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 invocations.

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_CHILD_VISIBLE

        Set by gtk_widget_set_child_visible, and if FALSE indicates that 

        the widget should not be mapped even if the parent is mapped

        and visible. Containers like GtkNotebook use this flag.

        A private flag, not a public flag, so if you need to check

        this flag, you should call gtk_widget_get_child_visible().

        (Should be very rarely necessary.)

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

	which share a global user style from the ones which got a certain

	style assign from outside the toolkit.

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_REALIZED 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 internally 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.

GTK_NEED_REQUEST:

	This flag is set if the widget doesn't have an up to date 

	requisition. If this flag is set, we must actually emit ::size-request

        when gtk_widget_size_request() is called. Otherwise, we can

        simply widget->requisition. We keep track of this all the time

        however, widgets with this flag set are only added to the resize 

	queue if they are viewable.

GTK_NEED_ALLOCATION:

	This flag is set if the widget doesn't have an up to date 

	allocation. If this flag is set, we must actually emit ::size-allocate

        when gtk_widget_size_allocate() is called, even if the new allocation

        is the same as the current allocation.

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_CHILD_VISIBLE (widget)

                              => GTK_WIDGET_REALIZED (widget)

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

     GTK_WIDGET_VISIBLE (widget) && GTK_WIDGET_CHILD_VISIBLE 

       => GTK_WIDGET_MAPPED (widget)

Note:, the definition

[  GTK_WIDGET_DRAWABLE = GTK_WIDGET_VISIBLE && GTK_WIDGET_MAPPED

   is made in gtkwidget.h, 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 descendant

  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_window (widget, window) if 

     the widget is being added to something other than container->window

  2) calls gtk_widget_set_parent (widget, container)

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 receives 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);

      g_object_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, CHILD_VISIBLE and !MAPPED. (A widget will only

     be !CHILD_VISIBLE if the container set it that way, so

     most containers will not have to check this.)

  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...)

 4) 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 section 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.