Constraint()Constraint()NameConstraint widget class - a widget that provides constraint resources
for its children.
Synopsis
Public Headers: <X11/StringDefs.h>
<X11/Constraint.h>
Private Header: <X11/ConstraintP.h>
Class Name: Constraint
Class Hierarchy: Core → Composite → Constraint
Class Pointer: constraintWidgetClass
Instantiation: Constraint is an Intrinsics meta-class, and is not
normally instantiated.
Functions/Macros: XtIsConstraint()DescriptionConstraint widgets are a subclass of compositeWidgetClass. Their name
is derived from the fact that they 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 as complicated as how other children
should change if this child is moved or resized. Constraint widgets
let a parent define 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.
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
is not a subclass of constraintWidgetClass, then the child's
constraints field is NULL.
Note that the constraint data structures are transparent to the child;
that is, when a child is managed by a parent that is a subclass of a
constraint widget, there is no difference, as far as the child is
concerned, from being managed by a normal composite widget.
See the "Background" section below for more information on Constraint
and constraint resources.
New ResourcesConstraint defines no new resources.
Inherited ResourcesConstraint inherits the following resources. The resources are listed
alphabetically, along with the superclass that defines them.
Resource Inherited From Resource Inherited From
XtNaccelerators Core XtNheight Core
XtNancestorSensitive Core XtNinitialResources- Core
Persistent
XtNbackground Core XtNinsertPosition Composite
XtNbackgroundPixmap Core XtNmappedWhen- Core
Managed
XtNborderColor Core XtNnumChildren Composite
XtNborderPixmap Core XtNscreen Core
XtNborderWidth Core XtNsensitive Core
XtNchildren Composite XtNtranslations Core
XtNcolormap Core XtNwidth Core
XtNdepth Core XtNx Core
XtNdestroyCallback Core XtNy Core
Class Structure
The Constraint class record is defined as follows:
typedef struct {
XtResourceList resources;/* constraint resource list */
Cardinal num_resources; /* number of constraints in list */
Cardinal constraint_size;/* size of constraint record */
XtInitProc initialize; /* constraint initialization */
XtWidgetProc destroy; /* constraint destroy proc */
XtSetValuesFunc set_values;/* constraint set_values proc */
XtPointer extension; /* pointer to extension record */
} ConstraintClassPart;
typedef struct {
CoreClassPart core_class;
CompositeClassPart composite_class;
ConstraintClassPart constraint_class;
} ConstraintClassRec, *ConstraintWidgetClass;
The fields of the Constraint class part have the following meanings:
resources
An array of XtResource, each of which defines a single constraint
resource for the widget class. See XtGetApplicationResources()
for an explanation of how to define resources and declare an
XtResourceList.
num_resources
The number of element in the resources array.
constraint_size
The size of the constraint instance record for this widget class.
A block this size will be automatically allocated for each child
of an instance of this widget class, and will be pointed to by
that child's Core constraints instance field. Use sizeof to
initialize this field.
initialize()
The constraint initialize() method, called to initialize the part
of constraint instance record added by this widget class. See the
reference page in Section 4. This is a chained method and cannot
be inherited. Use NULL if the constraint part defined by this
class does not need any initialization.
destroy()
The constraint destroy() method, called to deallocate any memory
or other resources associated with the part of the constraint
instance record added by this widget class when a child widget is
destroyed. See the reference page in Section 4. This is a
chained method and cannot be inherited. Use NULL if the
constraint part defined by this class does not need any special
deallocation.
set_values()
The constraint set_values() method, called when XtSetValues() is
called on a child widget. See the reference page in Section 4.
This is a chained method and cannot be inherited. Use NULL if the
constraint part defined by this class does not have any resources.
extension
A linked list of extension records, or NULL. There is currently
one extension defined for Constraint which adds a
get_values_hook() method; see below.
Extension Structure
The Intrinsics define one extension to Constraint:
typedef struct {
XtPointer next_extension; /* next record is linked list, or NULL */
XrmQuark record_type; /* NULLQUARK */
long version; /* XtConstraintExtensionVersion */
Cardinal record_size; /* use sizeof() */
XtArgsProc get_values_hook;
} ConstraintClassExtensionRec, *ConstraintClassExtension;
The record_type field should be NULLQUARK, the version field should be
XtConstraintExtensionVersion, and the get_values_hook() field should be
the get_values_hook() method for the constraint resources. See
Constraintget_values_hook(4).
Instance Structure
The Constraint instance structure defines no new fields:
typedef struct _ConstraintPart {
XtPointer dummy; /* No new fields, keep C compiler happy */
} ConstraintPart;
typedef struct {
CorePart core;
CompositePart composite;
ConstraintPart constraint;
} ConstraintRec, *ConstraintWidget;
Background
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 information 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 to
allocate a constraint record when a child is created. The
constraint_size field gives the number of bytes occupied by a
constraint record. 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() methods 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.
The XtGetValues() and XtSetValues() functions use the constraint
resources to get the values or set the values of constraint associated
with a child. XtSetValues() then calls the constraint set_values()
methods so that a parent can recompute derived constraint fields and
move or resize the child as appropriate.
If the Constraint widget or any of its superclasses have declared a
ConstraintClassExtension record in the constraint class part extension
fields with a record type of NULLQUARK, and if the get_values_hook()
field in the extension record is non-NULL, then XtGetValues() calls the
get_values_hook()method(s) to allow the parent to return derived
constraint fields.
The XtDestroyWidget() function calls the constraint destroy method to
deallocate any dynamic storage associated with a constraint record.
The constraint record itself must not be deallocated by the constraint
destroy method; XtDestroyWidget() does this automatically.
See AlsoCore(3), Composite(3),
Constraintdestroy(4), Constraintget_values_hook(4), Constraintinitialize(4), Constraintset_values(4).
Intrinsics ClassesConstraint()
