GTK has various widgets that can be visually adjusted by the user using the mouse or the keyboard, such as the range widgets, described in the Range Widgets section. There are also a few widgets that display some adjustable portion of a larger area of data, such as the text widget and the viewport widget.
Obviously, an application needs to be able to react to changes the user makes in range widgets. One way to do this would be to have each widget emit its own type of signal when its adjustment changes, and either pass the new value to the signal handler, or require it to look inside the widget's data structure in order to ascertain the value. But you may also want to connect the adjustments of several widgets together, so that adjusting one adjusts the others. The most obvious example of this is connecting a scrollbar to a panning viewport or a scrolling text area. If each widget has its own way of setting or getting the adjustment value, then the programmer may have to write their own signal handlers to translate between the output of one widget's signal and the "input" of another's adjustment setting function.
GTK solves this problem using the Adjustment object, which is not a widget but a way for widgets to store and pass adjustment information in an abstract and flexible form. The most obvious use of Adjustment is to store the configuration parameters and values of range widgets, such as scrollbars and scale controls. However, since Adjustments are derived from Object, they have some special powers beyond those of normal data structures. Most importantly, they can emit signals, just like widgets, and these signals can be used not only to allow your program to react to user input on adjustable widgets, but also to propagate adjustment values transparently between adjustable widgets.
You will see how adjustments fit in when you see the other widgets that incorporate them: Progress Bars, Viewports, Scrolled Windows, and others.
Many of the widgets which use adjustment objects do so automatically, but some cases will be shown in later examples where you may need to create one yourself. You create an adjustment using:
GtkObject *gtk_adjustment_new( gfloat value,
gfloat lower,
gfloat upper,
gfloat step_increment,
gfloat page_increment,
gfloat page_size );
The value argument is the initial value you want to give to the
adjustment, usually corresponding to the topmost or leftmost position
of an adjustable widget. The lower argument specifies the lowest
value which the adjustment can hold. The step_increment argument
specifies the "smaller" of the two increments by which the user can
change the value, while the page_increment is the "larger" one.
The page_size argument usually corresponds somehow to the visible
area of a panning widget. The upper argument is used to represent
the bottom most or right most coordinate in a panning widget's
child. Therefore it is not always the largest number that
value can take, since the page_size of such widgets is
usually non-zero.
The adjustable widgets can be roughly divided into those which use and
require specific units for these values and those which treat them as
arbitrary numbers. The group which treats the values as arbitrary
numbers includes the range widgets (scrollbars and scales, the
progress bar widget, and the spin button widget). These widgets are
all the widgets which are typically "adjusted" directly by the user
with the mouse or keyboard. They will treat the lower and
upper values of an adjustment as a range within which the user
can manipulate the adjustment's value. By default, they will only
modify the value of an adjustment.
The other group includes the text widget, the viewport widget, the
compound list widget, and the scrolled window widget. All of these
widgets use pixel values for their adjustments. These are also all
widgets which are typically "adjusted" indirectly using scrollbars.
While all widgets which use adjustments can either create their own
adjustments or use ones you supply, you'll generally want to let this
particular category of widgets create its own adjustments. Usually,
they will eventually override all the values except the value
itself in whatever adjustments you give them, but the results are, in
general, undefined (meaning, you'll have to read the source code to
find out, and it may be different from widget to widget).
Now, you're probably thinking, since text widgets and viewports insist
on setting everything except the value of their adjustments,
while scrollbars will only touch the adjustment's value, if
you share an adjustment object between a scrollbar and a text
widget, manipulating the scrollbar will automagically adjust the text
widget? Of course it will! Just like this:
/* creates its own adjustments */
text = gtk_text_new (NULL, NULL);
/* uses the newly-created adjustment for the scrollbar as well */
vscrollbar = gtk_vscrollbar_new (GTK_TEXT(text)->vadj);
Ok, you say, that's nice, but what if I want to create my own handlers
to respond when the user adjusts a range widget or a spin button, and
how do I get at the value of the adjustment in these handlers? To
answer these questions and more, let's start by taking a look at
struct _GtkAdjustment itself:
struct _GtkAdjustment
{
GtkData data;
gfloat lower;
gfloat upper;
gfloat value;
gfloat step_increment;
gfloat page_increment;
gfloat page_size;
};
The first thing you should know is that there aren't any handy-dandy
macros or accessor functions for getting the value out of an
Adjustment, so you'll have to (horror of horrors) do it like a
real C programmer. Don't worry - the GTK_ADJUSTMENT
(Object) macro does run-time type checking (as do all the GTK
type-casting macros, actually).
Since, when you set the value of an adjustment, you generally
want the change to be reflected by every widget that uses this
adjustment, GTK provides this convenience function to do this:
void gtk_adjustment_set_value( GtkAdjustment *adjustment,
gfloat value );
As mentioned earlier, Adjustment is a subclass of Object just
like all the various widgets, and thus it is able to emit signals.
This is, of course, why updates happen automagically when you share an
adjustment object between a scrollbar and another adjustable widget;
all adjustable widgets connect signal handlers to their adjustment's
value_changed signal, as can your program. Here's the definition
of this signal in struct _GtkAdjustmentClass:
void (* value_changed) (GtkAdjustment *adjustment);
The various widgets that use the Adjustment object will emit this
signal on an adjustment whenever they change its value. This happens
both when user input causes the slider to move on a range widget, as
well as when the program explicitly changes the value with
gtk_adjustment_set_value(). So, for example, if you have a scale
widget, and you want to change the rotation of a picture whenever its
value changes, you would create a callback like this:
void cb_rotate_picture (GtkAdjustment *adj, GtkWidget *picture)
{
set_picture_rotation (picture, adj->value);
...
and connect it to the scale widget's adjustment like this:
gtk_signal_connect (GTK_OBJECT (adj), "value_changed",
GTK_SIGNAL_FUNC (cb_rotate_picture), picture);
What about when a widget reconfigures the upper or lower
fields of its adjustment, such as when a user adds more text to a text
widget? In this case, it emits the changed signal, which looks
like this:
void (* changed) (GtkAdjustment *adjustment);
Range widgets typically connect a handler to this signal, which
changes their appearance to reflect the change - for example, the size
of the slider in a scrollbar will grow or shrink in inverse proportion
to the difference between the lower and upper values of its
adjustment.
You probably won't ever need to attach a handler to this signal, unless you're writing a new type of range widget. However, if you change any of the values in a Adjustment directly, you should emit this signal on it to reconfigure whatever widgets are using it, like this:
gtk_signal_emit_by_name (GTK_OBJECT (adjustment), "changed");
Now go forth and adjust!