Query: query_geometry
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Section: 4
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query_geometry() query_geometry()
Name
query_geometry - RectObj method called to request a widget's preferred size and position.
Synopsis
typedef XtGeometryResult (*XtGeometryHandler)(Widget, XtWidgetGeometry *,
XtWidgetGeometry *);
Widget w;
XtWidgetGeometry *request;
XtWidgetGeometry *preferred_return
Inputs
w Specifies the widget or object that is being queried.
request Specifies the geometry changes that the parent plans to make.
Outputs
preferred_return
Returns the widget or object's preferred geometry.
Returns
The widget's reply: XtGeometryYes, XtGeometryNo, or XtGeometryAlmost.
Description
The query_geometry() method is registered on the query_geometry field of the RectObj or Core class part structure. It is invoked by
XtQueryGeometry() when the widget's parent wants to determine the widget's preferred geometry. The parent may call this function when it
want to know the widget's ideal size in the absence of any constraints, or it may call it before changing some of the widget's geometry
fields in order to find out the widget's preference for the other fields.
The request argument indicates the geometry changes the parent plans to make. request->request_mode contains flags that indicate which of
the remaining fields are significant. (See the "Structures" section below for a description of the structure and a list of the flags.)
The query_geometry() method should examine this proposed change, set its preferred geometry in preferred_return, setting preferred_return->
request_mode to indicate which geometry field it cares about, and return one of the following values:
XtGeometryYes
The widget agrees to the change; the parent should make exactly the change it proposed. When query_geometry() returns this value, the
contents of preferred_return should be identical to the contents of request, for each flag bit set in preferred_return. This means
that the parent's proposed geometry is exactly the widget's preferred geometry (or that the widget has a flexible notion of "pre-
ferred" and feels that the proposal is good enough).
XtGeometryNo
This return value means that the widget's current geometry is its preferred geometry, and it would rather not be changed. When this
value is returned, query_geometry() doesn't have to fill in preferred_return, because XtQueryGeometry() will fill in any unspecified
fields with their current values, which in this case are the preferred values. It is of course up to the parent widget whether or not
to honor this request not to change.
XtGeometryAlmost
This return value means that the widget would like a geometry change, but that its preferred geometry differs from the proposed geome-
try. query_geometry() should return this value if at least one bit is set in the return flags that was not set in the proposed flags,
or if any of the returned values (that have their flag bit set) differ from the corresponding proposed values.
Note that query_geometry() need only return its preferences for the fields it cares about (generally width and height). XtQueryGeometry()
will fill in any unspecified fields with the widget's current value. Also, query_geometry() can ignore flag bits set in request for fields
that it does not care about. If a widget doesn't care about its location, for example, and its parent proposes to change its location
without changing its size, it should still place its preferred size in preferred_return, and follow the rules above to determine which
value to return.
It is up to the parent widget to decide what to do with the return value from this function (which it gets from XtQueryGeometry()). Some
parents will respect the widget's requests, but many will ignore them. Generally, if a parent sets a flag in request, it will change that
field, regardless of the widget's return value. The power of the query_geometry() method lies in the fact that the widget can propose
additional geometry changes with the XtGeometryAlmost return value. A parent that constrains the width of a widget may call XtQueryGeome-
try() to determine how high the widget would like to be, given its new width. A label widget that supported word wrapping, for example,
might request a larger height if its width were made smaller. A cooperating parent would make its change to the width, but also change the
height as the child requested.
The query_geometry() method is not chained. Widget classes that do not define a query_geometry() method can inherit the method from their
superclass by specifying XtInheritQueryGeometry on the query_geometry field of the RectObj or Core class part structure. Widgets that have
no preference about their geometry can set this field to NULL.
See the "Background" section below for more details on the query_geometry() method. See XtQueryGeometry(1) for the parent widget's per-
spective on this geometry negotiation process.
Usage
Like the geometry_manager() method, the query_geometry() method can be confusing, and can become almost arbitrarily complex. The
query_geometry() mechanism allows much more flexibility than most widgets need and more than most parents support. The fact that there are
three possible return values which do not have very clearly defined meanings (at least not well defined in practice) to the child or to the
parent makes the situation all the more confusing. There are some factors that can simplify the query_geometry() method, however.
Most widgets do not care about anything other than their width and height, and their query_geometry() methods never need to return pre-
ferred information about position, border width, or stacking order. Most parents won't bother to query these fields anyway.
Most widgets treat their width and height independently, and do not try to increase their height if their width shrinks, for example.
These widgets can use a simple method like the example shown below.
Some widgets do not even have a preference about their size. A label widget's preferred size is whatever it takes to display the string or
pixmap, and a composite widget's preferred size may be whatever it takes to lay out its children. A scrollbar, however, can make do with
whatever size it is given, and can simply inherit the query_geometry() method of its parent.
Example
The following procedure is the query_geometry() method from the Xaw Label widget. Most of the Xaw widgets that have a query_geometry()
method use almost identical procedures. The first thing it does is set its preferred width and height, which are the width and height of
the label plus internal margins. Then it checks to see if the parent's requested geometry had both width and height flags set (note that
it masks out any other bits for fields that it does not care about), and if so, and if the parent's request matches the preferred size, it
returns XtGeometryYes. Otherwise, if the preferred size matches the widget's current size, it returns XtGeometryNo, and otherwise it
returns XtGeometryAlmost.
Note that this procedure does not attempt to do anything special if only its width or only its height are constrained. The query_geome-
try() method of the Xaw Box widget does handle this case, and may be worth study if you will be writing a similar widget.
static XtGeometryResult QueryGeometry(w, intended, preferred)
Widget w;
XtWidgetGeometry *intended, *preferred;
{
register LabelWidget lw = (LabelWidget)w;
preferred->request_mode = CWWidth CWHeight;
preferred->width = (lw->label.label_width + 2 * lw->label.internal_width +
LEFT_OFFSET(lw));
preferred->height = lw->label.label_height + 2*lw->label.internal_height;
if ( ((intended->request_mode & (CWWidth CWHeight))
== (CWWidth CWHeight)) &&
intended->width == preferred->width &&
intended->height == preferred->height)
return XtGeometryYes;
else if (preferred->width == w->core.width &&
preferred->height == w->core.height)
return XtGeometryNo;
else
return XtGeometryAlmost;
}
Background
The query_geometry() method is expected to examine the bits set in request->request_mode, evaluate the proposed geometry for the widget,
and store the result in preferred_return (setting the bits in preferred_return-> request_mode corresponding to those geometry fields that
it cares about). If the proposed geometry change is acceptable without modification, the query_geometry() method should return XtGeome-
tryYes. If at least one field in preferred_return is different from the corresponding field in request or if a bit was set in pre-
ferred_return that was not set in request, the query_geometry() method should return XtGeometryAlmost. If the preferred geometry is iden-
tical to the current geometry, the query_geometry() method should return XtGeometryNo.
Note that the query_geometry() method may assume that no XtMakeResizeRequest() or XtMakeGeometryRequest() is in progress for the specified
widget; that is, it is not required to construct a reply consistent with the requested geometry if such a request were actually outstand-
ing.
After calling the query_geometry() method or if the query_geometry() field is NULL, XtQueryGeometry() examines all the unset bits in pre-
ferred_return-> request_mode and sets the corresponding fields in preferred_return to the current values from the widget instance. If
CWStackMode is not set, the stack_mode field is set to XtSMDontChange. XtQueryGeometry() returns either the value returned by the
query_geometry() method or XtGeometryYes if the query_geometry() field is NULL.
Therefore, the caller can interpret a return of XtGeometryYes as not needing to evaluate the contents of the reply and, more importantly,
not needing to modify its layout plans. A return of XtGeometryAlmost means either that both the parent and the child expressed interest in
at least one common field and the child's preference does not match the parent's intentions or that the child expressed interest in a field
that the parent might need to consider. A return value of XtGeometryNo means that both the parent and the child expressed interest in a
field and that the child suggests that the field's current value is its preferred value. In addition, whether or not the caller ignores
the return value or the reply mask, it is guaranteed that the preferred_return structure contains complete geometry information for the
child.
Parents are expected to call XtQueryGeometry() in their layout routine and wherever other information is significant after change_managed()
has been called. The change_managed() method may assume that the child's current geometry is its preferred geometry. Thus, the child is
still responsible for storing values into its own geometry during its initialize() method.
Structures
The possible return values of a query_geometry() method are defined as follows:
typedef enum {
XtGeometryYes, /* Request accepted */
XtGeometryNo, /* Request rejected */
XtGeometryAlmost,/* Compromise proposed */
XtGeometryDone /* never returned by query_geometry */
} XtGeometryResult;
The XtWidgetGeometry structure is similar to but not identical to the corresponding Xlib structure:
typedef unsigned long XtGeometryMask;
typedef struct {
XtGeometryMask request_mode;
Position x, y;
Dimension width, height;
Dimension border_width;
Widget sibling;
int stack_mode;
} XtWidgetGeometry;
XtQueryGeometry(), like the Xlib XConfigureWindow() function, uses request_mode to determine which fields in the XtWidgetGeometry structure
the parent and the child have set. The request_mode definitions are from <X11/X.h>:
#define CWX(1<<0)
#define CWY(1<<1)
#define CWWidth(1<<2)
#define CWHeight(1<<3)
#define CWBorderWidth(1<<4)
#define CWSibling(1<<5)
#define CWStackMode(1<<6)
The stack_mode definitions are from <X11/X.h>:
#define Above0
#define Below1
#define TopIf2
#define BottomIf3
#define Opposite4
The Intrinsics also support the following value:
#define XtSMDontChange5
For precise definitions of Above, Below, TopIf, BottomIf, and Opposite, see the reference page for XConfigureWindow() in Volume Two, Xlib
Reference Manual. XtSMDontChange indicates that the widget wants its current stacking order preserved.
See Also
XtQueryGeometry(1),
Core(3).
Xt - Intrinsics Methods query_geometry()