Chapter 3. Composite Widgets and Their Children

Table of Contents

Composite widgets (widgets whose class is a subclass of compositeWidgetClass) 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:

Overall management of children from creation to destruction.
Destruction of descendants when the composite widget is destroyed.
Physical arrangement (geometry management) of a displayable subset of children (that is, the managed children).
Mapping and unmapping of a subset of the managed children.

Overall management is handled by the generic procedures XtCreateWidget and XtDestroyWidget. XtCreateWidget adds children to their parent by calling the parent's insert_child procedure. XtDestroyWidget 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.

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.

Children are added to and removed from their parent's managed set by using XtManageChild, XtManageChildren, XtUnmanageChild, XtUnmanageChildren, and XtChangeManagedSet, which notify the parent to recalculate the physical layout of its children by calling the parent's change_managed procedure. The XtCreateManagedWidget convenience function calls XtCreateWidget and XtManageChild on the result.

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 map_when_managed field is False. The default is True and is changed by using XtSetMappedWhenManaged.

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:

Fixed boxes	Fixed boxes have a fixed number of children created by the parent. All these children are managed, and none ever makes geometry manager requests.
Homogeneous boxes	Homogeneous boxes treat all children equally and apply the same geometry constraints to each child. Many clients insert and delete widgets freely.
Heterogeneous boxes	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.
Shell boxes	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 `ConfigureNotify` events when the window size is changed by the window manager.

Addition of Children to a Composite Widget: The insert_child Procedure

To add a child to the parent's list of children, the XtCreateWidget function calls the parent's class routine insert_child. The insert_child procedure pointer in a composite widget is of type XtWidgetProc.

typedef void (*XtWidgetProc)(Widget w);

typedef void (*XtWidgetProc)(Widget w);

w	Passes the newly created child.

Most composite widgets inherit their superclass's operation. The insert_child routine in CompositeWidgetClass calls the insert_position procedure and inserts the child at the specified position in the children list, expanding it if necessary.

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 Chapter 12, Nonwidget Objects) must specify a CompositeClassExtension extension record as described in the section called “CompositeClassPart Structure” and set the accepts_objects field in this record to True. If the CompositeClassExtension record is not specified or the accepts_objects field is False, 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.

If there is not enough room to insert a new child in the children array (that is, num_children is equal to num_slots), the insert_child procedure must first reallocate the array and update num_slots. The insert_child procedure then places the child at the appropriate position in the array and increments the num_children field.

Insertion Order of Children: The insert_position Procedure

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.

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 (*XtOrderProc).

typedef Cardinal (*XtOrderProc)(Widget w);

w	Passes the newly created widget.

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 children 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.

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 num_children indicates that it should go after all other children. The default insert_position function returns num_children and can be overridden by a specific composite widget's resource list or by the argument list provided when the composite widget is created.

Deletion of Children: The delete_child Procedure

To remove the child from the parent's children list, the XtDestroyWidget function eventually causes a call to the Composite parent's class delete_child procedure. The delete_child procedure pointer is of type XtWidgetProc.

typedef void (*XtWidgetProc)(Widget w);

typedef void (*XtWidgetProc)(Widget w);

w	Passes the child being deleted.

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.

Adding and Removing Children from the Managed Set

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 XtWidgetProc. The widget argument specifies the composite widget whose managed child set has been modified.

Managing Children

To add a list of widgets to the geometry-managed (and hence displayable) subset of their Composite parent, use XtManageChildren.

typedef Widget *WidgetList;

void XtManageChildren(WidgetList children, Cardinal num_children);

children	Specifies a list of child widgets. Each child must be of class RectObj or any subclass thereof.
num_children	Specifies the number of children in the list.

The XtManageChildren function performs the following:

Issues an error if the children do not all have the same parent or if the parent's class is not a subclass of compositeWidgetClass.
Returns immediately if the common parent is being destroyed; otherwise, for each unique child on the list, XtManageChildren ignores the child if it already is managed or is being destroyed, and marks it if not.
If the parent is realized and after all children have been marked, it makes some of the newly managed children viewable:
- Calls the change_managed routine of the widgets' parent.
- Calls XtRealizeWidget on each previously unmanaged child that is unrealized.
- Maps each previously unmanaged child that has map_when_managed True.

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 managed field is True and should ignore all other children. Note that some composite widgets, especially fixed boxes, call XtManageChild from their insert_child procedure.

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 XtMoveWidget, which first updates the x and y fields and which then calls XMoveWindow.

If the composite widget wishes to change the size or border width of any of its children, it calls XtResizeWidget, which first updates the width, height, and border_width fields and then calls XConfigureWindow. Simultaneous repositioning and resizing may be done with XtConfigureWidget; see the section called “Widget Placement and Sizing”.

To add a single child to its parent widget's set of managed children, use XtManageChild.

void XtManageChild(Widget child);

void XtManageChild(Widget child);

child

Specifies the child. Must be of class RectObj or any subclass thereof.

The XtManageChild function constructs a WidgetList of length 1 and calls XtManageChildren.

To create and manage a child widget in a single procedure, use XtCreateManagedWidget or XtVaCreateManagedWidget.

Widget XtCreateManagedWidget(String name, WidgetClass widget_class, Widget parent, ArgList args, Cardinal num_args);

name	Specifies the resource instance name for the created widget.
widget_class	Specifies the widget class pointer for the created widget. (rC
parent	Specifies the parent widget. Must be of class Composite or any subclass thereof.
args	Specifies the argument list to override any other resource specifications.
num_args	Specifies the number of entries in the argument list.

The XtCreateManagedWidget function is a convenience routine that calls XtCreateWidget and XtManageChild.

Widget XtVaCreateManagedWidget(String name, WidgetClass widget_class, Widget parent);

name	Specifies the resource instance name for the created widget.
widget_class	Specifies the widget class pointer for the created widget. (rC
parent	Specifies the parent widget. Must be of class Composite or any subclass thereof.
...	Specifies the variable argument list to override any other resource specifications.

XtVaCreateManagedWidget is identical in function to XtCreateManagedWidget with the args and num_args parameters replaced by a varargs list, as described in Section 2.5.1.

Unmanaging Children

To remove a list of children from a parent widget's managed list, use XtUnmanageChildren.

void XtUnmanageChildren(WidgetList children, Cardinal num_children);

void XtUnmanageChildren(WidgetList children, Cardinal num_children);

children	Specifies a list of child widgets. Each child must be of class RectObj or any subclass thereof.
num_children	Specifies the number of children.

The XtUnmanageChildren function performs the following:

Returns immediately if the common parent is being destroyed.
Issues an error if the children do not all have the same parent or if the parent is not a subclass of compositeWidgetClass.
For each unique child on the list, XtUnmanageChildren ignores the child if it is unmanaged; otherwise it performs the following:
- Marks the child as unmanaged.
- If the child is realized and the map_when_managed field is True, it is unmapped.
If the parent is realized and if any children have become unmanaged, calls the change_managed routine of the widgets' parent.

XtUnmanageChildren 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, XtDestroyWidget should be called instead; see the section called “Exiting from an Application”.

To remove a single child from its parent widget's managed set, use XtUnmanageChild.

void XtUnmanageChild(Widget child);

child

Specifies the child. Must be of class RectObj or any subclass thereof.

The XtUnmanageChild function constructs a widget list of length 1 and calls XtUnmanageChildren.

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.

Bundling Changes to the Managed Set

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.

To simultaneously remove from and add to the geometry-managed set of children of a composite parent, use XtChangeManagedSet.

void XtChangeManagedSet(WidgetList unmanage_children, Cardinal num_unmanage_children, XtDoChangeProc do_change_proc, XtPointer client_data, WidgetList manage_children, Cardinal num_manage_children);

void XtChangeManagedSet(WidgetList unmanage_children, Cardinal num_unmanage_children, XtDoChangeProc do_change_proc, XtPointer client_data, WidgetList manage_children, Cardinal num_manage_children);

unmanage_children	Specifies the list of widget children to initially remove from the managed set.
num_unmanage_children	Specifies the number of entries in the unmanage_children list.
do_change_proc	Specifies a procedure to invoke between unmanaging and managing the children, or NULL.
client_data	Specifies client data to be passed to the do_change_proc.
manage_children	Specifies the list of widget children to finally add to the managed set.
num_manage_children	Specifies the number of entries in the manage_children list.

The XtChangeManagedSet function performs the following:

Returns immediately if num_unmanage_children and num_manage_children are both 0.
Issues a warning and returns if the widgets specified in the manage_children and the unmanage_children lists do not all have the same parent or if that parent is not a subclass of compositeWidgetClass.
Returns immediately if the common parent is being destroyed.
If do_change_proc is not NULL and the parent's CompositeClassExtension allows_change_managed_set field is False, then XtChangeManagedSet performs the following:
- Calls XtUnmanageChildren (unmanage_children, num_unmanage_children).
- Calls the do_change_proc.
- Calls XtManageChildren (manage_children, num_manage_children).
Otherwise, the following is performed:
- For each child on the unmanage_children list; if the child is already unmanaged it is ignored, otherwise it is marked as unmanaged, and if it is realized and its map_when_managed field is True, it is unmapped.
- If do_change_proc is non-NULL, the procedure is invoked.
- For each child on the manage_children list; if the child is already managed or is being destroyed, it is ignored; otherwise it is marked as managed.
- 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 XtRealizeWidget on each previously unmanaged child that is unrealized and mapping each previously unmanaged child that has map_when_managed True.

If no CompositeClassExtension record is found in the parent's composite class part extension field with record type NULLQUARK and version greater than 1, and if XtInheritChangeManaged was specified in the parent's class record during class initialization, the value of the allows_change_managed_set field is inherited from the superclass. The value inherited from compositeWidgetClass for the allows_change_managed_set field is False.

It is not an error to include a child in both the unmanage_children and the manage_children lists. The effect of such a call is that the child remains managed following the call, but the do_change_proc is able to affect the child while it is in an unmanaged state.

The do_change_proc is of type *XtDoChangeProc.

typedef void *XtDoChangeProc(Widget composite_parent, WidgetList unmange_children, Cardinal *num_unmanage_children, WidgetList manage_children, Cardinal *num_manage_children, XtPointer client_data);

typedef void *XtDoChangeProc(Widget composite_parent, WidgetList unmange_children, Cardinal *num_unmanage_children, WidgetList manage_children, Cardinal *num_manage_children, XtPointer client_data);

composite_parent	Passes the composite parent whose managed set is being altered.
unmanage_children	Passes the list of children just removed from the managed set.
num_unmanage_children	Passes the number of entries in the unmanage_children list.
manage_children	Passes the list of children about to be added to the managed set.
num_manage_children	Passes the number of entries in the manage_children list.
client_data	Passes the client data passed to `XtChangeManagedSet`.

The do_change_proc procedure is used by the caller of XtChangeManagedSet 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 XtChangeManagedSet 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 do_change_proc procedure changes the geometry of a child or of a descendant of a child, then that child should be included in the unmanage_children and manage_children lists.

Determining if a Widget Is Managed

To determine the managed state of a given child widget, use XtIsManaged.

Boolean XtIsManaged(Widget w);

w	Specifies the widget. Must be of class Object or any subclass thereof.

The XtIsManaged function returns True if the specified widget is of class RectObj or any subclass thereof and is managed, or False otherwise.

Controlling When Widgets Get Mapped

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 map_when_managed field to False.

To change the value of a given widget's map_when_managed field, use XtSetMappedWhenManaged.

void XtSetMappedWhenManaged(Widget w, Boolean map_when_managed);

void XtSetMappedWhenManaged(Widget w, Boolean map_when_managed);

w	Specifies the widget. Must be of class Core or any subclass thereof.
map_when_managed	Specifies a Boolean value that indicates the new value that is stored into the widget's map_when_managed field.

If the widget is realized and managed, and if map_when_managed is True, XtSetMappedWhenManaged maps the window. If the widget is realized and managed, and if map_when_managed is False, it unmaps the window. XtSetMappedWhenManaged is a convenience function that is equivalent to (but slightly faster than) calling XtSetValues and setting the new value for the XtNmappedWhenManaged resource then mapping the widget as appropriate. As an alternative to using XtSetMappedWhenManaged to control mapping, a client may set mapped_when_managed to False and use XtMapWidget and XtUnmapWidget explicitly.

To map a widget explicitly, use XtMapWidget.

 XtMapWidget(Widget w);

w	Specifies the widget. Must be of class Core or any subclass thereof.

To unmap a widget explicitly, use XtUnmapWidget.

 XtUnmapWidget(Widget w);

w	Specifies the widget. Must be of class Core or any subclass thereof.

Constrained Composite Widgets

The Constraint widget class is a subclass of compositeWidgetClass. 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.

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.

To make it easy for widgets and the Intrinsics to keep track of the constraints associated with a child, every widget has a constraints 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 constraintWidgetClass, then the child's constraints field is NULL.

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:

typedef struct {
Dimension max_width, max_height;
} MaxConstraintPart;
typedef struct {
MaxConstraintPart max;
} MaxConstraintRecord, *MaxConstraint;

A subclass of this widget class that also needs to maintain a minimum size would define its constraint record as follows:

typedef struct {
Dimension min_width, min_height;
} MinConstraintPart;
typedef struct {
MaxConstraintPart max;
MinConstraintPart min;
} MaxMinConstraintRecord, *MaxMinConstraint;

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 ConstraintClassPart 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.

The XtCreateWidget function uses the constraint_size field in the parent's class record to allocate a constraint record when a child is created. XtCreateWidget 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.

When the XtGetValues and XtSetValues 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. XtSetValues 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 ConstraintClassExtension record in the ConstraintClassPart extension fields with a record type of NULLQUARK and the get_values_hook field in the extension record is non-NULL, XtGetValues calls the get_values_hook procedure(s) to allow the parent to return derived constraint fields.

The XtDestroyWidget 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; XtDestroyWidget does this automatically.