Unregistered "static draw" handler for sequencer

Question

I'm trying to write a sequencer add-on. I need it to add a bgl UI on top of the sequencer timeline, without a real interactive part, so I'm trying to make it a kind of "static handler" (I'm no coder, I don't know the specific term for it) instead of a modal operator with timer.
So far, I have this working, but I can't find a way to unregister it correctly when deactivating the add-on...
Here's example code with "hello world":

bl_info = {  
 "name": "modaltest",  
 "author": "tonton",  
 "version": (0, 1),  
 "blender": (2, 7, 9),  
 "location": "Sequencer",  
 "description": "",  
  "wiki_url": "",  
 "tracker_url": "",  
 "category": "Sequencer"}

import bpy
import bgl
import blf


def draw_callback_px():

    font_id = 0  # XXX, need to find out how best to get this.

    # draw some text
    blf.position(font_id, 15, 30, 0)
    blf.size(font_id, 20, 72)
    blf.draw(font_id, "Hello Word ")


def register():
    bpy.types.SpaceSequenceEditor.draw_handler_add(draw_callback_px, (), 'WINDOW', 'POST_PIXEL')

def unregister():
    bpy.types.SpaceSequenceEditor.draw_handler_remove(draw_callback_px)

I tried a bunch of ways to unregister the handler, nothing so far has worked. Do you know what I am doing wrong? Thank you very much in advance! :D

Answer 1

Define a "drawing class".

Similarly to method used in this answer When registered the widgets are drawn on all VSE spaces. When unregistered the drawing handles are removed. Simple message and location passed to class. In script below two "widgets" are created drawing "Bob" and "Jack" at different locations on VSE.

bl_info = {  
 "name": "VSE blf test",  
 "author": "",  
 "version": (0, 1),  
 "blender": (2, 7, 9),  
 "location": "Sequencer",  
 "description": "",  
  "wiki_url": "",  
 "tracker_url": "",  
 "category": "Sequencer"}

import bpy
import blf

class DrawingClass:
    def __init__(self, msg, loc):
        self.msg = msg
        self.loc = loc
        self.handle = bpy.types.SpaceSequenceEditor.draw_handler_add(
                   self.draw_text_callback,(loc,),
                   'WINDOW', 'POST_PIXEL')

    def draw_text_callback(self, loc):
        font_id = 0  # XXX, need to find out how best to get this.

        # draw some text
        x, y = self.loc
        blf.position(font_id, x, y, 0)
        blf.size(font_id, 66, 72)
        blf.draw(font_id, "%s" % (self.msg))

    def remove_handle(self):
         bpy.types.SpaceSequenceEditor.draw_handler_remove(self.handle, 'WINDOW')

widgets = {}
def register():
    widgets["Bob"] = DrawingClass("Bob", (0, 0))
    widgets["Jack"] = DrawingClass("Jack", (140, 140))    

def unregister():
    for key, dc in widgets.items():
        dc.remove_handle()

if __name__ == "__main__":
    register()

Might find this handy for 2D coordinates in VSE.

PS. Initially forgot to change SpaceView3D type in handle_remove, and found another sure fire way to crash blender. Suggest actually passing the space type to class as well as demonstrated in BGLWidget class