Fix checking against frame_count in Screencopy.match

[hbatagelo]

self.frame_count accessed in match() could contain a frame number that is out of sync with the grabbed screenshot. This fix makes sure that the frame count checked in match() is the actual frame count when grab_screenshot() is called.

[RAOF]

Hm. I must be misunderstanding Python asynico, because I was under the impression that the async code is only run while in the await statement, so returning self.framecount from the async method should give you exactly the same result as when checking self.framecount immediately after the async method returns?

[AlanGriffiths]

I was under the impression that the async code is only run while in the await statement

My understanding is the same - in Python it is more like co-routines, than threading.

[hbatagelo]

I think this issue may be related to how the event loop schedules the callbacks. What I've observed is that ScreencopyTracker._frame_ready may be called just after grab_screenshot returns. If that happens, _frame_ready increases frame_count, but the returned image still refers to the previous frame.

When I log the values of frame_count and last_checked_frame_count in the while loop of Screencopy.match, sometimes I encounter the following scenario that motivates this PR:

# 1st iteration
screenshot = await asyncio.wait_for(self.grab_screenshot(), timeout) # In grab_screenshot, self.frame_count is some number, e.g., 10
# screenshot refers to the frame with self.frame_count==10
# But self_frame_count is now 11 because _frame_ready was scheduled to run after grab_screenshot was being waited for
if last_checked_frame_count != self.frame_count: # True because this is the first iteration and last_checked_frame_count==0
    last_checked_frame_count = self.frame_count # OK, last_checked_frame_count is now 11
    # find_template_in_image is called. Let's assume the template doesn't match with this screenshot
    
# 2nd iteration
screenshot = await asyncio.wait_for(self.grab_screenshot(), timeout) # In grab_screenshot, self.frame_count is 11
# screenshot refers to the frame with self.frame_count==11
# This time, self_frame_count is still 11 because there were no new frames
if last_checked_frame_count != self.frame_count: # False
    last_checked_frame_count = self.frame_count # Won't be reached, but screenshot may contain the matching template
    # ...
    
# If no additional frames are created, the loop will time out, even if screenshot contains the matching template

[RAOF]

Hm. I'm insufficiently pythonic to see precisely what's happening here, but returning (frame_count, screenshot) is definitely not incorrect, so let's do that.