Webagg backend: get rid of tornado

This allows using the webagg backend in JupyterLite, by replacing
tornado with asyncio

Author: martinRenou

lib/matplotlib/backends/backend_webagg_core.py

@@ -8,9 +8,8 @@
 #   way over a web socket.
 #
 # - `backend_webagg.py` contains a concrete implementation of a basic
-#   application, implemented with tornado.
+#   application, implemented with asyncio.
 
-import datetime
 from io import BytesIO, StringIO
 import json
 import logging
@@ -19,7 +18,7 @@
 
 import numpy as np
 from PIL import Image
-import tornado
+import asyncio
 
 from matplotlib import _api, backend_bases, backend_tools
 from matplotlib.backends import backend_agg
@@ -79,43 +78,43 @@ def _handle_key(key):
     return key
 
 
-class TimerTornado(backend_bases.TimerBase):
+class TimerAsyncio(backend_bases.TimerBase):
     def __init__(self, *args, **kwargs):
-        self._timer = None
+        self._task = None
         super().__init__(*args, **kwargs)
 
+    async def _timer_task(self, interval):
+        while True:
+            try:
+                await asyncio.sleep(interval)
+                self._on_timer()
+
+                if self._single:
+                    break
+            except asyncio.CancelledError:
+                break
+
     def _timer_start(self):
         self._timer_stop()
-        if self._single:
-            ioloop = tornado.ioloop.IOLoop.instance()
-            self._timer = ioloop.add_timeout(
-                datetime.timedelta(milliseconds=self.interval),
-                self._on_timer)
-        else:
-            self._timer = tornado.ioloop.PeriodicCallback(
-                self._on_timer,
-                max(self.interval, 1e-6))
-            self._timer.start()
+
+        self._task = asyncio.ensure_future(
+            self._timer_task(max(self.interval / 1_000., 1e-6))
+        )
 
     def _timer_stop(self):
-        if self._timer is None:
-            return
-        elif self._single:
-            ioloop = tornado.ioloop.IOLoop.instance()
-            ioloop.remove_timeout(self._timer)
-        else:
-            self._timer.stop()
-        self._timer = None
+        if self._task is not None:
+            self._task.cancel()
+        self._task = None
 
     def _timer_set_interval(self):
         # Only stop and restart it if the timer has already been started
-        if self._timer is not None:
+        if self._task is not None:
             self._timer_stop()
             self._timer_start()
 
 
 class FigureCanvasWebAggCore(backend_agg.FigureCanvasAgg):
-    _timer_cls = TimerTornado
+    _timer_cls = TimerAsyncio
     # Webagg and friends having the right methods, but still
     # having bugs in practice.  Do not advertise that it works until
     # we can debug this.