How to automatize adding draw handler to space?

Question

I need to add a draw handler when starting blender. problem is that handler uses context data for drawing. because of this, an error occurs when adding handler in addon register() function:

I found out that such an error happens due to the use of context during the registration of the addon. I do not know any other automation methods other than addon registration, so I will be happy to listen to your suggestions

Answer 1

You can't add draw handler in register() function in your addon, because it runs too early.

Use bpy.app.handlers instead
https://docs.blender.org/api/current/bpy.app.handlers.html

bl_info = {
    "name" : "Add draw handler to Viewport when start",
    "author" : "me",
    "version" : (23, 8, 1),
    "blender" : (3, 4),
    "location" : "View3d > Tool",
    "warning" : "",
    "description" : "Test",
    "wiki_url" : "",
    "category" : "3D View",
}

#### copy from https://docs.blender.org/api/3.6/gpu.html
import bpy
import gpu
from gpu_extras.batch import batch_for_shader

vertices = (
    (100, 100), (300, 100),
    (100, 200), (300, 200))

indices = (
    (0, 1, 2), (2, 1, 3))

shader = gpu.shader.from_builtin('2D_UNIFORM_COLOR')
batch = batch_for_shader(shader, 'TRIS', {"pos": vertices}, indices=indices)


def draw():
    shader.uniform_float("color", (0, 0.5, 0.5, 1.0))
    batch.draw(shader)
####


from bpy.app.handlers import persistent

@persistent
def when_version_update_end(dummy):
    print("when_version_update_end")
    bpy.types.SpaceView3D.draw_handler_add(draw, (), 'WINDOW', 'POST_PIXEL')


def register():
    bpy.app.handlers.version_update.append(when_version_update_end)

def unregister():
    bpy.types.SpaceView3D.draw_handler_remove(draw, 'WINDOW')

if __name__ == "__main__":
    register()