9.7. Rulers
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## 9.7. Rulers

`Ruler` widgets are used to indicate the location of the mouse pointer in a given window. A window can have a horizontal ruler spanning across the width and a vertical 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 the functions:

      hruler = gtk.HRuler()    # horizontal ruler

      vruler = gtk.VRuler()    # 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(gtk.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 9.8, “Rulers Example” illustrates the result:

Figure 9.8. Rulers Example

The rulers.py source code is:

        1	#!/usr/bin/env python
        2
        3	# example rulers.py
        4
        5	import pygtk
        6	pygtk.require('2.0')
        7	import gtk
        8
        9	class RulersExample:
       10	    XSIZE = 400
       11	    YSIZE = 400
       12
       13	    # This routine gets control when the close button is clicked
       14	    def close_application(self, widget, event, data=None):
       15	        gtk.main_quit()
       16	        return False
       17
       18	    def __init__(self):
       19	        window = gtk.Window(gtk.WINDOW_TOPLEVEL)
       20	        window.connect("delete_event", self.close_application)
       21	        window.set_border_width(10)
       22
       23	        # Create a table for placing the ruler and the drawing area
       24	        table = gtk.Table(3, 2, False)
       25	        window.add(table)
       26
       27	        area = gtk.DrawingArea()
       28	        area.set_size_request(self.XSIZE, self.YSIZE)
       29	        table.attach(area, 1, 2, 1, 2,
       30	                     gtk.EXPAND|gtk.FILL, gtk.FILL, 0, 0 )
       31	        area.set_events(gtk.gdk.POINTER_MOTION_MASK |
       32	                        gtk.gdk.POINTER_MOTION_HINT_MASK )
       33
       34	        # The horizontal ruler goes on top. As the mouse moves across the
       35	        # drawing area, a motion_notify_event is passed to the
       36	        # appropriate event handler for the ruler.
       37	        hrule = gtk.HRuler()
       38	        hrule.set_metric(gtk.PIXELS)
       39	        hrule.set_range(7, 13, 0, 20)
       40	        def motion_notify(ruler, event):
       41	            return ruler.emit("motion_notify_event", event)
       42	        area.connect_object("motion_notify_event", motion_notify, hrule)
       43	        table.attach(hrule, 1, 2, 0, 1,
       44	                     gtk.EXPAND|gtk.SHRINK|gtk.FILL, gtk.FILL, 0, 0 )
       45
       46	        # The vertical ruler goes on the left. As the mouse moves across
       47	        # the drawing area, a motion_notify_event is passed to the
       48	        # appropriate event handler for the ruler.
       49	        vrule = gtk.VRuler()
       50	        vrule.set_metric(gtk.PIXELS)
       51	        vrule.set_range(0, self.YSIZE, 10, self.YSIZE)
       52	        area.connect_object("motion_notify_event", motion_notify, vrule)
       53	        table.attach(vrule, 0, 1, 1, 2,
       54	                     gtk.FILL, gtk.EXPAND|gtk.SHRINK|gtk.FILL, 0, 0 )
       55
       56	        # Now show everything
       57	        area.show()
       58	        hrule.show()
       59	        vrule.show()
       60	        table.show()
       61	        window.show()
       62
       63	def main():
       64	    gtk.main()
       65	    return 0
       66
       67	if __name__ == "__main__":
       68	    RulersExample()
       69	    main()

Lines 42 and 52 connect the `motion_notify`() callback to the area but passing `hrule` in line 42 and `vrule` in line 52 as user data. The `motion_notify`() callback will be called twice each time the mouse moves - once with `hrule` and once with `vrule`.

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