ENH: make tkagg blocking work

The issue is that we need a tk window to grab the mainloop.  In the upstream
tkagg backend we get a manager from Gcf, but in mpl-gui we are not populating
Gcf so we need to patch the tkagg backend.

To get the run-time patching to work as we also needed to add a path to
directly use the Backend classes in the modules rather than re-constituting
another class!

Author: tacaswell

mpl_gui/__init__.py

@@ -17,6 +17,7 @@
 from itertools import count
 
 from matplotlib.backend_bases import FigureCanvasBase as _FigureCanvasBase
+from matplotlib.cbook import _setattr_cm
 
 from ._figure import Figure  # noqa: F401
 
@@ -78,7 +79,8 @@ def show(figs, *, block=None, timeout=0):
 
     if block and len(managers):
         if timeout == 0:
-            backend.Show().mainloop()
+            with _setattr_cm(backend, get_active_managers=lambda: managers):
+                backend.mainloop()
         elif len(managers):
             manager, *_ = managers
             manager.canvas.start_event_loop(timeout=timeout)

mpl_gui/_manage_backend.py

@@ -89,10 +89,17 @@ def select_gui_toolkit(newbackend=None):
         # creating a "class" that inherits from backend_bases._Backend and whose
         # body is filled with the module's globals.
 
-        backend_name = cbook._backend_module_name(newbackend)
+        if newbackend.lower() == 'tkagg':
+            backend_name = f'mpl_gui._patched_backends.{newbackend.lower()}'
+        else:
+            backend_name = cbook._backend_module_name(newbackend)
 
-        class backend_mod(matplotlib.backend_bases._Backend):
-            locals().update(vars(importlib.import_module(backend_name)))
+        mod = importlib.import_module(backend_name)
+        if hasattr(mod, 'Backend'):
+            backend_mod = mod.Backend
+        else:
+            class backend_mod(matplotlib.backend_bases._Backend):
+                locals().update(vars())
 
         rc_params_string = newbackend
 

mpl_gui/_patched_backends/__init__.py

@@ -0,0 +1,25 @@
+from matplotlib.backends.backend_tkagg import _BackendTkAgg
+
+
+@_BackendTkAgg.export
+class _PatchedBackendTkAgg(_BackendTkAgg):
+    @staticmethod
+    def get_active_managers():
+        raise RuntimeError("This method should never actually be called")
+
+    @classmethod
+    def mainloop(cls):
+        managers = cls.get_active_managers()
+        if managers:
+            first_manager = managers[0]
+            manager_class = type(first_manager)
+            if manager_class._owns_mainloop:
+                return
+            manager_class._owns_mainloop = True
+            try:
+                first_manager.window.mainloop()
+            finally:
+                manager_class._owns_mainloop = False
+
+
+Backend = _PatchedBackendTkAgg