Size Negotiation

the section called Size Allocation in the chapter called GTK+ Basics describes the size negotiation process; be sure you're familiar with it before reading this section.

There's no obvious "right" size for GtkEv, so the size request method requests an arbitrary size that looks nice:

static void 
gtk_ev_size_request   (GtkWidget        *widget,
                       GtkRequisition   *requisition)
{
  g_return_if_fail(widget != NULL);
  g_return_if_fail(GTK_IS_EV(widget));

  /* 
   * GtkEv always wants to be the same fixed size.
   */
  
  requisition->width  = 450;
  requisition->height = 300;
}
      

GTK+ takes care of storing a widget's last size request in widget->requisition.

If GtkEv were a real-life widget rather than an illustrative example, it would be unnecessary to implement a size request method. The default GtkWidget method simply returns the current value of widget->requisition, so GtkEv could initialize widget->requisition in gtk_ev_init() and use the default method.

Alternatively, the size request method could be implemented more elaborately; GtkEv could attempt to predict the maximum width of the text to be displayed, for example.

Once a widget's parent container decides how much space is actually available, the widget receives a size allocation. The size allocation method should do the following:

• Assign the new allocation to widget->allocation; this does not happen automatically, as it does for widget->requisition.

• Divide the allocation among any child widgets.

• Resize any GdkWindows, if the widget is realized.

• Perform any widget-specific tasks; for example, GtkEv updates the two GdkRectangles representing its internal layout.

Here is the GtkEv size allocation method; it should be self-explanatory:

static void 
gtk_ev_size_allocate  (GtkWidget        *widget,
                       GtkAllocation    *allocation)
{
  static const gint spacing = 10; 
  GtkEv* ev;

  g_return_if_fail(widget != NULL);
  g_return_if_fail(GTK_IS_EV(widget));
  
  ev = GTK_EV(widget);

  widget->allocation = *allocation;

  ev->event_window_rect.width = 
    MAX(allocation->width - spacing*2, 0);
  ev->event_window_rect.height = 
    MAX(allocation->height / 5 - spacing / 2, 0);

  ev->event_window_rect.x = 
    (allocation->width - ev->event_window_rect.width)/2;
  ev->event_window_rect.y = 
    MIN(spacing, allocation->height);

  ev->description_rect.x = ev->event_window_rect.x;
  ev->description_rect.y = 
    ev->event_window_rect.y + ev->event_window_rect.height + spacing;
  ev->description_rect.width = 
    ev->event_window_rect.width;
  ev->description_rect.height = 
    MAX((allocation->height - ev->event_window_rect.height - spacing*3), 0);

  if (GTK_WIDGET_REALIZED (widget))
    {
      gdk_window_move_resize (widget->window,
                              allocation->x, 
                              allocation->y,
                              allocation->width, 
                              allocation->height);

      gdk_window_move_resize (ev->event_window,
                              ev->event_window_rect.x, 
                              ev->event_window_rect.y,
                              ev->event_window_rect.width,
                              ev->event_window_rect.height);      
    }
}