gtk_tut-22.html

GTK development guide

                     yc - s*dial->radius);
    }

  /* Draw pointer */

  s = sin(dial->angle);
  c = cos(dial->angle);


  points[0].x = xc + s*dial->pointer_width/2;
  points[0].y = yc + c*dial->pointer_width/2;
  points[1].x = xc + c*dial->radius;
  points[1].y = yc - s*dial->radius;
  points[2].x = xc - s*dial->pointer_width/2;
  points[2].y = yc - c*dial->pointer_width/2;

  gtk_draw_polygon (widget->style,
                    widget->window,
                    GTK_STATE_NORMAL,
                    GTK_SHADOW_OUT,
                    points, 3,
                    TRUE);
  
  return FALSE;
}
</PRE>
</CODE></BLOCKQUOTE>
<P>
<H3>Event handling</H3>

<P>The rest of the widget's code handles various types of events, and
isn't too different from what would be found in many GTK
applications. Two types of events can occur - either the user can
click on the widget with the mouse and drag to move the pointer, or
the value of the Adjustment object can change due to some external
circumstance. 
<P>When the user clicks on the widget, we check to see if the click was
appropriately near the pointer, and if so, store the button that the
user clicked with in the <CODE>button</CODE> field of the widget
structure, and grab all mouse events with a call to
<CODE>gtk_grab_add()</CODE>. Subsequent motion of the mouse causes the
value of the control to be recomputed (by the function
<CODE>gtk_dial_update_mouse</CODE>). Depending on the policy that has been
set, "value_changed" events are either generated instantly
(<CODE>GTK_UPDATE_CONTINUOUS</CODE>), after a delay in a timer added with
<CODE>gtk_timeout_add()</CODE> (<CODE>GTK_UPDATE_DELAYED</CODE>), or only when the
button is released (<CODE>GTK_UPDATE_DISCONTINUOUS</CODE>).
<P>
<BLOCKQUOTE><CODE>
<PRE>
static gint
gtk_dial_button_press (GtkWidget      *widget,
                       GdkEventButton *event)
{
  GtkDial *dial;
  gint dx, dy;
  double s, c;
  double d_parallel;
  double d_perpendicular;

  g_return_val_if_fail (widget != NULL, FALSE);
  g_return_val_if_fail (GTK_IS_DIAL (widget), FALSE);
  g_return_val_if_fail (event != NULL, FALSE);

  dial = GTK_DIAL (widget);

  /* Determine if button press was within pointer region - we 
     do this by computing the parallel and perpendicular distance of
     the point where the mouse was pressed from the line passing through
     the pointer */
  
  dx = event->x - widget->allocation.width / 2;
  dy = widget->allocation.height / 2 - event->y;
  
  s = sin(dial->angle);
  c = cos(dial->angle);
  
  d_parallel = s*dy + c*dx;
  d_perpendicular = fabs(s*dx - c*dy);
  
  if (!dial->button &amp;&amp;
      (d_perpendicular &lt; dial->pointer_width/2) &amp;&amp;
      (d_parallel > - dial->pointer_width))
    {
      gtk_grab_add (widget);

      dial->button = event->button;

      gtk_dial_update_mouse (dial, event->x, event->y);
    }

  return FALSE;
}

static gint
gtk_dial_button_release (GtkWidget      *widget,
                          GdkEventButton *event)
{
  GtkDial *dial;

  g_return_val_if_fail (widget != NULL, FALSE);
  g_return_val_if_fail (GTK_IS_DIAL (widget), FALSE);
  g_return_val_if_fail (event != NULL, FALSE);

  dial = GTK_DIAL (widget);

  if (dial->button == event->button)
    {
      gtk_grab_remove (widget);

      dial->button = 0;

      if (dial->policy == GTK_UPDATE_DELAYED)
        gtk_timeout_remove (dial->timer);
      
      if ((dial->policy != GTK_UPDATE_CONTINUOUS) &amp;&amp;
          (dial->old_value != dial->adjustment->value))
        gtk_signal_emit_by_name (GTK_OBJECT (dial->adjustment), "value_changed");
    }

  return FALSE;
}

static gint
gtk_dial_motion_notify (GtkWidget      *widget,
                         GdkEventMotion *event)
{
  GtkDial *dial;
  GdkModifierType mods;
  gint x, y, mask;

  g_return_val_if_fail (widget != NULL, FALSE);
  g_return_val_if_fail (GTK_IS_DIAL (widget), FALSE);
  g_return_val_if_fail (event != NULL, FALSE);

  dial = GTK_DIAL (widget);

  if (dial->button != 0)
    {
      x = event->x;
      y = event->y;

      if (event->is_hint || (event->window != widget->window))
        gdk_window_get_pointer (widget->window, &amp;x, &amp;y, &amp;mods);

      switch (dial->button)
        {
        case 1:
          mask = GDK_BUTTON1_MASK;
          break;
        case 2:
          mask = GDK_BUTTON2_MASK;
          break;
        case 3:
          mask = GDK_BUTTON3_MASK;
          break;
        default:
          mask = 0;
          break;
        }

      if (mods &amp; mask)
        gtk_dial_update_mouse (dial, x,y);
    }

  return FALSE;
}

static gint
gtk_dial_timer (GtkDial *dial)
{
  g_return_val_if_fail (dial != NULL, FALSE);
  g_return_val_if_fail (GTK_IS_DIAL (dial), FALSE);

  if (dial->policy == GTK_UPDATE_DELAYED)
    gtk_signal_emit_by_name (GTK_OBJECT (dial->adjustment), "value_changed");

  return FALSE;
}

static void
gtk_dial_update_mouse (GtkDial *dial, gint x, gint y)
{
  gint xc, yc;
  gfloat old_value;

  g_return_if_fail (dial != NULL);
  g_return_if_fail (GTK_IS_DIAL (dial));

  xc = GTK_WIDGET(dial)->allocation.width / 2;
  yc = GTK_WIDGET(dial)->allocation.height / 2;

  old_value = dial->adjustment->value;
  dial->angle = atan2(yc-y, x-xc);

  if (dial->angle &lt; -M_PI/2.)
    dial->angle += 2*M_PI;

  if (dial->angle &lt; -M_PI/6)
    dial->angle = -M_PI/6;

  if (dial->angle > 7.*M_PI/6.)
    dial->angle = 7.*M_PI/6.;

  dial->adjustment->value = dial->adjustment->lower + (7.*M_PI/6 - dial->angle) *
    (dial->adjustment->upper - dial->adjustment->lower) / (4.*M_PI/3.);

  if (dial->adjustment->value != old_value)
    {
      if (dial->policy == GTK_UPDATE_CONTINUOUS)
        {
          gtk_signal_emit_by_name (GTK_OBJECT (dial->adjustment), "value_changed");
        }
      else
        {
          gtk_widget_draw (GTK_WIDGET(dial), NULL);

          if (dial->policy == GTK_UPDATE_DELAYED)
            {
              if (dial->timer)
                gtk_timeout_remove (dial->timer);

              dial->timer = gtk_timeout_add (SCROLL_DELAY_LENGTH,
                                             (GtkFunction) gtk_dial_timer,
                                             (gpointer) dial);
            }
        }
    }
}
</PRE>
</CODE></BLOCKQUOTE>
<P>Changes to the Adjustment by external means are communicated to our
widget by the "changed" and "value_changed" signals. The handlers
for these functions call <CODE>gtk_dial_update()</CODE> to validate the
arguments, compute the new pointer angle, and redraw the widget (by
calling <CODE>gtk_widget_draw()</CODE>).
<P>
<BLOCKQUOTE><CODE>
<PRE>
static void
gtk_dial_update (GtkDial *dial)
{
  gfloat new_value;
  
  g_return_if_fail (dial != NULL);
  g_return_if_fail (GTK_IS_DIAL (dial));

  new_value = dial->adjustment->value;
  
  if (new_value &lt; dial->adjustment->lower)
    new_value = dial->adjustment->lower;

  if (new_value > dial->adjustment->upper)
    new_value = dial->adjustment->upper;

  if (new_value != dial->adjustment->value)
    {
      dial->adjustment->value = new_value;
      gtk_signal_emit_by_name (GTK_OBJECT (dial->adjustment), "value_changed");
    }

  dial->angle = 7.*M_PI/6. - (new_value - dial->adjustment->lower) * 4.*M_PI/3. /
    (dial->adjustment->upper - dial->adjustment->lower);

  gtk_widget_draw (GTK_WIDGET(dial), NULL);
}

static void
gtk_dial_adjustment_changed (GtkAdjustment *adjustment,
                              gpointer       data)
{
  GtkDial *dial;

  g_return_if_fail (adjustment != NULL);
  g_return_if_fail (data != NULL);

  dial = GTK_DIAL (data);

  if ((dial->old_value != adjustment->value) ||
      (dial->old_lower != adjustment->lower) ||
      (dial->old_upper != adjustment->upper))
    {
      gtk_dial_update (dial);

      dial->old_value = adjustment->value;
      dial->old_lower = adjustment->lower;
      dial->old_upper = adjustment->upper;
    }
}

static void
gtk_dial_adjustment_value_changed (GtkAdjustment *adjustment,
                                    gpointer       data)
{
  GtkDial *dial;

  g_return_if_fail (adjustment != NULL);
  g_return_if_fail (data != NULL);

  dial = GTK_DIAL (data);

  if (dial->old_value != adjustment->value)
    {
      gtk_dial_update (dial);

      dial->old_value = adjustment->value;
    }
}
</PRE>
</CODE></BLOCKQUOTE>
<P>
<H3>Possible Enhancements</H3>

<P>The Dial widget as we've described it so far runs about 670 lines of
code. Although that might sound like a fair bit, we've really
accomplished quite a bit with that much code, especially since much of
that length is headers and boilerplate. However, there are quite a few
more enhancements that could be made to this widget:
<P>
<UL>
<LI> If you try this widget out, you'll find that there is some
flashing as the pointer is dragged around. This is because the entire
widget is erased every time the pointer is moved before being
redrawn. Often, the best way to handle this problem is to draw to an
offscreen pixmap, then copy the final results onto the screen in one
step. (The ProgressBar widget draws itself in this fashion.)
</LI>
<LI> The user should be able to use the up and down arrow keys to
increase and decrease the value.
</LI>
<LI> It would be nice if the widget had buttons to increase and
decrease the value in small or large steps. Although it would be
possible to use embedded Button widgets for this, we would also like
the buttons to auto-repeat when held down, as the arrows on a
scrollbar do. Most of the code to implement this type of behavior can
be found in the Range widget.
</LI>
<LI> The Dial widget could be made into a container widget with a
single child widget positioned at the bottom between the buttons
mentioned above. The user could then add their choice of a label or
entry widget to display the current value of the dial.
</LI>
</UL>
<P>
<H2><A NAME="ss22.5">22.5 Learning More</A>
</H2>

<P>Only a small part of the many details involved in creating widgets
could be described above. If you want to write your own widgets, the
best source of examples is the GTK source itself. Ask yourself some
questions about the widget you want to write: IS it a Container
widget? Does it have its own window? Is it a modification of an
existing widget? Then find a similar widget, and start making changes.
Good luck!
<P>
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