Question

Toolkits usually provide only the most common widgets like buttons, text widgets, sliders etc. No toolkit can provide all possible widgets.

More specialised widget must be created by client programmers. They do it by using the drawing tools provided by the toolkit. There are two possibilities: a programmer can modify or enhance an existing widget, or he can create a custom widget from scratch.

Burning widget

This is an example of a widget that we create from scratch. This widget can be found in various media burning applications, like Nero Burning ROM.

burning.py

#!/usr/bin/python
# -*- coding: utf-8 -*-

# ZetCode PyGTK tutorial 
#
# This example creates a burning
# custom widget
#
# author: Jan Bodnar
# website: zetcode.com 
# last edited: April 2011


import gtk
import cairo

class Burning(gtk.DrawingArea):

  def __init__(self, parent):
    
    self.par = parent
    super(Burning, self).__init__()
 
    self.num = ( "75", "150", "225", "300", 
      "375", "450", "525", "600", "675" )
 
    self.set_size_request(-1, 30)
    self.connect("expose-event", self.expose)
  

  def expose(self, widget, event):
    
    cr = widget.window.cairo_create()
    cr.set_line_width(0.8)

    cr.select_font_face("Courier", 
      cairo.FONT_SLANT_NORMAL, cairo.FONT_WEIGHT_NORMAL)
    cr.set_font_size(11)

    width = self.allocation.width
   
    self.cur_width = self.par.get_cur_value()

    step = round(width / 10.0)

    till = (width / 750.0) * self.cur_width
    full = (width / 750.0) * 700

    if (self.cur_width >= 700):
      
      cr.set_source_rgb(1.0, 1.0, 0.72)
      cr.rectangle(0, 0, full, 30)
      cr.save()
      cr.clip()
      cr.paint()
      cr.restore()
      
      cr.set_source_rgb(1.0, 0.68, 0.68)
      cr.rectangle(full, 0, till-full, 30)
      cr.save()
      cr.clip()
      cr.paint()
      cr.restore()

    else:   
      cr.set_source_rgb(1.0, 1.0, 0.72)
      cr.rectangle(0, 0, till, 30)
      cr.save()
      cr.clip()
      cr.paint()
      cr.restore()
     

    cr.set_source_rgb(0.35, 0.31, 0.24)
    
    for i in range(1, len(self.num) + 1):
      cr.move_to(i*step, 0)
      cr.line_to(i*step, 5)
      cr.stroke()
      
      (x, y, width, height, dx, dy) = cr.text_extents(self.num[i-1])
      cr.move_to(i*step-width/2, 15)
      cr.text_path(self.num[i-1])
      cr.stroke()
     
    
 
class PyApp(gtk.Window): 

  def __init__(self):
    super(PyApp, self).__init__()
    
    self.set_title("Burning")
    self.set_size_request(350, 200)    
    self.set_position(gtk.WIN_POS_CENTER)
    self.connect("destroy", gtk.main_quit)

    self.cur_value = 0
     
    vbox = gtk.VBox(False, 2)
    
    scale = gtk.HScale()
    scale.set_range(0, 750)
    scale.set_digits(0)
    scale.set_size_request(160, 40)
    scale.set_value(self.cur_value)
    scale.connect("value-changed", self.on_changed)
        
    fix = gtk.Fixed()
    fix.put(scale, 50, 50)
    
    vbox.pack_start(fix)
    
    self.burning = Burning(self)
    vbox.pack_start(self.burning, False, False, 0)

    self.add(vbox)
    self.show_all()
    
    
  def on_changed(self, widget):
    self.cur_value = widget.get_value()
    self.burning.queue_draw()
  
  
  def get_cur_value(self):
    return self.cur_value
  

PyApp()
gtk.main()

We put a DrawingArea on the bottom of the window and draw the entire widget manually. All the important code resides in the expose() method of the Burning class. This widget shows graphically the total capacity of a medium and the free space available to us. The widget is controlled by a scale widget. The minimum value of our custom widget is 0, the maximum is 750. If we reach value 700, we began drawing in red colour. This normally indicates overburning.

self.num = ( "75", "150", "225", "300", 
  "375", "450", "525", "600", "675" )

These numbers are shown on the burning widget. They show the capacity of the medium.

self.cur_width = self.par.get_cur_value()

These two lines get the current number from the scale widget. We get the parent widget and from the parent widget, we get the current value.

till = (width / 750.0) * self.cur_width
full = (width / 750.0) * 700

The till parameter determines the total size to be drawn. This value comes from the slider widget. It is a proportion of the whole area. The full parameter determines the point, where we begin to draw in red colour.

cr.set_source_rgb(1.0, 1.0, 0.72)
cr.rectangle(0, 0, till, 30)
cr.save()
cr.clip()
cr.paint()
cr.restore()

This code here, draws a yellow rectangle up to point, where the medium is full.

(x, y, width, height, dx, dy) = cr.text_extents(self.num[i-1])
cr.move_to(i*step-width/2, 15)
cr.text_path(self.num[i-1])
cr.stroke()

This code here draws the numbers on the burning widget. We calculate the TextExtents to position the text correctly.

def on_changed(self, widget):
  self.cur_value = widget.get_value()
  self.burning.queue_draw()

We get the value from the scale widget, store it in the cur_value variable for later use. We redraw the burning widget.

Figure: Burning widget

In this chapter, we created a custom widget in PyGTK.