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<H2><A NAME="sec_Range_Widgets"></A> <A NAME="s8">8. Range Widgets</A></H2>
<P>The category of range widgets includes the ubiquitous scrollbar widget
and the less common "scale" widget. Though these two types of widgets
are generally used for different purposes, they are quite similar in
function and implementation. All range widgets share a set of common
graphic elements, each of which has its own X window and receives
events. They all contain a "trough" and a "slider" (what is sometimes
called a "thumbwheel" in other GUI environments). Dragging the slider
with the pointer moves it back and forth within the trough, while
clicking in the trough advances the slider towards the location of the
click, either completely, or by a designated amount, depending on
which mouse button is used.
<P>As mentioned in
<A HREF="gtk_tut-7.html#sec_Adjustment">Adjustments</A> above,
all range widgets are associated with an adjustment object, from which
they calculate the length of the slider and its position within the
trough. When the user manipulates the slider, the range widget will
change the value of the adjustment.
<P>
<H2><A NAME="ss8.1">8.1 Scrollbar Widgets</A>
</H2>
<P>These are your standard, run-of-the-mill scrollbars. These should be
used only for scrolling some other widget, such as a list, a text box,
or a viewport (and it's generally easier to use the scrolled window
widget in most cases). For other purposes, you should use scale
widgets, as they are friendlier and more featureful.
<P>There are separate types for horizontal and vertical scrollbars.
There really isn't much to say about these. You create them with the
following functions, defined in <CODE><gtk/gtkhscrollbar.h></CODE>
and <CODE><gtk/gtkvscrollbar.h></CODE>:
<P>
<BLOCKQUOTE><CODE>
<PRE>
GtkWidget *gtk_hscrollbar_new( GtkAdjustment *adjustment );
GtkWidget *gtk_vscrollbar_new( GtkAdjustment *adjustment );
</PRE>
</CODE></BLOCKQUOTE>
<P>and that's about it (if you don't believe me, look in the header
files!). The <CODE>adjustment</CODE> argument can either be a pointer to an
existing Adjustment, or NULL, in which case one will be created for
you. Specifying NULL might actually be useful in this case, if you
wish to pass the newly-created adjustment to the constructor function
of some other widget which will configure it for you, such as a text
widget.
<H2><A NAME="ss8.2">8.2 Scale Widgets</A>
</H2>
<P>Scale widgets are used to allow the user to visually select and
manipulate a value within a specific range. You might want to use a
scale widget, for example, to adjust the magnification level on a
zoomed preview of a picture, or to control the brightness of a color,
or to specify the number of minutes of inactivity before a screensaver
takes over the screen.
<P>
<H3>Creating a Scale Widget</H3>
<P>As with scrollbars, there are separate widget types for horizontal and
vertical scale widgets. (Most programmers seem to favour horizontal
scale widgets.) Since they work essentially the same way, there's no
need to treat them separately here. The following functions, defined
in <CODE><gtk/gtkvscale.h></CODE> and
<CODE><gtk/gtkhscale.h></CODE>, create vertical and horizontal scale
widgets, respectively:
<P>
<BLOCKQUOTE><CODE>
<PRE>
GtkWidget *gtk_vscale_new( GtkAdjustment *adjustment );
GtkWidget *gtk_hscale_new( GtkAdjustment *adjustment );
</PRE>
</CODE></BLOCKQUOTE>
<P>The <CODE>adjustment</CODE> argument can either be an adjustment which has
already been created with <CODE>gtk_adjustment_new()</CODE>, or <CODE>NULL</CODE>, in
which case, an anonymous Adjustment is created with all of its
values set to <CODE>0.0</CODE> (which isn't very useful in this case). In
order to avoid confusing yourself, you probably want to create your
adjustment with a <CODE>page_size</CODE> of <CODE>0.0</CODE> so that its <CODE>upper</CODE>
value actually corresponds to the highest value the user can select.
(If you're <EM>already</EM> thoroughly confused, read the section on
<A HREF="gtk_tut-7.html#sec_Adjustment">Adjustments</A> again for an explanation of
what exactly adjustments do and how to create and manipulate them.)
<P>
<H3>Functions and Signals (well, functions, at least)</H3>
<P>Scale widgets can display their current value as a number beside the
trough. The default behaviour is to show the value, but you can change
this with this function:
<P>
<BLOCKQUOTE><CODE>
<PRE>
void gtk_scale_set_draw_value( GtkScale *scale,
gint draw_value );
</PRE>
</CODE></BLOCKQUOTE>
<P>As you might have guessed, <CODE>draw_value</CODE> is either <CODE>TRUE</CODE> or
<CODE>FALSE</CODE>, with predictable consequences for either one.
<P>The value displayed by a scale widget is rounded to one decimal point
by default, as is the <CODE>value</CODE> field in its GtkAdjustment. You can
change this with:
<P>
<BLOCKQUOTE><CODE>
<PRE>
void gtk_scale_set_digits( GtkScale *scale,
gint digits );
</PRE>
</CODE></BLOCKQUOTE>
<P>where <CODE>digits</CODE> is the number of decimal places you want. You can
set <CODE>digits</CODE> to anything you like, but no more than 13 decimal
places will actually be drawn on screen.
<P>Finally, the value can be drawn in different positions
relative to the trough:
<P>
<BLOCKQUOTE><CODE>
<PRE>
void gtk_scale_set_value_pos( GtkScale *scale,
GtkPositionType pos );
</PRE>
</CODE></BLOCKQUOTE>
<P>The argument <CODE>pos</CODE> is of type <CODE>GtkPositionType</CODE>, which is
defined in <CODE><gtk/gtkenums.h></CODE>, and can take one of the
following values:
<P>
<BLOCKQUOTE><CODE>
<PRE>
GTK_POS_LEFT
GTK_POS_RIGHT
GTK_POS_TOP
GTK_POS_BOTTOM
</PRE>
</CODE></BLOCKQUOTE>
<P>If you position the value on the "side" of the trough (e.g., on the
top or bottom of a horizontal scale widget), then it will follow the
slider up and down the trough.
<P>All the preceding functions are defined in
<CODE><gtk/gtkscale.h></CODE>. The header files for all GTK widgets
are automatically included when you include
<CODE><gtk/gtk.h></CODE>. But you should look over the header files
of all widgets that interest you,
<P>
<H2><A NAME="sec_Range_Functions"></A> <A NAME="ss8.3">8.3 Common Range Functions </A>
</H2>
<P>The Range widget class is fairly complicated internally, but, like
all the "base class" widgets, most of its complexity is only
interesting if you want to hack on it. Also, almost all of the
functions and signals it defines are only really used in writing
derived widgets. There are, however, a few useful functions that are
defined in <CODE><gtk/gtkrange.h></CODE> and will work on all range
widgets.
<P>
<H3>Setting the Update Policy</H3>
<P>The "update policy" of a range widget defines at what points during
user interaction it will change the <CODE>value</CODE> field of its
Adjustment and emit the "value_changed" signal on this
Adjustment. The update policies, defined in
<CODE><gtk/gtkenums.h></CODE> as type <CODE>enum GtkUpdateType</CODE>,
are:
<P>
<UL>
<LI>GTK_UPDATE_POLICY_CONTINUOUS - This is the default. The
"value_changed" signal is emitted continuously, i.e., whenever the
slider is moved by even the tiniest amount.</LI>
<LI>GTK_UPDATE_POLICY_DISCONTINUOUS - The "value_changed" signal is
only emitted once the slider has stopped moving and the user has
released the mouse button.</LI>
<LI>GTK_UPDATE_POLICY_DELAYED - The "value_changed" signal is emitted
when the user releases the mouse button, or if the slider stops moving
for a short period of time.</LI>
</UL>
<P>The update policy of a range widget can be set by casting it using the
<CODE>GTK_RANGE (Widget)</CODE> macro and passing it to this function:
<P>
<BLOCKQUOTE><CODE>
<PRE>
void gtk_range_set_update_policy( GtkRange *range,
GtkUpdateType policy) ;
</PRE>
</CODE></BLOCKQUOTE>
<P>
<H3>Getting and Setting Adjustments</H3>
<P>Getting and setting the adjustment for a range widget "on the fly" is
done, predictably, with:
<P>
<BLOCKQUOTE><CODE>
<PRE>
GtkAdjustment* gtk_range_get_adjustment( GtkRange *range );
void gtk_range_set_adjustment( GtkRange *range,
GtkAdjustment *adjustment );
</PRE>
</CODE></BLOCKQUOTE>
<P><CODE>gtk_range_get_adjustment()</CODE> returns a pointer to the adjustment to
which <CODE>range</CODE> is connected.
<P><CODE>gtk_range_set_adjustment()</CODE> does absolutely nothing if you pass it
the adjustment that <CODE>range</CODE> is already using, regardless of whether
you changed any of its fields or not. If you pass it a new
Adjustment, it will unreference the old one if it exists (possibly
destroying it), connect the appropriate signals to the new one, and
call the private function <CODE>gtk_range_adjustment_changed()</CODE>, which
will (or at least, is supposed to...) recalculate the size and/or
position of the slider and redraw if necessary. As mentioned in the
section on adjustments, if you wish to reuse the same Adjustment,
when you modify its values directly, you should emit the "changed"
signal on it, like this:
<P>
<BLOCKQUOTE><CODE>
<PRE>
gtk_signal_emit_by_name (GTK_OBJECT (adjustment), "changed");
</PRE>
</CODE></BLOCKQUOTE>
<H2><A NAME="ss8.4">8.4 Key and Mouse bindings</A>
</H2>
<P>All of the GTK range widgets react to mouse clicks in more or less
the same way. Clicking button-1 in the trough will cause its
adjustment's <CODE>page_increment</CODE> to be added or subtracted from its
<CODE>value</CODE>, and the slider to be moved accordingly. Clicking mouse
button-2 in the trough will jump the slider to the point at which the
button was clicked. Clicking any button on a scrollbar's arrows will
cause its adjustment's value to change <CODE>step_increment</CODE> at a time.
<P>It may take a little while to get used to, but by default, scrollbars
as well as scale widgets can take the keyboard focus in GTK. If you
think your users will find this too confusing, you can always disable
this by unsetting the <CODE>GTK_CAN_FOCUS</CODE> flag on the scrollbar, like
this:
<P>
<BLOCKQUOTE><CODE>
<PRE>
GTK_WIDGET_UNSET_FLAGS (scrollbar, GTK_CAN_FOCUS);
</PRE>
</CODE></BLOCKQUOTE>
<P>The key bindings (which are, of course, only active when the widget
has focus) are slightly different between horizontal and vertical
range widgets, for obvious reasons. They are also not quite the same
for scale widgets as they are for scrollbars, for somewhat less
obvious reasons (possibly to avoid confusion between the keys for
horizontal and vertical scrollbars in scrolled windows, where both
operate on the same area).
<P>
<H3>Vertical Range Widgets</H3>
<P>All vertical range widgets can be operated with the up and down arrow
keys, as well as with the <CODE>Page Up</CODE> and <CODE>Page Down</CODE> keys. The
arrows move the slider up and down by <CODE>step_increment</CODE>, while
<CODE>Page Up</CODE> and <CODE>Page Down</CODE> move it by <CODE>page_increment</CODE>.
<P>The user can also move the slider all the way to one end or the other
of the trough using the keyboard. With the VScale widget, this is
done with the <CODE>Home</CODE> and <CODE>End</CODE> keys, whereas with the
VScrollbar widget, this is done by typing <CODE>Control-Page Up</CODE>
and <CODE>Control-Page Down</CODE>.
<P>
<H3>Horizontal Range Widgets</H3>
<P>The left and right arrow keys work as you might expect in these
widgets, moving the slider back and forth by <CODE>step_increment</CODE>. The
<CODE>Home</CODE> and <CODE>End</CODE> keys move the slider to the ends of the trough.
For the HScale widget, moving the slider by <CODE>page_increment</CODE> is
accomplished with <CODE>Control-Left</CODE> and <CODE>Control-Right</CODE>,
while for HScrollbar, it's done with <CODE>Control-Home</CODE> and
<CODE>Control-End</CODE>.
<H2><A NAME="sec_Range_Example"></A> <A NAME="ss8.5">8.5 Example</A>
</H2>
<P>This example is a somewhat modified version of the "range controls"
test from <CODE>testgtk.c</CODE>. It basically puts up a window with three
range widgets all connected to the same adjustment, and a couple of
controls for adjusting some of the parameters mentioned above and in
the section on adjustments, so you can see how they affect the way
these widgets work for the user.
<P>
<BLOCKQUOTE><CODE>
<PRE>
/* example-start rangewidgets rangewidgets.c */
#include <gtk/gtk.h>
GtkWidget *hscale, *vscale;
void cb_pos_menu_select( GtkWidget *item,
GtkPositionType pos )
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