Constantly running modal operator

Question

TL;DR: Skip middle paragraph.

I'm trying to make an addon that draws certain info in the view, and have it do that basically all the time that the addon is on.

The way I've been told bgl drawing works is that you put the drawing code in a modal operator. However, all examples I've seen involve starting it from the invoke() method, which is run when the user executes the operator - not what I want. I've also tried putting the bpy.context.window_manager.modal_handler_add(self) code in the register() class method, but because that method does not have access to self, I only got errors with the various arguments I tried (which were: none, the class (its general name), the cls argument that register(cls) gets, and some others. If needed I'll expand on this).

How do I make the operator run "on startup", or how else can I achieve my goal?

Answer 1

I am afraid this is a limitation and you cannot interactively draw with bgl outside of modal operator. There is no way to initialize the drawing handler because it needs the modal operator for access to events and the modal loop where tag redraw would happen.

Tag redraw could be done but the events are the bummer. Linking similar: Get events without running modal

From blender api user events outside of modal can't be caught. There is probably a possibility to catch them with python in monitor space and from blender areas and window positions get them local (like mouse coords).

From user perspective how blender is made it seems the philosophy is that user should have control over modal operators and start/end them manually ideally with button in ui. So something doesn't run in the background without your knowledge. There is somewhere an answer from Brecht on something similar but couln't find it.

There is possibility to call an operator from handlers, but mostly you will get another limitation:

 "RuntimeError: maximum recursion depth exceeded"

Calling the operator from load_pre handler should be fine though. See similar: My custom timer is cancelled when loading a *.blend file

Answer 2

It seems impossible to make a modal operator run "on startup", without the user starting it. However, it also seems that the following code adds a drawing function on startup (when the addon is registered) and that that function gets executed indefinitely:

def draw_callback_px():
    #draw whatever you want here, just thought I'd add something

    bgl.glEnable(bgl.GL_BLEND)
    bgl.glColor4f(0.0, 0.0, 0.0, 0.5)
    bgl.glLineWidth(2)

    bgl.glBegin(bgl.GL_LINE_STRIP)

    bgl.glVertex3f(0, 0, 0)
    bgl.glVertex3f(1, 0, 0)
    bgl.glVertex3f(1, 1, 0)
    bgl.glVertex3f(1, 1, 1)

    bgl.glEnd()

    # restore opengl defaults
    bgl.glLineWidth(1)
    bgl.glDisable(bgl.GL_BLEND)
    bgl.glColor4f(0.0, 0.0, 0.0, 1.0)

def register():
    bpy.types.SpaceView3D.draw_handler_add(draw_callback_px, (), 'WINDOW', 'POST_VIEW')

If I now install this addon, the drawing function is appended there both on startup, when installing the addon; even if we're recovering the previous session. At least according to my testing. If someone could test this out to confirm, please do.

...one tiny caveat tho. This does not ever tag the drawing area for redrawing, so to see the drawings, one sometimes has to interact with the 3Dview are at least once after installing the addon.