9.7. Rulers

Ruler widgets are used to indicate the location of the mouse pointer in a given window. A window can have a vertical ruler spanning across the width and a horizontal ruler spanning down the height. A small triangular indicator on the ruler shows the exact location of the pointer relative to the ruler.

A ruler must first be created. Horizontal and vertical rulers are created using
 
hruler = GtkHRuler()    # horizontal ruler

vruler = GtkVRuler()    # vertical ruler

Once a ruler is created, we can define the unit of measurement. Units of measure for rulers can be PIXELS, INCHES or CENTIMETERS. This is set using the method:
 
    ruler.set_metric(metric)

The default measure is PIXELS.
 
    ruler.set_metric(PIXELS)

Other important characteristics of a ruler are how to mark the units of scale and where the position indicator is initially placed. These are set for a ruler using the method:
 
    ruler.set_range(lower, upper, position, max_size)

The lower and upper arguments define the extent of the ruler, and max_size is the largest possible number that will be displayed. Position defines the initial position of the pointer indicator within the ruler.

A vertical ruler can span an 800 pixel wide window thus
 
    vruler.set_range(0, 800, 0, 800)

The markings displayed on the ruler will be from 0 to 800, with a number for every 100 pixels. If instead we wanted the ruler to range from 7 to 16, we would code
 
    vruler.set_range(7, 16, 0, 20)

The indicator on the ruler is a small triangular mark that indicates the position of the pointer relative to the ruler. If the ruler is used to follow the mouse pointer, the motion_notify_event signal should be connected to the motion_notify_event method of the ruler. We need to setup a motion_notify_event callback for the area and use connect_object() to get the ruler to emit a motion_notify_signal:
 
    def motion_notify(ruler, event):
        return ruler.emit("motion_notify_event", event)

    area.connect_object("motion_notify_event", motion_notify, ruler)

The rulers.py example program creates a drawing area with a horizontal ruler above it and a vertical ruler to the left of it. The size of the drawing area is 600 pixels wide by 400 pixels high. The horizontal ruler spans from 7 to 13 with a mark every 100 pixels, while the vertical ruler spans from 0 to 400 with a mark every 100 pixels. Placement of the drawing area and the rulers is done using a table. Figure 9.7 illustrates the result:

Figure 9.7 Rulers Example

The rulers.py source code is:
 
    1   #!/usr/bin/env python
    2   
    3   # example rulers.py
    4   
    5   import gtk
    6   import GDK
    7   
    8   class RulersExample:
    9       XSIZE = 600
   10       YSIZE = 400
   11   
   12       # This routine gets control when the close button is clicked
   13       def close_application(self, widget, event, data=None):
   14           gtk.mainquit()
   15           return gtk.FALSE
   16   
   17       def __init__(self):
   18           window = gtk.GtkWindow(gtk.WINDOW_TOPLEVEL)
   19           window.connect("delete_event", self.close_application)
   20           window.set_border_width(10)
   21   
   22           # Create a table for placing the ruler and the drawing area
   23           table = gtk.GtkTable(3, 2, gtk.FALSE)
   24           window.add(table)
   25   
   26           area = gtk.GtkDrawingArea()
   27           area.size(self.XSIZE, self.YSIZE)
   28           table.attach(area, 1, 2, 1, 2,
   29                        gtk.EXPAND|gtk.FILL, gtk.FILL, 0, 0 )
   30           area.set_events(GDK.POINTER_MOTION_MASK |
   31                                    GDK.POINTER_MOTION_HINT_MASK )
   32   
   33           # The horizontal ruler goes on top. As the mouse moves across the
   34           # drawing area, a motion_notify_event is passed to the
   35           # appropriate event handler for the ruler.
   36           hrule = gtk.GtkHRuler()
   37           hrule.set_metric(gtk.PIXELS)
   38           hrule.set_range(7, 13, 0, 20)
   39           def motion_notify(ruler, event):
   40               return ruler.emit("motion_notify_event", event)
   41           area.connect_object("motion_notify_event", motion_notify, hrule)
   42           table.attach(hrule, 1, 2, 0, 1,
   43                        gtk.EXPAND|gtk.SHRINK|gtk.FILL, gtk.FILL, 0, 0 )
   44       
   45           # The vertical ruler goes on the left. As the mouse moves across
   46           # the drawing area, a motion_notify_event is passed to the
   47           # appropriate event handler for the ruler.
   48           vrule = gtk.GtkVRuler()
   49           vrule.set_metric(gtk.PIXELS)
   50           vrule.set_range(0, self.YSIZE, 10, self.YSIZE)
   51           area.connect_object("motion_notify_event", motion_notify, vrule)
   52           table.attach(vrule, 0, 1, 1, 2,
   53                        gtk.FILL, gtk.EXPAND|gtk.SHRINK|gtk.FILL, 0, 0 )
   54   
   55           # Now show everything
   56           area.show()
   57           hrule.show()
   58           vrule.show()
   59           table.show()
   60           window.show()
   61   
   62   def main():
   63       gtk.mainloop()
   64       return 0
   65   
   66   if __name__ == "__main__":
   67       RulersExample()
   68       main()

Lines 41 and 51 connect the motion_notify() callback to the area but passing hrule in line 41 and vrule in line 55 as user data. The motion_notify() callback will be called twice each time the mouse moves - once with hrule and once with vrule.