I can't get a SpaceView3D draw callback to work

Question

I'm trying to add a callback which runs at every SpaceView3D draw. As suggested from Change Current Frame acc. to Active Layer.

class DrawClass:
    def __init__(self, context):
       self.handle = bpy.types.SpaceView3D.draw_handler_add(self.draw_callback, (context,), 'WINDOW', 'POST_PIXEL')

    def draw_callback(self):
        print("Testing...")

if __name__ == "__main__":
    DrawClass(bpy.context)

This is a script I wrote to test if my callback function gets added to the draw handler. I would expect an output from print function on the console I started Blender from. But I get nothing. Am I doing something wrong here?

Answer 1

All possible callbacks

Prints to console when callbacks fired

So much info in the python console, autocomplete on draw handler add. Will see that args is a tuple of arguments passed to the callback method. Error in question was passing 1 arg to a method with none. Will show up in system console as an error.

>>> bpy.types.SpaceView3D.draw_handler_add(
draw_handler_add(callback, args, region_type, draw_type)
.. method:: draw_handler_add(callback, args, region_type, draw_type)
Add a new draw handler to this space type.
It will be called every time the specified region in the space type will be drawn.
Note: All arguments are positional only for now.
:param callback:
   A function that will be called when the region is drawn.
   It gets the specified arguments as input.
:type callback: function
:param args: Arguments that will be passed to the callback.
:type args: tuple
:param region_type: The region type the callback draws in; usually ``WINDOW``. (:class:`bpy.types.Region.type`)
:type region_type: str
:param draw_type: Usually ``POST_PIXEL`` for 2D drawing and ``POST_VIEW`` for 3D drawing. In some cases ``PRE_VIEW`` can be used. ``BACKDROP`` can be used for backdrops in the node editor.
:type draw_type: str
:return: Handler that can be removed later on.
:rtype: object

Attempt to add every draw call back possible. Has to be a redraw of the type specified to fire the callback. I doesn't relentlessly fire.

import bpy
from bpy.types import SpaceView3D, Region
class DrawClass:
    def __init__(self, context):
        print("INIT")
        for rt in  Region.bl_rna.properties['type'].enum_items.keys():
            for d in ('POST_PIXEL', 'POST_VIEW', 'PRE_VIEW', 'BACKDROP'):

                try:
                    SpaceView3D.draw_handler_add(
                        self.dcb, (rt, d,), rt, d)
                except:
                    print("No dcb for", rt, d)


    def dcb(self, rt, d):
        print("Testing dcb:", rt, d)


if __name__ == "__main__":
    DrawClass(bpy.context)