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8. Range Widgets

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.

As mentioned in Adjustments 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.

8.1 Scrollbar Widgets

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.

There are separate classes for horizontal and vertical scrollbars. There really isn't much to say about these. You create them with the following creation procedures, defined in GTK_HSCROLLBAR and GTK_VSCROLLBAR

make ( adjustment : GTK_ADJUSTMENT )

and that's about it (if you don't believe me, look in the source files!). The adjustment argument can either be a an existing GTK_ADJUSTMENT, or Void, in which case one will be created for you. Specifying Void might actually be useful in this case, if you wish to pass the newly-created adjustment to the creation procedure of some other widget which will configure it for you, such as a text widget.

8.2 Scale Widgets

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 colour, or to specify the number of minutes of inactivity before a screensaver takes over the screen.

Creating a Scale Widget

As with scrollbars, there are separate widget classes 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 creation procedures, are defined in classes GTK_VSCALE and GTK_HSCALE

make ( adjustment : GTK_ADJUSTMENT )

The adjustment argument can either be an adjustment which has already been created , or Void, in which case, an anonymous GTK_ADJUSTMENT is created with all of its values set to 0.0 (which isn't very useful in this case). In order to avoid confusing yourself, you probably want to create your adjustment with a page_size of 0.0 so that its upper value actually corresponds to the highest value the user can select. (If you're already thoroughly confused, read the section on Adjustments again for an explanation of what exactly adjustments do and how to create and manipulate them).

Functions and Signals (well, functions, at least)

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 procedure:

set_draw_value ( draw_value : BOOLEAN )

As you might have guessed, draw_value is either True or False, with predictable consequences for either one.

The value displayed by a scale widget is rounded to one decimal point by default, as is the value field in its GTK_ADJUSTMENT. You can change this with:

set_digits ( digits : INTEGER )

where digits is the number of decimal places you want. You can set digits to anything you like, but no more than 13 decimal places will actually be drawn on screen.

Finally, the value can be drawn in different positions relative to the trough:

set_value_pos ( pos : GTK_POSITION_TYPE ) is

The argument pos is of class GTK_POSITION_TYPE, which is defined in GTK_CONSTANTS, and can take one of the following values:

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.

8.3 Common Functions

The GTK_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 procedures that are defined in class GTK_RANGE and will work on all range widgets.

Setting the Update Policy

The "update policy" of a range widget defines at what points during user interaction it will change the value field of its GTK_ADJUSTMENT and emit the "value_changed" signal on this GTK_ADJUSTMENT. The update policies, defined in GTK_UPDATE_TYPE are:

The update policy of a range widget can be set by passing it to this procedure:

set_update_policy ( policy : GTK_UPDATE_TYPE )

Getting and Setting Adjustments

Getting and setting the adjustment for a range widget "on the fly" is done, predictably, with:

get_adjustment : GTK_ADJUSTMENT
set_adjustment ( adjustment : GTK_ADJUSTMENT )

get_adjustment() returns an adjustment to which range is connected.

set_adjustment() does absolutely nothing if you pass it the adjustment that range is already using, regardless of whether you changed any of its fields or not. If you pass it a new GTK_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 procedure adjustment_changed(), 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 GTK_ADJUSTMENT, when you modify its values directly, you should emit the "changed" signal on it, like this:

signal_emit_by_name (adjustment, "changed");

8.4 Key and Mouse bindings

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 page_increment to be added or subtracted from its value, 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 step_increment at a time.

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 GTK_CAN_FOCUS flag on the scrollbar, like this:

scrollbar.unset_flags (GTK_CAN_FOCUS);

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).

Vertical Range Widgets

All vertical range widgets can be operated with the up and down arrow keys, as well as with the Page Up and Page Down keys. The arrows move the slider up and down by step_increment, while Page Up and Page Down move it by page_increment.

The user can also move the slider all the way to one end or the other of the trough using the keyboard. With the GtkVScale widget, this is done with the Home and End keys, whereas with the GtkVScrollbar widget, this is done by typing Control-Page Up and Control-Page Down.

Horizontal Range Widgets

The left and right arrow keys work as you might expect in these widgets, moving the slider back and forth by step_increment. The Home and End keys move the slider to the ends of the trough. For the GtkHScale widget, moving the slider by page_increment is accomplished with Control-Left and Control-Right, while for GtkHScrollbar, it's done with Control-Home and Control-End.

8.5 Example

This example is a somewhat modified version of the "range controls" test from testgtk. 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 seciton on adjustments, so you can see how they affect the way these widgets work for the user.

-- example-start rangewidgets rangewidgets.e
class RANGEWIDGETS
inherit
   -- All constants we need are defined in the class below.
   -- eg gtk_window_toplevel etc.
   GTK_CONSTANTS
   -- This class contains common features like vegtk_init and so
   VEGTK_MAIN
   -- If we need to handle callbacks we must inherit from the below
   -- class
   VEGTK_CALLBACK_HANDLER
creation
   make
feature -- Callback handlers
   close_application is
   -- This callback quits the program
   do
       gtk_main_quit;
   end
   cb_pos_menu_select (data : ANY; cb_data : VEGTK_CALLBACK_DATA) is
   local
      pos : GTK_POSITION_TYPE;
   do
       pos ?= data;
       -- Set the value position on both scale widgets
       hscale.set_value_pos (pos);
       vscale.set_value_pos (pos);
   end

   cb_update_menu_select (data : ANY) is
   local
      policy : GTK_UPDATE_TYPE;
   do
       policy ?= data;
       -- Set the update policy for both scale widgets
       hscale.set_update_policy (policy);
       vscale.set_update_policy (policy);
   end

   cb_digits_scale (data : ANY; cb_data : VEGTK_CALLBACK_DATA) is
   local
      adj : GTK_ADJUSTMENT;
   do
       adj ?= cb_data.get_widget;
       -- Set the number of decimal places to which adj.value is rounded
       hscale.set_digits (adj.value.rounded);
       vscale.set_digits (adj.value.rounded);
   end

   cb_page_size (data : ANY; cb_data : VEGTK_CALLBACK_DATA) is
   local
      get,set : GTK_ADJUSTMENT;
--      signals : expanded GTK_SIGNAL_EXTERNALS;
   do
       get ?= cb_data.get_widget;
       set ?= data;
       -- Set the page size and page increment size of the sample
       -- adjustment to the value specified by the "Page Size" scale

       set.set_page_size (get.value);
       set.set_page_increment (get.value);
       -- Now emit the "changed" signal to reconfigure all the widgets that
       -- are attached to this adjustment
       set.changed;
--       signals.gtk_signal_emit_by_name(set.to_external,("changed").to_external);
   end

   cb_draw_value (data : ANY; cb_data : VEGTK_CALLBACK_DATA) is
   local
      button : GTK_CHECK_BUTTON;
   do
       button ?= cb_data.get_widget;
       -- Turn the value display on the scale widgets off or on depending
       --  on the state of the checkbutton
       hscale.set_draw_value (button.get_active);
       vscale.set_draw_value (button.get_active);
   end
feature -- Convenience procedures

   make_menu_item(name : STRING; callback : POINTER ;
                  data : ANY) : GTK_MENU_ITEM is
   do
       !!Result.make_with_label(name);
       signal_connect_with_data (Result, "activate",    callback, data);
       Result.show;
   end

   scale_set_default_values (scale : GTK_SCALE) is
   do
       scale.set_update_policy (Gtk_update_continuous);
       scale.set_digits (1);
       scale.set_value_pos (Gtk_pos_top);
       scale.set_draw_value (True);
   end

   create_range_controls is
      -- makes the sample window
   local
       window : GTK_WINDOW;
       box1, box2, box3 : GTK_BOX;
       button : GTK_BUTTON;
       check_button : GTK_CHECK_BUTTON;
       scrollbar : GTK_HSCROLLBAR;
       separator : GTK_HSEPARATOR;
       opt : GTK_OPTION_MENU
       menu : GTK_MENU;
       item : GTK_MENU_ITEM;
       label : GTK_LABEL;
       scale : GTK_HSCALE;
       adj1, adj2 : GTK_ADJUSTMENT;
   do
       -- Standard window-creating stuff
       !!window.make(Gtk_window_toplevel);
       signal_connect (window, "destroy",$close_application);
       window.set_title ("range controls");

       !GTK_VBOX!box1.make (False, 0);
       window.add (box1);
       box1.show;

       !GTK_HBOX!box2.make (False, 10);
       box2.set_border_width (10);
       box1.pack_start (box2, True, True, 0);
       box2.show;

       -- calue, lower, upper, step_increment, page_increment, page_size
       -- Note that the page_size value only makes a difference for
       -- scrollbar widgets, and the highest value you'll get is actually
       -- (upper - page_size).
       !!adj1.make(0.0, 0.0, 101.0, 0.1, 1.0, 1.0);

       !!vscale.make (adj1);
       scale_set_default_values (vscale);
       box2.pack_start (vscale, True, True, 0);
       vscale.show;

       !GTK_VBOX!box3.make (False, 10);
       box2.pack_start (box3, True, True, 0);
       box3.show;

       -- Reuse the same adjustment
       !!hscale.make(adj1);
       hscale.set_usize (200, 30);
       scale_set_default_values (hscale);
       box3.pack_start (hscale, True, True, 0);
       hscale.show;

       -- Reuse the same adjustment again
       !!scrollbar.make (adj1);
       -- Notice how this causes the scales to always be updated
       -- continuously when the scrollbar is moved
       scrollbar.set_update_policy (Gtk_update_continuous);
       box3.pack_start (scrollbar, True, True, 0);
       scrollbar.show;

       !GTK_HBOX!box2.make (False, 10);
       box2.set_border_width (10);
       box1.pack_start (box2, True, True, 0);
       box2.show;

       -- A checkbutton to control whether the value is displayed or not
       !!check_button.make_with_label("Display value on scale widgets");
       check_button.set_active (True);
       signal_connect (check_button, "toggled",$cb_draw_value);
       box2.pack_start ( check_button, True, True, 0);
       check_button.show;

       !GTK_HBOX!box2.make (False, 10);
       box2.set_border_width (10);

       -- An option menu to change the position of the value
       !!label.make ("Scale Value Position:");
       box2.pack_start (label, False, False, 0);
       label.show;

       !!opt.make;
       !!menu.make;

       item := make_menu_item ("Top",$cb_pos_menu_select,Gtk_pos_top);
       menu.append (item);

       item := make_menu_item ("Bottom",$cb_pos_menu_select,Gtk_pos_bottom);
       menu.append (item);

       item := make_menu_item ("Left", $cb_pos_menu_select,Gtk_pos_left);
       menu.append (item);

       item := make_menu_item ("Right", $cb_pos_menu_select,Gtk_pos_right);
       menu.append (item);

       opt.set_menu (menu);
       box2.pack_start (opt, True, True, 0);
       opt.show;

       box1.pack_start (box2, True, True, 0);
       box2.show;

       !GTK_HBOX!box2.make (False, 10);
       box2.set_border_width (10);

       -- Yet another option menu, this time for the update policy of the
       -- scale widgets
       !!label.make ("Scale Update Policy:");
       box2.pack_start (label, False, False, 0);
       label.show;

       !!opt.make;
       !!menu.make;

       item := make_menu_item ("Continuous",$cb_update_menu_select,
                               Gtk_update_continuous);
       menu.append (item);

       item := make_menu_item ("Discontinuous",$cb_update_menu_select,
                               Gtk_update_discontinuous);
       menu.append (item);

       item := make_menu_item ("Delayed",$cb_update_menu_select,
                              Gtk_update_delayed);
       menu.append (item);

       opt.set_menu (menu);
       box2.pack_start (opt, True, True, 0);
       opt.show;

       box1.pack_start (box2, True, True, 0);
       box2.show;

       !GTK_HBOX!box2.make (False, 10);
       box2.set_border_width (10);

       -- A GTK_HSCALE widget for adjusting the number of digits on the
       -- sample scales.
       !!label.make ("Scale Digits:");
       box2.pack_start (label, False, False, 0);
       label.show;

       !!adj2.make (1.0, 0.0, 5.0, 1.0, 1.0, 0.0);
       signal_connect (adj2, "value_changed",$cb_digits_scale);
       !!scale.make (adj2);
       scale.set_digits (0);
       box2.pack_start (scale, True, True, 0);
       scale.show;

       box1.pack_start (box2, True, True, 0);
       box2.show;

       !GTK_HBOX!box2.make (False, 10);
       box2.set_border_width (10);

       -- And, one last GTK_HSCALE widget for adjusting the page size of the
       -- scrollbar.
       !!label.make ("Scrollbar Page Size:");
       box2.pack_start (label, False, False, 0);
       label.show;

       !!adj2.make (1.0, 1.0, 101.0, 1.0, 1.0, 0.0);
       signal_connect_with_data (adj2, "value_changed",$cb_page_size, adj1);
       !!scale.make(adj2);
       scale.set_digits (0);
       box2.pack_start (scale, True, True, 0);
       scale.show;

       box1.pack_start (box2, True, True, 0);
       box2.show;

       !!separator.make;
       box1.pack_start (separator, False, True, 0);
       separator.show;

       !GTK_VBOX!box2.make (False, 10);
       box2.set_border_width (10);
       box1.pack_start (box2, False, True, 0);
       box2.show;

       !!button.make_with_label ("Quit");
       signal_connect (button, "clicked",$close_application)

       box2.pack_start (button, True, True, 0);
       button.set_flags (Gtk_can_default);
       button.grab_default;
       button.show;

       window.show;
   end -- create_range_controls

feature -- Creation
   make is
   local
      window : GTK_WINDOW;
      rbutton : GTK_RADIO_BUTTON;
      button : GTK_BUTTON;
      box1,box2   : GTK_VBOX;
      separator : GTK_HSEPARATOR;
   do
      -- This is called in all GTK applications. Arguments are parsed
      -- from the command line and are returned to the application.
      vegtk_init;

      create_range_controls;

      gtk_main;
   end -- make
feature {NONE}
   hscale : GTK_HSCALE;
   vscale : GTK_HSCALE;
end -- class RANGEWIDGETS
-- example-end

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