How not to get a gtkdrawingarea cleared in the draw event? - python-3.x

Question

My drawingarea is cleared everytime the draw event is called.
How to avoid a drawingarea to be cleared ?
Thanks
#!/usr/bin/env python3
import gi
gi.require_version('Gtk','3.0')
from gi.repository import Gtk, Gdk
import cairo
import math
class MouseButtons:
LEFT_BUTTON = 1
RIGHT_BUTTON = 3
class Example(Gtk.Window):
def __init__(self):
super(Example, self).__init__()
self.init_ui()
def init_ui(self):
self.darea = Gtk.DrawingArea()
self.darea.connect("draw", self.on_draw)
self.darea.set_events(Gdk.EventMask.BUTTON_PRESS_MASK)
self.add(self.darea)
self.set_title("Fill & stroke")
self.resize(230, 150)
self.set_position(Gtk.WindowPosition.CENTER)
self.connect("delete-event", Gtk.main_quit)
self.darea.connect("button-press-event", self.on_button_press)
self.coords = []
self.show_all()
def on_draw(self, wid, cr):
cr.set_source_rgb(0.6, 0.6, 0.6)
cr.arc(self.coords[0], self.coords[1], 40, 0, 2*math.pi)
cr.fill()
def on_button_press(self, w, e):
if e.type == Gdk.EventType.BUTTON_PRESS \
and e.button == MouseButtons.LEFT_BUTTON:
self.coords = [e.x, e.y]
self.darea.queue_draw()
def main():
app = Example()
Gtk.main()
if __name__ == "__main__":
main()
In this example, each time I click on the drawingarea, a circle is drawn. I would like to draw the new circle but without to redrawing the previous one.
Is it possible ?

Answer 1

I would recommend adjusting your mental model of the drawing area; don't think of it as being "cleared" every time the draw handler is called. Rather, think of it like this: the draw handler is called every time the drawing area needs to be redrawn from scratch (among other reasons: because some other window moved in front of it, or because your program asked for a draw update). The drawing area's contents, once drawn, are not persisted anywhere.
If you need persistent window contents, then you should use a backing store and draw that onto the screen in the draw handler, or you could use a canvas library if you want to treat existing drawn objects as independently existing.

Answer 2

I found he answer :
#!/usr/bin/env python3
import gi
gi.require_version('Gtk','3.0')
from gi.repository import Gtk, Gdk
import cairo
import math
class MouseButtons:
LEFT_BUTTON = 1
RIGHT_BUTTON = 3
class Example(Gtk.Window):
def __init__(self):
super(Example, self).__init__()
self.init_ui()
def init_ui(self):
self.darea = Gtk.DrawingArea()
self.darea.connect("draw", self.on_draw)
self.darea.set_events(Gdk.EventMask.BUTTON_PRESS_MASK)
self.add(self.darea)
self.set_title("Fill & stroke")
self.resize(230, 150)
self.set_position(Gtk.WindowPosition.CENTER)
self.connect("delete-event", Gtk.main_quit)
self.darea.connect("button-press-event", self.on_button_press)
self.show_all()
a = self.darea.get_allocation()
print (a.x, a.y, a.width, a.height)
self.img = cairo.ImageSurface(cairo.Format.RGB24, a.width, a.height)
def on_draw(self, wid, cr):
cr.set_source_surface(self.img, 0, 0)
cr.paint()
def on_button_press(self, w, e):
if e.type == Gdk.EventType.BUTTON_PRESS \
and e.button == MouseButtons.LEFT_BUTTON:
cr = cairo.Context(self.img)
cr.set_source_rgb(0.6, 0.6, 0.6)
cr.arc(e.x, e.y, 40, 0, 2*math.pi)
cr.fill()
self.darea.queue_draw()
def main():
app = Example()
Gtk.main()
if __name__ == "__main__":
main()