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7. Adjustments

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.

7.1 Creating an Adjustment

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:

function gtk_adjustment_new (value : gdouble;
                             lower : gdouble;
                             upper : gdouble;
                             step_increment : gdouble;
                             page_increment : gdouble;
                             page_size : gdouble) : PGtkObject;

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.

7.2 Using Adjustments the Easy Way

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 }
   viewport := gtk_viewport_new(nil, nil);
   { Uses the newly-created adjustment for the scrollbar as well }
   vscrollbar := gtk_vscrollbar_new(gtk_viewport_get_vadjustment(viewport));

7.3 Adjustment Internals

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 {
   GtkObject parent_instance;

   gdouble lower;
   gdouble upper;
   gdouble value;
   gdouble step_increment;
   gdouble page_increment;
   gdouble page_size;

This translates to:

   TGtkAdjustment = record
                       parent_instance : GtkObject;
                       lower : gdouble;
                       upper : gdouble;
                       value : gdouble;
                       step_increment : gdouble;
                       page_increment : gdouble;
                       page_size : gdouble;

If you don't like to poke directly at struct internals like a real C programmer, you can use the following accessor to inspect the value of an adjustment:

function gtk_adjustment_get_value (adjustment : PGtkAdjustment) : gdouble;

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 procedure to do this:

procedure gtk_adjustment_set_value (adjustment : PGtkAdjustment;
                                    value : gdouble);

As mentioned earlier, Adjustment is a sub-class 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:

procedure cb_rotate_picture (adj : PGtkAdjustment;
                             picture : PGtkWidget); cdecl;
   set_picture_rotation(picture, gtk_adjustment_get_value(adj));

and connect it to the scale widget's adjustment like this:

   g_signal_connect(adj, 'value_changed',
                    G_CALLBACK(@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 an Adjustment directly, you should emit this signal on it to reconfigure whatever widgets are using it, like this:

   g_signal_emit_by_name(adjustment, 'changed');

Now go forth and adjust!

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