= B)


1)  GTK_WIDGET_DESTROYED (widget) => !GTK_WIDGET_REALIZED (widget)
                                  => !GTK_WIDGET_VISIBLE (widget)
[ The latter is not currently in place, but it should be ] 
 
2) GTK_WIDGET_MAPPED (widget) => GTK_WIDGET_REALIZED (widget)

3) if GTK_WIDGET_TOPLEVEL (widget):
   GTK_WIDGET_VISIBLE (widget) <=> GTK_WIDGET_MAPPED (widget)

4) if !GTK_WIDGET_TOPLEVEL (widget):
  widget->parent && GTK_WIDGET_REALIZED (widget->parent) <=>
     GTK_WIDGET_REALIZED (widget)

5) if !GTK_WIDGET_TOPLEVEL (widget):

   GTK_WIDGET_MAPPED (widget) => GTK_WIDGET_VISIBLE (widget)
                              => GTK_WIDGET_REALIZED (widget)

   widget->parent && GTK_WIDGET_MAPPED (widget->parent) && 
     GTK_WIDGET_VISIBLE (widget) => GTK_WIDGET_MAPPED (widget)

Note:, the definition

[  GTK_WIDGET_DRAWABLE = GTK_WIDGET_VISIBLE && GTK_WIDGET_MAPPED
   is made in gtkwidget.c, but by 3) and 5), 
     
      GTK_WIDGET_MAPPED => GTK_WIDGET_VISIBLE
]

6) GTK_REDRAW_PENDING => GTK_WIDGET_REALIZED
   GTK_RESIZE_PENDING =>         "
   GTK_LEAVE_PENDING  =>         "
   GTK_RESIZE_NEEDED  =>         "

III. How states are changed:
----------------------------

How can the user control the state of a widget:
-----------------------------------------------

(In the following, set flag means set the flag, do appropriate
actions, and enforce above invariants) 

gtk_widget_show: 
 if !GTK_DESTROYED sets GTK_VISIBLE

gtk_widget_hide:
 if !GTK_VISIBLE for widget

gtk_widget_destroy:
 sets GTK_DESTROYED
 For a top-level widget

gtk_widget_realize:
 if !GTK_DESTROYED sets GTK_REALIZED
- Calling gtk_widget_realize when the widget is not a descendent
  of a toplevel is an ERROR.

gtk_container_add (container, widget) [ and container-specific variants ]
 Sets widget->parent 

gtk_container_remove (container, widget)
 unsets widget->parent

gtk_widget_reparent (widget, new_parent)
 Equivalent to removing widget from old parent and adding it to
 the new parent, except that the widget will not be temporarily 
 unrealized if both the old parent and the new parent are realized.


gtk_widget_unrealize
gtk_widget_map
gtk_widget_unmap

These functions are not meant to be used by applications - they
are used only by GTK and widgets to enforce invariants on the
state.

When The X window corresponding to a GTK window is destroyed:
-------------------------------------------------------------

gtk_widget_destroy is called (as above).



IV. Responsibilities of widgets
--------------------------------

Adding to a container
---------------------

When a widget is added to a container, the container:

  1) calls gtk_widget_set_parent (widget, container)
  2) calls gtk_widget_set_parent_window (widget, window) if 
     the widget is being added to something other than container->window
  3) if container is realized, and not widget, realizes widget
  4) if container is mapped, and not widget and widget is GTK_VISIBLE,
     maps widget
  5) Queues a resize if the widget is mapped

Note: It would be nice to remove 3) and 4) out of widget specific code
  since they are of the invariant-enforcing nature, but it is
  a bit hard, since they can't be done until after 2)


Removing from a container
-------------------------

When a widget is removed to a container, the container:

  1) Calls gtk_widget_unparent (widget)
  2) Queues a resize.

Notes:

 gtk_widget_unparent unrealizes the widget except in the 
   special case GTK_IN_REPARENT is set.


At widget creation
------------------

Widgets are created in an unrealized state. 

 1) The widget should allocate and initialize needed data structures


The Realize signal
------------------

When a widget recieves the "realize" signal it should:

 NO_WINDOW widgets: (probably OK to use default handler)

  1) set the realized flag
  2) set widget->window
      widget->window = gtk_widget_get_parent_window (widget);
      gdk_window_ref (widget->window);
  3) attach the widget's style

  widget->style = gtk_style_attach (widget->style, widget->window);

 widget with window(s)

  1) set the REALIZED flag
  2) create windows with the parent obtained from
      gtk_widget_get_parent_window (widget);
  3) attach the widget's style
  4) set the background color for the new window based on the style

The Map signal
--------------

  1) Set the MAPPED flag
  2) If the widget has any windows, gdk_window_show those windows
  3) call gtk_widget_map for all child widgets that are 
     VISIBLE and !MAPPED.
  3) Do any other functions related to putting the widget onscreen.
     (for instance, showing extra popup windows...)

The Unmap signal
----------------

When a widget receives the unmap signal, it must:

 1) If the widget has a window, gdk_window_hide that window, 
 2) If the widget does not have a window, unmap all child widgets
 3) Do any other functions related to taking the widget offscreen
     (for instance, removing popup windows...)
 3) Unset GTK_MAPPED


The Unrealize signal
--------------------

When a widget receives the unrealize signal, it must

 1) For any windows other than widget->window do:

    gdk_window_set_user_data (window, NULL);
    gdk_window_destroy (window);

 2) Call the parent's unrealize handler


The Widget class unrealize handler will take care of unrealizing
all children if necessary. [should this be made consistent with
unmap???]


The Destroy Signal
------------------

Commentary:

  The destroy signal probably shouldn't exist at all. A widget
  should merely be unrealized and removed from its parent
  when the user calls gtk_widget_destroy or a GDK_DESTROY event
  is received. However, a large body of code depends on
  getting a definitive signal when a widget goes away.

  That could be put in the finalization step, but, especially
  with language bindings, the cleanup step may need to refer
  back to the widget. (To use gtk_widget_get_data, for instance)
  If it does so via a pointer in a closure (natural for
  Scheme, or Perl), then the finalization procedure will never
  be called. 

  Also, if we made that the finalization step, we would have
  to propagate the GDK_DESTROY event in any case, since it is
  at that point at which user-visible actions need to be taken.


When a widget receives the destroy signal, it must:

  1) If the widget "owns" any widgets other than its child
     widgets, (for instance popup windows) it should
     call gtk_widget_destroy () for them.

  2) Call the parent class's destroy handler.


The "destroy" signal will only be received once. A widget
will never receive any other signals after the destroy
signal (but see the sectionalize on "Finalize" below)

The widget must handle calls to all publically accessible
functions in an innocuous manner even after a "destroy"
signal. (A widget can assume that it will not be realized 
after a "destroy" signal is received, which may simplify
handling this requirement)


The Finalize Pseudo-signal
--------------------------

The finalize pseudo-signal is received after all references
to the widget have been removed. The finalize callback
cannot make any GTK calls with the widget as a parameter.

1) Free any memory allocated by the widget. (But _not_
   the widget structure itself.

2) Call the parent class's finalize signal


A note on chaining "destroy" signals and finalize signals:
---------------------------------------------------------

This is done by code like:

  if (GTK_OBJECT_CLASS (parent_class)->destroy)
    (* GTK_OBJECT_CLASS (parent_class)->destroy) (object);

It may not be completely obvious why this works. Note
that parent_class is a static variable on a per-class
basis. So say: we have

  GtkFoo <- GtkBar <- GtkWidget <-GtkObject

And that Foo, Widget, and Object all have destructors, but
not Bar.

Then gtk_foo_destroy will call gtk_widget_destroy (because
it was not overridden in the Bar class structure) and
gtk_widget_destroy will call gtk_object_destroy because
the parent_class variable referenced by gtk_foo_destroy is the 
static variable in gtkwidget.c: GtkObjectClass.