Changed that CanvasGroup's loop can be set to None, instead of settin…

…g _rc_canvas_group to None

docs/backendapi.rst

@@ -11,22 +11,22 @@ Also see https://github.com/pygfx/rendercanvas/blob/main/rendercanvas/stub.py.
     with each version without warning.
 
 
-.. autoclass:: rendercanvas.stub.StubCanvasGroup
+.. autoclass:: rendercanvas.stub.StubLoop
     :members:
     :private-members:
     :member-order: bysource
 
 
-.. autoclass:: rendercanvas.stub.StubRenderCanvas
+.. autoclass:: rendercanvas.stub.StubCanvasGroup
     :members:
     :private-members:
     :member-order: bysource
 
 
-.. autoclass:: rendercanvas.base.WrapperRenderCanvas
-
-
-.. autoclass:: rendercanvas.stub.StubLoop
+.. autoclass:: rendercanvas.stub.StubRenderCanvas
     :members:
     :private-members:
     :member-order: bysource
+
+
+.. autoclass:: rendercanvas.base.WrapperRenderCanvas

rendercanvas/base.py

@@ -30,30 +30,32 @@ class BaseCanvasGroup:
 
     def __init__(self, default_loop):
         self._canvases = weakref.WeakSet()
-        self._loop = default_loop
+        self._loop = None
         self.select_loop(default_loop)
 
     def _register_canvas(self, canvas, task):
         """Used by the canvas to register itself."""
         self._canvases.add(canvas)
         loop = self.get_loop()
-        loop._register_canvas_group(self)
-        loop.add_task(task, name="scheduler-task")
+        if loop is not None:
+            loop._register_canvas_group(self)
+            loop.add_task(task, name="scheduler-task")
 
     def select_loop(self, loop):
         """Select the loop to use for this group of canvases."""
-        if not isinstance(loop, BaseLoop):
-            raise TypeError("select_loop() requires a loop instance.")
+        if not (loop is None or isinstance(loop, BaseLoop)):
+            raise TypeError("select_loop() requires a loop instance or None.")
         elif len(self._canvases):
             raise RuntimeError("Cannot select_loop() when live canvases exist.")
         elif loop is self._loop:
             pass
         else:
-            self._loop._unregister_canvas_group(self)
+            if self._loop is not None:
+                self._loop._unregister_canvas_group(self)
             self._loop = loop
 
     def get_loop(self):
-        """Get the currently associated loop."""
+        """Get the currently associated loop (can be None for canvases that don't run a scheduler)."""
         return self._loop
 
     def get_canvases(self):
@@ -88,7 +90,6 @@ class BaseRenderCanvas:
     _rc_canvas_group = None
     """Class attribute that refers to the ``CanvasGroup`` instance to use for canvases of this class.
     It specifies what loop is used, and enables users to changing the used loop.
-    Set to None to not use a loop.
     """
 
     @classmethod
@@ -132,14 +133,19 @@ def __init__(
             "fps": "?",
             "backend": self.__class__.__name__,
             "loop": self._rc_canvas_group.get_loop().__class__.__name__
-            if self._rc_canvas_group
+            if (self._rc_canvas_group and self._rc_canvas_group.get_loop())
             else "no-loop",
         }
 
         # Events and scheduler
         self._events = EventEmitter()
         self.__scheduler = None
-        if self._rc_canvas_group is not None:
+        if self._rc_canvas_group is None:
+            pass  # No scheduling, not even grouping
+        elif self._rc_canvas_group.get_loop() is None:
+            # Group, but no loop: no scheduling
+            self._rc_canvas_group._register_canvas(self, None)
+        else:
             self.__scheduler = Scheduler(
                 self,
                 self._events,
@@ -583,6 +589,8 @@ class WrapperRenderCanvas(BaseRenderCanvas):
     wrapped canvas and set it as ``_subwidget``.
     """
 
+    _rc_canvas_group = None  # No grouping for these wrappers
+
     @classmethod
     def select_loop(cls, loop):
         m = sys.modules[cls.__module__]

rendercanvas/offscreen.py

@@ -6,7 +6,11 @@
 
 import time
 
-from .base import BaseRenderCanvas, BaseLoop
+from .base import BaseCanvasGroup, BaseRenderCanvas, BaseLoop
+
+
+class OffscreenCanvasGroup(BaseCanvasGroup):
+    pass
 
 
 class ManualOffscreenRenderCanvas(BaseRenderCanvas):
@@ -15,7 +19,7 @@ class ManualOffscreenRenderCanvas(BaseRenderCanvas):
     Call the ``.draw()`` method to perform a draw and get the result.
     """
 
-    _rc_canvas_group = None  # unmanaged, no loop
+    _rc_canvas_group = OffscreenCanvasGroup(None)  # no loop, no scheduling
 
     def __init__(self, *args, pixel_ratio=1.0, **kwargs):
         super().__init__(*args, **kwargs)

rendercanvas/stub.py

@@ -37,6 +37,9 @@ def _rc_call_later(self, delay, callback):
         raise NotImplementedError()
 
 
+loop = StubLoop()
+
+
 class StubCanvasGroup(BaseCanvasGroup):
     """
     The ``CanvasGroup`` representss a group of canvas objects from the same class, that share a loop.
@@ -75,7 +78,7 @@ def _draw_frame_and_present(self):
 
     # Must be implemented by subclasses.
 
-    _rc_canvas_group = None
+    _rc_canvas_group = StubCanvasGroup(loop)
 
     def _rc_gui_poll(self):
         raise NotImplementedError()