how to run bpy callback on workspace tools change - event-handling

Question

How to add a pre-draw hook to current context workspace.tools change?
I attempted to get there using bpy.types.SpaceView3D.draw_handler_add(...) which as it runs on every draw, checks if workspace.tools changed, and if it changed, run my callback, but my callback wants to add its own SpaceView3D.draw_handler_add and doing it this way adds it a frame-too-late, leaving the view port undrawn until a user event repaints the screen.

Answer 1

I found this post online
https://devtalk.blender.org/t/update-property-when-active-tool-changes/11467/12
summary: maybe there is a mainline callback new
https://developer.blender.org/D10635
AFWS Jan '20
#kaio
This seem like a better solution. It’s kind of a mystery code, cause I
couldn’t figure out where you got that code info ,but then started
looking at the space_toolsystem_common.py file. kaio AFWS Jan '20
Just realized there might be a cleaner way of getting callbacks for
active tools using the msgbus. Since workspace tools aren’t rna
properties themselves, figured it’s possible to monitor the
bpy_prop_collection which changes with the tool.
The handle is the workspace itself, so shouldn’t have to worry about
keeping a reference. The subscription lasts until a new file is
loaded, so add a load_post callback which reapplies it.
Note this doesn’t proactively subscribe to workspaces added
afterwards. Might need a separate callback for that :joy:
import bpy
def rna_callback(workspace):
idname = workspace.tools[-1].idname
print(idname)
def subscribe(workspace):
bpy.msgbus.subscribe_rna(
key=ws.path_resolve("tools", False),
owner=workspace,
args=(workspace,),
notify=rna_callback)
if __name__ == "__main__":
ws = bpy.context.workspace
subscribe(bpy.context.workspace)
# Subscribe to all workspaces: if 0:
for ws in bpy.data.workspaces:
subscribe(bpy.context.workspace)
# Clear all workspace subscriptions if 0:
for ws in bpy.data.workspaces:
bpy.msgbus.clear_by_owner(ws)