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4.1 Displaying a Form

After having defined the forms the application program can use them to interact with the user. As a first step the program has to display the forms with which it wants the user to interact. This is done using the routine

Window fl_show_form(FL_FORM *form, int place, int border,
                    const char *name);

It opens a (top-level) window on the screen in which the form is shown. The parameter name is the title of the form (and its associated icon if any). The routine returns the ID of the forms window. You normally never need this. Immediately after the form becomes visible, a full draw of all objects on the form is performed. Due to the two way buffering mechanism of Xlib, if fl_show_form() is followed by something that blocks (e.g., waiting for a device other than X devices to come online), the output buffer might not be properly flushed, resulting in the form only being partially drawn. If your program works this way, use the following function after fl_show_form()

void fl_update_display(int blocking);

where blocking is false (0), the function flushes the X buffer so the drawing requests are on their way to the server. When blocking is true (1), the function flushes the buffer and waits until all the events are received and processed by the server. For typical programs that use fl_do_forms() or fl_check_forms() after fl_show_form(), flushing is not necessary as the output buffer is flushed automatically. Excessive call to fl_update_display() degrades performace.

The location and size of the window to be shown on the call of fl_show_form() are determined by the place argument. The following possibilities exist:

FL_PLACE_SIZE

The user can control the position but the size is fixed. Interactive resizing is not allowed once the form becomes visible.

FL_PLACE_POSITION

Initial position used will be the one set via fl_set_form_position(). Interactive resizing is possible.

FL_PLACE_GEOMETRY

Place at the latest position and size (see also below) or the geometry set via fl_set_form_geometry(). A form so shown will have a fixed size and interactive resizing is not allowed.

FL_PLACE_ASPECT

Allows interactive resizing but any new size will have the aspect ratio as that of the initial size.

FL_PLACE_MOUSE

The form is placed centered below the mouse. Interactive resizing will not be allowed unless this option is accompanied by FL_FREE_SIZE as in FL_PLACE_MOUSE|FL_FREE_SIZE.

FL_PLACE_CENTER

The form is placed in the center of the screen. If FL_FREE_SIZE is also specified, interactive resizing will be allowed.

FL_PLACE_FULLSCREEN

The form is scaled to cover the full screen. If FL_FREE_SIZE is also specified, interactive resizing will be allowed.

FL_PLACE_FREE

Both the position and size are completely free. The initial size used is the designed size. Initial position, if setvia fl_set_form_position(), will be used otherwise interactive positioning may be possible if the window manager allows it.

FL_PLACE_HOTSPOT

The form is placed so that mouse is on the form’s "hotspot". If FL_FREE_SIZE is also specified, interactive resizing will be allowed.

FL_PLACE_CENTERFREE

Same as FL_PLACE_CENTER|FL_FREE_SIZE, i.e., place the form at the center of the screen and allow resizing.

FL_PLACE_ICONIC

The form is shown initially iconified. The size and location used are the window manager’s default.

As mentioned above, some of the settings will result in a fixed size of the form (i.e., a size that can’t be changed by the user per default). In some cases this can be avoided by OR’ing the value with FL_FREE_SIZE as a modifier.

If no size was specified, the designed (or later scaled) size will be used. Note that the initial position is dependent upon the window manager used. Some window managers allow interactive placement of the windows but some don’t.

You can set the position or size to be used via the following calls

void fl_set_form_position(FL_FORM *form, FL_Coord x, FL_Coord y);

and

void fl_set_form_size(FL_FORM *form, FL_Coord w, FL_Coord h);

or, combining both these two functions,

void fl_set_form_geometry(FL_FORM form*, FL_Coord x, FL_Coord y,
                          FL_Coord w, FL_Coord h);

before placing the form on the screen. (Actually the routines can also be called while the form is being displayed. They will change the position and/or size of the form.) x, y, w and h indicate the position of the form on the screen and its size4. The position is measured from the top-left corner of the screen. When the position is negative the distance from the right or the bottom is indicated. Next the form should be placed on the screen using FL_PLACE_GEOMETRY, FL_PLACE_FREE. E.g., to place a form at the lower-right corner of the screen use

fl_set_form_position(form, -1, -1);
fl_show_form(form, FL_PLACE_GEOMETRY, FL_TRANSIENT, "formName");

(Following the X convention for specifying geometries a negative x-position specifies the distance of the right eside of the form from the right side of the screen and a negative y-position the distance of the bottom of the form from the bottom of the screen.)

To show a form so that a particular object or point is under the mouse, use one of the following two routines to set the "hotspot"

void fl_set_form_hotspot(FL_FORM *form, FL_Coord x, FL_Coord y);
void fl_set_form_hotobject(FL_FORM *form, FL_OBJECT *obj);

and then use FL_PLACE_HOTSPOT for the place argument in the call of fl_show_form(). The coordinates x and y are relative to the upper-left hand corner of the form (within the window decorations).

In the call fl_show_form() the argument border indicates whether or not to request window manager’s decoration. border should take one of the following values:

FL_FULLBORDER

Full border decorations.

FL_TRANSIENT

Borders with (possibly) less decorations.

FL_NOBORDER

No decoration at all.

For some dialogs, such as demanding an answer etc., you probably do not want the window manager’s full decorations. Use FL_TRANSIENT for this.

A window border is useful to let the user iconify a form, move it around or resize it. If a form is transient or has no border, it is normally more difficult (or even impossible) to move the form. A transient form typically should have less decoration, but not necessarily so. It depends on the window managers as well as their options. FL_NOBORDER is guaranteed to have no border5 and is immune to iconification request. Because of this, borderless forms can be hostile to other applications6, so use this only if absolutely necessary.

There are other subtle differences between the different decoration requests. For instance, (small) transient forms always have save_under (see XSetWindowAttributes()) set to true by default. Some window properties, WM_COMMAND in particular, are only set for full-bordered forms and will only migrate to other full-bordered forms when the original form having the property becomes unmapped.

The library has a notion of a "main form" of an application, roughly the form that would be on the screen the longest. By default, the first full-bordered form shown becomes the main form of the application. All transient windows shown afterwards will stay on top of the main form. The application can set or change the main form anytime using the following routine

void fl_set_app_mainform(FL_FORM *form);

Setting the main form of an application will cause the WM_COMMAND property set for the form if no other form has this property.

Sometimes it is necessary to have access to the window resource ID before the window is mapped (shown). For this, the following routine can be used

Window fl_prepare_form_window(FL_FORM *form, int place, int border,
                              const char *name);

This routine creates a window that obeys any and all constraints just as fl_show_form() does but remains unmapped. To map such a window, the following must be used

Window fl_show_form_window(FL_FORM *form);

Between these two calls, the application program has full access to the window and can set all attributes, such as icon pixmaps etc., that are not set by fl_show_form().

You can also scale the form and all objects on it programmatically using the following routine

void fl_scale_form(FL_FORM *form, double xsc, double ysc);

where you indicate a scaling factor in the x- and y-direction with respect to the current size. See rescale.c for an example.

When a form is scaled, either programmatically or interactively, all objects on the form per default will also be scaled. This includes both the sizes and positions of the objects. For most cases, this default behavior is adequate. In some cases, e.g., to keep a group of objects together, more control is needed. To this end, the following routines can be used

void fl_set_object_resize(FL_OBJECT *obj, unsigned how_resize);
void fl_set_object_gravity(FL_OBJECT *obj,
                           unsigned nw_gravity, unsigned se_gravity);

The how_resize argument of fl_set_object_resize() can be one of

FL_RESIZE_NONE

don’t resize the object at all

FL_RESIZE_X

resize it in x- (horizontal) direction only

FL_RESIZE_Y

resize it in y- (vertical) direction only

FL_RESIZE_ALL

is an alias for FL_RESIZE_X|FL_RESIZE_Y and makes the object resizable in both dimension.

The arguments nw_gravity and se_gravity of fl_set_object_gravity() control the positioning of the upper-left and lower-right corner of the object and work analogously to the win_gravity in Xlib. The details are as follows: Let P be the corner the gravity applies to, (dx1,dy1) the distance to the upper-left corner of the form, (dx2,dy2) the distance to the lower-right corner of the form, then,

ValueEffect
FL_NoGravityDefault linear scaling, see below
FL_NorthWestdx1, dy1 constant
FL_Northdy1 constant
FL_NorthEastdy1, dx2 constant
FL_Westdx1 constant
FL_Eastdx2 constant
FL_SouthWestdx1, dy2 constant
FL_Southdy2 constant
FL_SouthEastdx2, dy2 constant
ForgetGravitydon’t consider the setting for this argument
xforms_images/gravity

Default for all object is FL_RESIZE_ALL and ForgetGravity. Note that the three parameters are not orthogonal and the positioning request will always override the scaling request in case of conflict. This means the resizing settings for an object are considered only if one (or both) of the gravities is FL_NoGravity.

For the special case where how_resize is FL_RESIZE_NONE and both gravities are set to ForgetGravity, the object is left un-scaled, but the object is moved so that the new position keeps the center of gravity of the object constant relative to the form.

Again, since all sizing requests go though the window manager, there is no guarantee that your request will be honored. If a form is placed with FL_PLACE_GEOMETRY or other size-restricting options, resizing it later via fl_set_form_size() will likely be rejected.

To determine the gravity and resize settings for an object use the functions

void fl_get_object_gravity(FL_OBJECT *obj,
                           unsigned int *nw, unsigned int *se);
void fl_get_object_resize(FL_OBJECT *obj, unsigned int *resize );

Sometimes, you may want to change an attribute for all objects on a particular form, to this end, the following iterator is available

void fl_for_all_objects(FL_FORM *form,
                        int (*operate)(FL_OBJECT *obj, void *data),
                        void *data);

where function operate is called for every object of the form form unless operate() returns nonzero, which terminates the iterator.

Multiple forms can be shown at the same moment and the system will interact with all of them simultaneously.

The graphical mode in which the form is shown depends on the type of machine. In general, the visual chosen by XForms is the one that has the most colors. Application programs have many ways to change this default, either through command line options, resources or programmatically. See the Part V for details.

If for any reason, you would like to change the form title (as well as its associated icon) after it is shown, the following functions can be used

void fl_set_form_title(FL_FORM *form, const char *name)
void fl_set_form_title_f(FL_FORM *form, const char *fmt, ...)

To set or change the icon shown when a form is iconified, use the following routine

void fl_set_form_icon(FL_FORM *form, Pixmap icon, Pixmap mask);

where icon and mask can be any valid Pixmap ID. (See Other Pixmap Routines for some of the routines that can be used to create Pixmaps.) Note that an icon previously setvia this function (if it exists) is not freed or modified in anyway. See the demo program iconify.c for an example.

If the application program wants to stop interacting with a form and remove it from the screen, it has to use the call

void fl_hide_form(FL_FORM *form);

To check if a form is visible or not, use the following call

int fl_form_is_visible(FL_FORM *form);

The function returns one of

FL_INVISIBLE

if the form is not visible (0),

FL_VISIBLE

if the form is visible (1) and

FL_BEING_HIDDEN

if the form is visible but is in the process of being hidden (-1).

Note that if you don’t need a form anymore you can deallocate its memory using the call fl_free_form() described earlier.

Window managers typically have a menu entry labeled "delete" or "close" meant to terminate an application program gently by informing the application program with a WM_DELETE_WINDOW protocol message. Although the Forms Library catches this message, it does not do anything except terminating the application. This can cause problems if the application has to do some record keeping before exiting. To perform record keeping or to elect to ignore this message, register a callback function using the following routine

int fl_set_atclose(int (*at_close)(FL_FORM *, void *), void *data);

The callback function at_close will be called before the Forms Library terminates the application. The first parameter of the callback function is the form that received the WM_DELETE_WINDOW message. To prevent the Forms Library from terminating the application, the callback function should return the constant FL_IGNORE. Any other value (e.g., FL_OK) will result in the termination of the application.

Similar mechanism exists for individual forms

int fl_set_form_atclose(FL_FORM *,
                        int (*at_close)(FL_FORM *, void *),
                        void *data);

except that FL_OK does not terminate the application, it results in the form being closed. Of course, if you’d like to terminate the application, you can always call exit(3) yourself within the callback function.


Footnotes

(4)

The parameters should be sensitive to the coordinate unit in effect at the time of the call, but at present, they are not, i.e., the function takes only values in pixel units.

(5)

Provided the window manager is compliant. If the window manager isn’t compliant all bets are off.

(6)

Actually, they are also hostile to their sibling forms. See Overview of Main Functions.


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