path: root/specs/CH03.xml

<chapter id='Composite_Widgets_and_Their_Children'>
<title>Composite Widgets and Their Children</title>
<para>
Composite widgets (widgets whose class is a subclass of
<function>compositeWidgetClass</function>)
can have an arbitrary number of children.
Consequently, they are responsible for much more than primitive widgets.
Their responsibilities (either implemented directly by the widget class
or indirectly by Intrinsics functions) include:
</para>
<itemizedlist spacing='compact'>
  <listitem>
    <para>
Overall management of children from creation to destruction.
    </para>
  </listitem>
  <listitem>
    <para>
Destruction of descendants when the composite widget is destroyed.
    </para>
  </listitem>
  <listitem>
    <para>
Physical arrangement (geometry management) of a displayable subset of
children (that is, the managed children).
    </para>
  </listitem>
  <listitem>
    <para>
Mapping and unmapping of a subset of the managed children.
    </para>
  </listitem>
</itemizedlist>
<para>
Overall management is handled by the generic procedures
<xref linkend='XtCreateWidget' xrefstyle='select: title'/>
and
<xref linkend='XtDestroyWidget' xrefstyle='select: title'/>.
<xref linkend='XtCreateWidget' xrefstyle='select: title'/>
adds children to their parent by calling the parent's insert_child
procedure.
<xref linkend='XtDestroyWidget' xrefstyle='select: title'/>
removes children from their parent by calling the parent's delete_child
procedure and ensures that all children of a destroyed composite widget
also get destroyed.
</para>

<para>
Only a subset of the total number of children is actually managed by
the geometry manager and hence possibly visible.
For example, a composite editor widget
supporting multiple editing buffers might allocate one child
widget for each file buffer,
but it might display only a small number of the existing buffers.
Widgets that are in this displayable subset are called managed widgets
and enter into geometry manager calculations.
The other children are called unmanaged widgets
and, by definition, are not mapped by the Intrinsics.
</para>

<para>
Children are added to and removed from their parent's managed set by using
<xref linkend='XtManageChild' xrefstyle='select: title'/>,
<xref linkend='XtManageChildren' xrefstyle='select: title'/>,
<xref linkend='XtUnmanageChild' xrefstyle='select: title'/>,
<xref linkend='XtUnmanageChildren' xrefstyle='select: title'/>,
and
<xref linkend='XtChangeManagedSet' xrefstyle='select: title'/>,
which notify the parent to recalculate the physical layout of its children
by calling the parent's change_managed procedure.
The
<xref linkend='XtCreateManagedWidget' xrefstyle='select: title'/>
convenience function calls
<xref linkend='XtCreateWidget' xrefstyle='select: title'/>
and
<xref linkend='XtManageChild' xrefstyle='select: title'/>
on the result.
</para>

<para>
Most managed children are mapped,
but some widgets can be in a state where they take up physical space
but do not show anything.
Managed widgets are not mapped automatically
if their <emphasis remap='I'>map_when_managed</emphasis> field is
<function>False</function>.
The default is
<function>True</function>
and is changed by using
<xref linkend='XtSetMappedWhenManaged' xrefstyle='select: title'/>.
</para>

<para>
Each composite widget class declares a geometry manager,
which is responsible for figuring out where the managed children
should appear within the composite widget's window.
Geometry management techniques fall into four classes:
<variablelist>
  <varlistentry>
    <term>Fixed boxes</term>
    <listitem>
      <para>
Fixed boxes have a fixed number of children created by the parent.
All these children are managed,
and none ever makes geometry manager requests.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>Homogeneous boxes</term>
    <listitem>
      <para>
Homogeneous boxes treat all children equally and apply the same geometry
constraints to each child.
Many clients insert and delete widgets freely.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>Heterogeneous boxes</term>
    <listitem>
      <para>
Heterogeneous boxes have a specific location where each child is placed.
This location usually is not specified in pixels,
because the window may be resized, but is expressed rather
in terms of the relationship between a child
and the parent or between the child and other specific children.
The class of heterogeneous boxes is usually a subclass of
Constraint.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>Shell boxes</term>
    <listitem>
      <para>
Shell boxes typically have only one child,
and the child's size is usually
exactly the size of the shell.
The geometry manager must communicate with the window manager, if it exists,
and the box must also accept
<function>ConfigureNotify</function>
events when the window size is changed by the window manager.
    </para>
  </listitem>
  </varlistentry>
</variablelist>
</para>
<sect1 id="Addition_of_Children_to_a_Composite_Widget_The_insert_child_Procedure">
<title>Addition of Children to a Composite Widget: The insert_child Procedure</title>
<para>
To add a child to
the parent's list of children, the
<xref linkend='XtCreateWidget' xrefstyle='select: title'/>
function calls the parent's class routine insert_child.
The insert_child procedure pointer in a composite widget is of type
<xref linkend='XtWidgetProc' xrefstyle='select: title'/>.
</para>

<funcsynopsis id='XtWidgetProc_2'>
<funcprototype>
<funcdef>typedef void <function>(*XtWidgetProc)</function></funcdef>
   <paramdef>Widget <parameter>w</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>w</emphasis>
    </term>
    <listitem>
      <para>
Passes the newly created child.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
Most composite widgets inherit their superclass's operation.
The insert_child routine in
<function>CompositeWidgetClass calls the insert_position procedure</function>
and inserts the child at the specified position
in the <emphasis remap='I'>children</emphasis> list, expanding it if necessary.
</para>

<para>
Some composite widgets define their own insert_child routine
so that they can order their children in some convenient way,
create companion controller widgets for a new widget,
or limit the number or class of their child widgets.
A composite widget class that wishes
to allow nonwidget children (see <xref linkend='Nonwidget_Objects' />) must specify a
<function>CompositeClassExtension</function>
extension record as described
in <xref linkend='CompositeClassPart_Structure' />
and set the <emphasis remap='I'>accepts_objects</emphasis> field in this record to
<function>True</function>.
If the
<function>CompositeClassExtension</function>
record is not specified or the
<emphasis remap='I'>accepts_objects</emphasis> field is
<function>False</function>,
the composite widget can assume that all its children are of a subclass of Core
without an explicit subclass test in the insert_child procedure.
</para>

<para>
If there is not enough room to insert a new child in the <emphasis remap='I'>children</emphasis> array
(that is, <emphasis remap='I'>num_children</emphasis> is equal to <emphasis remap='I'>num_slots</emphasis>),
the insert_child procedure must first reallocate the array
and update <emphasis remap='I'>num_slots</emphasis>.
The insert_child procedure then places the child at the appropriate position
in the array and increments the <emphasis remap='I'>num_children</emphasis> field.
</para>
</sect1>

<sect1 id="Insertion_Order_of_Children_The_insert_position_Procedure">
<title>Insertion Order of Children: The insert_position Procedure</title>
<para>
Instances of composite widgets sometimes need to specify more about the order in which
their children are kept.
For example,
an application may want a set of command buttons in some logical order
grouped by function,
and it may want buttons that represent file names to be kept
in alphabetical order without constraining the order in which the
buttons are created.
</para>

<para>
An application controls the presentation order of a set of children by
supplying an
XtNinsertPosition
resource.
The insert_position procedure pointer in a composite widget instance is of type
<xref linkend='XtOrderProc' xrefstyle='select: title'/>.
</para>

<funcsynopsis id='XtOrderProc'>
<funcprototype>
<funcdef>typedef Cardinal <function>(*XtOrderProc)</function></funcdef>

   <paramdef>Widget <parameter>w</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>w</emphasis>
    </term>
    <listitem>
      <para>
Passes the newly created widget.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
Composite widgets that allow clients to order their children (usually
homogeneous boxes) can call their widget instance's insert_position
procedure from the class's insert_child procedure to determine where a new
child should go in its <emphasis remap='I'>children</emphasis> array.
Thus, a client using a composite class can apply different sorting criteria
to widget instances of the class, passing in a different insert_position
procedure resource when it creates each composite widget instance.
</para>

<para>
The return value of the insert_position procedure
indicates how many children should go before the widget.
Returning zero indicates that the widget should go before all other children,
and returning <emphasis remap='I'>num_children</emphasis> indicates that it should go after all other children.
The default insert_position function returns <emphasis remap='I'>num_children</emphasis>
and can be overridden by a specific composite widget's resource list
or by the argument list provided when the composite widget is created.
</para>
</sect1>

<sect1 id="Deletion_of_Children_The_delete_child_Procedure">
<title>Deletion of Children: The delete_child Procedure</title>

<para>
To remove the child from the parent's <emphasis remap='I'>children</emphasis> list, the
<xref linkend='XtDestroyWidget' xrefstyle='select: title'/>
function eventually causes a call to the Composite parent's class delete_child
procedure.
The delete_child procedure pointer is of type
<xref linkend='XtWidgetProc' xrefstyle='select: title'/>.
</para>

<funcsynopsis id='_XtWidgetProc'>
<funcprototype>
<funcdef>typedef void <function>(*XtWidgetProc)</function></funcdef>
   <paramdef>Widget <parameter>w</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>w</emphasis>
    </term>
    <listitem>
      <para>
Passes the child being deleted.
      </para>
    </listitem>
  </varlistentry>
</variablelist>


<para>
Most widgets inherit the delete_child procedure from their superclass.
Composite widgets that create companion widgets define their own
delete_child procedure to remove these companion widgets.
</para>
</sect1>

<sect1 id="Adding_and_Removing_Children_from_the_Managed_Set">
<title>Adding and Removing Children from the Managed Set</title>
<para>
The Intrinsics provide a set of generic routines to permit the addition of
widgets to or the removal of widgets from a composite widget's managed set.
These generic routines eventually call the composite widget's change_managed
procedure if the procedure pointer is non-NULL.
The change_managed procedure pointer is of type
<xref linkend='XtWidgetProc' xrefstyle='select: title'/>.
The widget argument specifies the composite widget whose managed child
set has been modified.
</para>

<sect2 id="Managing_Children">
<title>Managing Children</title>
<para>
To add a list of widgets to the geometry-managed (and hence displayable)
subset of their Composite parent, use
<xref linkend='XtManageChildren' xrefstyle='select: title'/>.
</para>

<para>typedef Widget *WidgetList;</para>

<funcsynopsis id='XtManageChildren'>
<funcprototype>
<funcdef>void <function>XtManageChildren</function></funcdef>
   <paramdef>WidgetList <parameter>children</parameter></paramdef>
   <paramdef>Cardinal <parameter>num_children</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>children</emphasis>
    </term>
    <listitem>
      <para>
Specifies a list of child widgets.  Each child must be of class
RectObj or any subclass thereof.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>num_children</emphasis>
    </term>
    <listitem>
      <para>
Specifies the number of children in the list.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
The
<xref linkend='XtManageChildren' xrefstyle='select: title'/>
function performs the following:
</para>
<itemizedlist spacing='compact'>
  <listitem>
    <para>
Issues an error if the children do not all have the same parent or
if the parent's class is not a subclass of
<function>compositeWidgetClass</function>.
    </para>
  </listitem>
  <listitem>
    <para>
Returns immediately if the common parent is being destroyed;
otherwise, for each unique child on the list,
<xref linkend='XtManageChildren' xrefstyle='select: title'/>
ignores the child if it already is managed or is being destroyed,
and marks it if not.
    </para>
  </listitem>
  <listitem>
    <para>
If the parent is realized and after all children have been marked,
it makes some of the newly managed children viewable:
    </para>
  </listitem>
  <listitem>
    <itemizedlist spacing='compact'>
      <listitem>
        <para>
Calls the change_managed routine of the widgets' parent.
        </para>
      </listitem>
      <listitem>
        <para>
Calls
<xref linkend='XtRealizeWidget' xrefstyle='select: title'/>
on each previously unmanaged child that is unrealized.
        </para>
      </listitem>
      <listitem>
        <para>
Maps each previously unmanaged child that has <emphasis remap='I'>map_when_managed</emphasis>
<function>True</function>.
        </para>
      </listitem>
    </itemizedlist>
  </listitem>
</itemizedlist>
<para>
Managing children is independent of the ordering of children and
independent of creating and deleting children.
The layout routine of the parent
should consider children whose <emphasis remap='I'>managed</emphasis> field is
<function>True</function>
and should ignore all other children.
Note that some composite widgets, especially fixed boxes, call
<xref linkend='XtManageChild' xrefstyle='select: title'/>
from their insert_child procedure.
</para>

<para>
If the parent widget is realized,
its change_managed procedure is called to notify it
that its set of managed children has changed.
The parent can reposition and resize any of its children.
It moves each child as needed by calling
<xref linkend='XtMoveWidget' xrefstyle='select: title'/>,
which first updates the <emphasis remap='I'>x</emphasis> and <emphasis remap='I'>y</emphasis> fields and which then calls
<function>XMoveWindow</function>.
</para>

<para>
If the composite widget wishes to change the size or border width of any of
its children, it calls
<xref linkend='XtResizeWidget' xrefstyle='select: title'/>,
which first updates the
<emphasis remap='I'>width</emphasis>, <emphasis remap='I'>height</emphasis>, and <emphasis remap='I'>border_width</emphasis>
fields and then calls
<function>XConfigureWindow</function>.
Simultaneous repositioning and resizing may be done with
<xref linkend='XtConfigureWidget' xrefstyle='select: title'/>;
see <xref linkend='Widget_Placement_and_Sizing' />.
</para>

<para>
To add a single child to its parent widget's set of managed children, use
<xref linkend='XtManageChild' xrefstyle='select: title'/>.
</para>

<funcsynopsis id='XtManageChild'>
<funcprototype>
<funcdef>void <function>XtManageChild</function></funcdef>
   <paramdef>Widget <parameter>child</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>child</emphasis>
    </term>
    <listitem>
      <para>
Specifies the child.  Must be of class RectObj or any subclass thereof.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
The
<xref linkend='XtManageChild' xrefstyle='select: title'/>
function constructs a
<function>WidgetList</function>
of length 1 and calls
<xref linkend='XtManageChildren' xrefstyle='select: title'/>.
</para>

<para>
To create and manage a child widget in a single procedure, use
<xref linkend='XtCreateManagedWidget' xrefstyle='select: title'/>
or
<xref linkend='XtVaCreateManagedWidget' xrefstyle='select: title'/>.
</para>

<funcsynopsis id='XtCreateManagedWidget'>
<funcprototype>
<funcdef>Widget <function>XtCreateManagedWidget</function></funcdef>
   <paramdef>const char * <parameter>name</parameter></paramdef>
   <paramdef>WidgetClass <parameter>widget_class</parameter></paramdef>
   <paramdef>Widget <parameter>parent</parameter></paramdef>
   <paramdef>ArgList <parameter>args</parameter></paramdef>
   <paramdef>Cardinal <parameter>num_args</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>name</emphasis>
    </term>
    <listitem>
      <para>
Specifies the resource instance name for the created widget.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>widget_class</emphasis>
    </term>
    <listitem>
      <para>
Specifies the widget class pointer for the created widget.  (rC
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>parent</emphasis>
    </term>
    <listitem>
      <para>
Specifies the parent widget.  Must be of class Composite or any
subclass thereof.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>args</emphasis>
    </term>
    <listitem>
      <para>
Specifies the argument list to override any other resource specifications.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>num_args</emphasis>
    </term>
    <listitem>
      <para>
Specifies the number of entries in the argument list.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
The
<xref linkend='XtCreateManagedWidget' xrefstyle='select: title'/>
function is a convenience routine that calls
<xref linkend='XtCreateWidget' xrefstyle='select: title'/>
and
<xref linkend='XtManageChild' xrefstyle='select: title'/>.
</para>

<funcsynopsis id='XtVaCreateManagedWidget'>
<funcprototype>
<funcdef>Widget <function>XtVaCreateManagedWidget</function></funcdef>
   <paramdef>const char * <parameter>name</parameter></paramdef>
   <paramdef>WidgetClass <parameter>widget_class</parameter></paramdef>
   <paramdef>Widget <parameter>parent</parameter></paramdef>
   <paramdef>...</paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>name</emphasis>
    </term>
    <listitem>
      <para>
Specifies the resource instance name for the created widget.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>widget_class</emphasis>
    </term>
    <listitem>
      <para>
Specifies the widget class pointer for the created widget.  (rC
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>parent</emphasis>
    </term>
    <listitem>
      <para>
Specifies the parent widget.  Must be of class Composite or any
subclass thereof.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      ...
    </term>
    <listitem>
      <para>
Specifies the variable argument list to override any other
resource specifications.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
<xref linkend='XtVaCreateManagedWidget' xrefstyle='select: title'/>
is identical in function to
<xref linkend='XtCreateManagedWidget' xrefstyle='select: title'/>
with the <emphasis remap='I'>args</emphasis> and <emphasis remap='I'>num_args</emphasis> parameters replaced
by a varargs list, as described in Section 2.5.1.
</para>
</sect2>

<sect2 id="Unmanaging_Children">
<title>Unmanaging Children</title>
<para>
To remove a list of children from a parent widget's managed list, use
<xref linkend='XtUnmanageChildren' xrefstyle='select: title'/>.
</para>

<funcsynopsis id='XtUnmanageChildren'>
<funcprototype>
<funcdef>void <function>XtUnmanageChildren</function></funcdef>
   <paramdef>WidgetList <parameter>children</parameter></paramdef>
   <paramdef>Cardinal <parameter>num_children</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>children</emphasis>
    </term>
    <listitem>
      <para>
Specifies a list of child widgets.  Each child must be of class
RectObj or any subclass thereof.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>num_children</emphasis>
    </term>
    <listitem>
      <para>
Specifies the number of children.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
The
<xref linkend='XtUnmanageChildren' xrefstyle='select: title'/>
function performs the following:
</para>
<itemizedlist spacing='compact'>
  <listitem>
    <para>
Returns immediately if the common parent is being destroyed.
    </para>
  </listitem>
  <listitem>
    <para>
Issues an error if the children do not all have the same parent
or if the parent is not a subclass of
<function>compositeWidgetClass</function>.
    </para>
  </listitem>
  <listitem>
    <para>
For each unique child on the list,
<xref linkend='XtUnmanageChildren' xrefstyle='select: title'/>
ignores the child if it is unmanaged; otherwise it performs the following:
    </para>
  </listitem>
  <listitem>
    <itemizedlist spacing='compact'>
      <listitem>
        <para>
Marks the child as unmanaged.
        </para>
      </listitem>
      <listitem>
        <para>
If the child is realized and the <emphasis remap='I'>map_when_managed</emphasis> field is
<function>True</function>,
it is unmapped.
        </para>
      </listitem>
    </itemizedlist>
  </listitem>
  <listitem>
    <para>
If the parent is realized and if any children have become unmanaged,
calls the change_managed routine of the widgets' parent.
    </para>
  </listitem>
</itemizedlist>
<para>
<xref linkend='XtUnmanageChildren' xrefstyle='select: title'/>
does not destroy the child widgets.
Removing widgets from a parent's managed set is often a temporary banishment,
and some time later the client may manage the children again.
To destroy widgets entirely,
<xref linkend='XtDestroyWidget' xrefstyle='select: title'/>
should be called instead;
see <xref linkend='Exiting_from_an_Application' />.
</para>

<para>
To remove a single child from its parent widget's managed set, use
<xref linkend='XtUnmanageChild' xrefstyle='select: title'/>.
</para>

<funcsynopsis id='XtUnmanageChild'>
<funcprototype>
<funcdef>void <function>XtUnmanageChild</function></funcdef>
   <paramdef>Widget <parameter>child</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>child</emphasis>
    </term>
    <listitem>
      <para>
Specifies the child.  Must be of class RectObj or any subclass thereof.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
The
<xref linkend='XtUnmanageChild' xrefstyle='select: title'/>
function constructs a widget list
of length 1 and calls
<xref linkend='XtUnmanageChildren' xrefstyle='select: title'/>.
</para>

<para>
These functions are low-level routines that are used by generic
composite widget building routines.
In addition, composite widgets can provide widget-specific,
high-level convenience procedures.
</para>
</sect2>

<sect2 id="Bundling_Changes_to_the_Managed_Set">
<title>Bundling Changes to the Managed Set</title>
<para>
A client may simultaneously unmanage and manage children
with a single call to the Intrinsics.  In this same call the
client may provide a callback procedure that can modify the
geometries of one or more children.  The composite widget class
defines whether this single client call results in separate invocations
of the change_managed method, one to unmanage and the other to
manage, or in just a single invocation.
</para>

<para>
To simultaneously remove from and add to the geometry-managed
set of children of a composite parent, use
<xref linkend='XtChangeManagedSet' xrefstyle='select: title'/>.
</para>

<funcsynopsis id='XtChangeManagedSet'>
<funcprototype>
<funcdef>void <function>XtChangeManagedSet</function></funcdef>
   <paramdef>WidgetList <parameter>unmanage_children</parameter></paramdef>
   <paramdef>Cardinal <parameter>num_unmanage_children</parameter></paramdef>
   <paramdef>XtDoChangeProc <parameter>do_change_proc</parameter></paramdef>
   <paramdef>XtPointer <parameter>client_data</parameter></paramdef>
   <paramdef>WidgetList <parameter>manage_children</parameter></paramdef>
   <paramdef>Cardinal <parameter>num_manage_children</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>unmanage_children</emphasis>
    </term>
    <listitem>
      <para>
Specifies the list of widget children to initially remove from the managed set.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>num_unmanage_children</emphasis>
    </term>
    <listitem>
      <para>
Specifies the number of entries in the <emphasis remap='I'>unmanage_children</emphasis> list.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>do_change_proc</emphasis>
    </term>
    <listitem>
      <para>
Specifies a procedure to invoke between unmanaging
and managing the children, or NULL.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>client_data</emphasis>
    </term>
    <listitem>
      <para>
Specifies client data to be passed to the do_change_proc.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>manage_children</emphasis>
    </term>
    <listitem>
      <para>
Specifies the list of widget children to finally add to the managed set.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>num_manage_children</emphasis>
    </term>
    <listitem>
      <para>
Specifies the number of entries in the <emphasis remap='I'>manage_children</emphasis> list.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
The
<xref linkend='XtChangeManagedSet' xrefstyle='select: title'/>
function performs the following:
</para>
<itemizedlist spacing='compact'>
  <listitem>
    <para>
Returns immediately if <emphasis remap='I'>num_unmanage_children</emphasis> and
<emphasis remap='I'>num_manage_children</emphasis> are both 0.
    </para>
  </listitem>
  <listitem>
    <para>
Issues a warning and returns if the widgets specified in the
<emphasis remap='I'>manage_children</emphasis> and
the <emphasis remap='I'>unmanage_children</emphasis> lists do not all have the same parent or if
that parent is not a subclass of
<function>compositeWidgetClass</function>.
    </para>
  </listitem>
  <listitem>
    <para>
Returns immediately if the common parent is being destroyed.
    </para>
  </listitem>
  <listitem>
    <para>
If <emphasis remap='I'>do_change_proc</emphasis> is not NULL and the parent's
<function>CompositeClassExtension</function>
<emphasis remap='I'>allows_change_managed_set</emphasis> field is
<function>False</function>,
then
<xref linkend='XtChangeManagedSet' xrefstyle='select: title'/>
performs the following:
    </para>
  </listitem>
  <listitem>
    <itemizedlist spacing='compact'>
      <listitem>
        <para>
Calls
<xref linkend='XtUnmanageChildren' xrefstyle='select: title'/>
(<emphasis remap='I'>unmanage_children</emphasis>, <emphasis remap='I'>num_unmanage_children</emphasis>).
        </para>
      </listitem>
      <listitem>
    <para>
Calls the <emphasis remap='I'>do_change_proc</emphasis>.
        </para>
      </listitem>
      <listitem>
        <para>
Calls
<xref linkend='XtManageChildren' xrefstyle='select: title'/>
(<emphasis remap='I'>manage_children</emphasis>, <emphasis remap='I'>num_manage_children</emphasis>).
        </para>
      </listitem>
    </itemizedlist>
  </listitem>
  <listitem>
    <para>
Otherwise, the following is performed:
    </para>
  </listitem>
  <listitem>
    <itemizedlist spacing='compact'>
      <listitem>
        <para>
For each child on the <emphasis remap='I'>unmanage_children</emphasis> list; if the child is
already unmanaged it is ignored, otherwise it is marked as unmanaged,
and if it is realized and its <emphasis remap='I'>map_when_managed</emphasis> field is
<function>True</function>,
it is unmapped.
        </para>
      </listitem>
      <listitem>
        <para>
If <emphasis remap='I'>do_change_proc</emphasis> is non-NULL, the procedure is invoked.
        </para>
      </listitem>
      <listitem>
        <para>
For each child on the <emphasis remap='I'>manage_children</emphasis> list; if the child is already
managed or is being destroyed, it is ignored; otherwise it is
marked as managed.
        </para>
      </listitem>
      <listitem>
        <para>
If the parent is realized and after all children have been marked,
the change_managed method of the parent is invoked, and subsequently
some of the newly managed children are made viewable by calling
<xref linkend='XtRealizeWidget' xrefstyle='select: title'/>
on each previously unmanaged child that is unrealized and
mapping each previously unmanaged child that has <emphasis remap='I'>map_when_managed</emphasis>
<function>True</function>.
        </para>
      </listitem>
    </itemizedlist>
  </listitem>
</itemizedlist>
<para>
If no
<function>CompositeClassExtension</function>
record is found in the parent's composite class part <emphasis remap='I'>extension</emphasis> field
with record type
<emphasis role='strong'>NULLQUARK</emphasis>
and version greater than 1, and if
<function>XtInheritChangeManaged</function>
was specified in the parent's class record during class initialization,
the value of the <emphasis remap='I'>allows_change_managed_set</emphasis>
field is inherited from the superclass.  The value inherited from
<function>compositeWidgetClass</function>
for the <emphasis remap='I'>allows_change_managed_set</emphasis> field is
<function>False</function>.
</para>

<para>
It is not an error to include a child in both the <emphasis remap='I'>unmanage_children</emphasis>
and the <emphasis remap='I'>manage_children</emphasis> lists.  The effect of such a call is that
the child remains managed following the call, but the <emphasis remap='I'>do_change_proc</emphasis> is
able to affect the child while it is in an unmanaged state.
</para>

<para>
The <emphasis remap='I'>do_change_proc</emphasis> is of type
<xref linkend='XtDoChangeProc' xrefstyle='select: title'/>.
</para>

<funcsynopsis id='XtDoChangeProc'>
<funcprototype>
<funcdef>typedef void <function>*XtDoChangeProc</function></funcdef>

   <paramdef>Widget <parameter>composite_parent</parameter></paramdef>
   <paramdef>WidgetList <parameter>unmange_children</parameter></paramdef>
   <paramdef>Cardinal *<parameter>num_unmanage_children</parameter></paramdef>
   <paramdef>WidgetList <parameter>manage_children</parameter></paramdef>
   <paramdef>Cardinal *<parameter>num_manage_children</parameter></paramdef>
   <paramdef>XtPointer <parameter>client_data</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>composite_parent</emphasis>
    </term>
    <listitem>
      <para>
Passes the composite parent whose managed set is being altered.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>unmanage_children</emphasis>
    </term>
    <listitem>
      <para>
Passes the list of children just removed from the managed set.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>num_unmanage_children</emphasis>
    </term>
    <listitem>
      <para>
Passes the number of entries in the <emphasis remap='I'>unmanage_children</emphasis> list.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>manage_children</emphasis>
    </term>
    <listitem>
      <para>
Passes the list of children about to be added to the managed set.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>num_manage_children</emphasis>
    </term>
    <listitem>
      <para>
Passes the number of entries in the <emphasis remap='I'>manage_children</emphasis> list.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>client_data</emphasis>
    </term>
    <listitem>
      <para>
Passes the client data passed to
<xref linkend='XtChangeManagedSet' xrefstyle='select: title'/>.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
The <emphasis remap='I'>do_change_proc</emphasis> procedure is used by the caller of
<xref linkend='XtChangeManagedSet' xrefstyle='select: title'/>
to make changes to one or more children at the point when the
managed set contains the fewest entries.  These changes may
involve geometry requests, and in this case the caller of
<xref linkend='XtChangeManagedSet' xrefstyle='select: title'/>
may take advantage of the fact that the Intrinsics internally grant
geometry requests made by unmanaged children without invoking
the parent's geometry manager.  To achieve this advantage, if
the <emphasis remap='I'>do_change_proc</emphasis> procedure
changes the geometry of a child or of a descendant of a child, then
that child should be included in the <emphasis remap='I'>unmanage_children</emphasis> and
<emphasis remap='I'>manage_children</emphasis> lists.
</para>
</sect2>

<sect2 id="Determining_if_a_Widget_Is_Managed">
<title>Determining if a Widget Is Managed</title>
<para>
To determine the managed state of a given child widget, use
<xref linkend='XtIsManaged' xrefstyle='select: title'/>.
</para>

<funcsynopsis id='XtIsManaged'>
<funcprototype>
<funcdef>Boolean <function>XtIsManaged</function></funcdef>
   <paramdef>Widget <parameter>w</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>w</emphasis>
    </term>
    <listitem>
      <para>
Specifies the widget.  Must be of class Object or any subclass thereof.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
The
<xref linkend='XtIsManaged' xrefstyle='select: title'/>
function returns
<function>True</function>
if the specified widget is of class RectObj or any subclass thereof
and is managed, or
<function>False</function>
otherwise.
</para>
</sect2>
</sect1>

<sect1 id="Controlling_When_Widgets_Get_Mapped">
<title>Controlling When Widgets Get Mapped</title>
<para>
A widget is normally mapped if it is managed.
However,
this behavior can be overridden by setting the XtNmappedWhenManaged resource
for the widget when it is created
or by setting the <emphasis remap='I'>map_when_managed</emphasis> field to
<function>False</function>.
</para>

<para>
To change the value of a given widget's <emphasis remap='I'>map_when_managed</emphasis> field, use
<xref linkend='XtSetMappedWhenManaged' xrefstyle='select: title'/>.
</para>

<funcsynopsis id='XtSetMappedWhenManaged'>
<funcprototype>
<funcdef>void <function>XtSetMappedWhenManaged</function></funcdef>
   <paramdef>Widget <parameter>w</parameter></paramdef>
   <paramdef>Boolean <parameter>map_when_managed</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>w</emphasis>
    </term>
    <listitem>
      <para>
Specifies the widget.  Must be of class Core or any subclass thereof.
      </para>
    </listitem>
  </varlistentry>
  <varlistentry>
    <term>
      <emphasis remap='I'>map_when_managed</emphasis>
    </term>
    <listitem>
      <para>
Specifies a Boolean value that indicates the new value
that is stored into the widget's <emphasis remap='I'>map_when_managed</emphasis>
field.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
If the widget is realized and managed,
and if <emphasis remap='I'>map_when_managed</emphasis> is
<function>True</function>,
<xref linkend='XtSetMappedWhenManaged' xrefstyle='select: title'/>
maps the window.
If the widget is realized and managed,
and if <emphasis remap='I'>map_when_managed</emphasis> is
<function>False</function>,
it unmaps the window.
<xref linkend='XtSetMappedWhenManaged' xrefstyle='select: title'/>
is a convenience function that is equivalent to (but slightly faster than)
calling
<xref linkend='XtSetValues' xrefstyle='select: title'/>
and setting the new value for the XtNmappedWhenManaged resource
then mapping the widget as appropriate.
As an alternative to using
<xref linkend='XtSetMappedWhenManaged' xrefstyle='select: title'/>
to control mapping,
a client may set <emphasis remap='I'>mapped_when_managed</emphasis> to
<function>False</function>
and use
<xref linkend='XtMapWidget' xrefstyle='select: title'/>
and
<xref linkend='XtUnmapWidget' xrefstyle='select: title'/>
explicitly.
</para>

<para>
To map a widget explicitly, use
<xref linkend='XtMapWidget' xrefstyle='select: title'/>.
</para>

<funcsynopsis id='XtMapWidget'>
<funcprototype>
<funcdef>void <function>XtMapWidget</function></funcdef>
   <paramdef>Widget <parameter>w</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>w</emphasis>
    </term>
    <listitem>
      <para>
Specifies the widget.  Must be of class Core or any subclass thereof.
    </para>
  </listitem>
  </varlistentry>
</variablelist>

<para>
To unmap a widget explicitly, use
<xref linkend='XtUnmapWidget' xrefstyle='select: title'/>.
</para>

<funcsynopsis id='XtUnmapWidget'>
<funcprototype>
<funcdef>void <function>XtUnmapWidget</function></funcdef>
   <paramdef>Widget <parameter>w</parameter></paramdef>
</funcprototype>
</funcsynopsis>

<variablelist>
  <varlistentry>
    <term>
      <emphasis remap='I'>w</emphasis>
    </term>
    <listitem>
      <para>
Specifies the widget.  Must be of class Core or any subclass thereof.
    </para>
  </listitem>
  </varlistentry>
</variablelist>


</sect1>

<sect1 id="Constrained_Composite_Widgets">
<title>Constrained Composite Widgets</title>
<para>
The Constraint
widget class is a subclass of
<function>compositeWidgetClass</function>.
The name is derived from the fact that constraint widgets
may manage the geometry
of their children based on constraints associated with each child.
These constraints can be as simple as the maximum width and height
the parent will allow the child to occupy or can be as complicated as
how other children should change if this child is moved or resized.
Constraint
widgets let a parent define constraints as resources that are supplied for their children.
For example, if the
Constraint
parent defines the maximum sizes for its children,
these new size resources are retrieved for each child as if they were
resources that were defined by the child widget's class.
Accordingly,
constraint resources may be included in the argument list or resource file just
like any other resource for the child.
</para>

<para>
Constraint
widgets have all the responsibilities of normal composite widgets
and, in addition, must process and act upon the constraint information
associated with each of their children.
</para>

<para>
To make it easy for widgets and the Intrinsics to keep track of the
constraints associated with a child,
every widget has a <emphasis remap='I'>constraints</emphasis> field,
which is the address of a parent-specific structure that contains
constraint information about the child.
If a child's parent does not belong to a subclass of
<function>constraintWidgetClass</function>,
then the child's <emphasis remap='I'>constraints</emphasis> field is NULL.
</para>

<para>
Subclasses of
Constraint
can add constraint data to the constraint record defined by their superclass.
To allow this, widget writers should define the constraint
records in their private .h file by using the same conventions as used for
widget records.
For example, a widget class that needs to maintain a maximum
width and height for each child might define its constraint record as
follows:
</para>
<literallayout>
<code>typedef struct {</code>
<code>        Dimension max_width, max_height;</code>
<code>} MaxConstraintPart;</code>
<code>typedef struct {</code>
<code>        MaxConstraintPart max;</code>
<code>} MaxConstraintRecord, *MaxConstraint;</code>
</literallayout>
<para>
A subclass of this widget class that also needs to maintain a minimum size would
define its constraint record as follows:
</para>
<literallayout>
<code>typedef struct {</code>
<code>        Dimension min_width, min_height;</code>
<code>} MinConstraintPart;</code>
<code>typedef struct {</code>
<code>        MaxConstraintPart max;</code>
<code>        MinConstraintPart min;</code>
<code>} MaxMinConstraintRecord, *MaxMinConstraint;</code>
</literallayout>
<para>
Constraints are allocated, initialized, deallocated, and otherwise maintained
insofar as possible by the Intrinsics.
The Constraint class record part has several entries that facilitate this.
All entries in
<function>ConstraintClassPart</function>
are fields and procedures that are defined and implemented by the parent,
but they are called whenever actions are performed on the parent's children.
</para>

<para>
The
<xref linkend='XtCreateWidget' xrefstyle='select: title'/>
function uses the <emphasis remap='I'>constraint_size</emphasis> field in the parent's class record
to allocate a constraint record when a child is created.
<xref linkend='XtCreateWidget' xrefstyle='select: title'/>
also uses the constraint resources to fill in resource fields in the
constraint record associated with a child.
It then calls the constraint initialize procedure so that the parent
can compute constraint fields that are derived from constraint resources
and can possibly move or resize the child to conform to the given constraints.
</para>

<para>
When the
<xref linkend='XtGetValues' xrefstyle='select: title'/>
and
<xref linkend='XtSetValues' xrefstyle='select: title'/>
functions are executed
on a child, they use the constraint resources to get the values or
set the values of constraints associated with that child.
<xref linkend='XtSetValues' xrefstyle='select: title'/>
then calls the constraint set_values procedures so that the parent can
recompute derived constraint fields and move or resize the child
as appropriate.
If a
Constraint
widget class or any of its superclasses have declared a
<function>ConstraintClassExtension</function>
record in the
<function>ConstraintClassPart</function>
<emphasis remap='I'>extension</emphasis>
fields with a record type of
<emphasis role='strong'>NULLQUARK</emphasis>
and the <emphasis remap='I'>get_values_hook</emphasis> field in
the extension record is non-NULL,
<xref linkend='XtGetValues' xrefstyle='select: title'/>
calls the get_values_hook
procedure(s) to allow the parent to return derived constraint fields.
</para>

<para>
The
<xref linkend='XtDestroyWidget' xrefstyle='select: title'/>
function calls the constraint destroy procedure to deallocate any
dynamic storage associated with a constraint record.
The constraint record itself must not be deallocated by the constraint
destroy procedure;
<xref linkend='XtDestroyWidget' xrefstyle='select: title'/>
does this automatically.
</para>
</sect1>
</chapter>