Simplify GridHelper invalidation.

GridHelperBase.invalidate/valid and Axes.invalidate_grid_helper are
never used externally and appear to be internal.  But invalidation
doesn't need to be stored in a separate flag (`_force_update`), one can
simply reset `_old_limits` (which only serves this purpose) and unify
that with `_old_values` (which does the same).

The only `_update` method that was previously called to regenerate
invalidated grid info was `GridHelperCurveLinear._update`, which, other
than the redundant check of staleness (already done in `_update_lim`),
just forwarded to `_update_grid`, so get rid of `_update` and just
stick to `_update_grid`.

Author: anntzer

doc/api/next_api_changes/deprecations/19042-AL.rst

@@ -0,0 +1,5 @@
+``GridHelperBase`` invalidation
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+The ``GridHelperBase.invalidate``, ``GridHelperBase.valid``, and
+``axislines.Axes.invalidate_grid_helper`` methods are considered internal
+and deprecated.

lib/mpl_toolkits/axisartist/axislines.py

@@ -315,25 +315,26 @@ def _f(locs, labels):
 class GridHelperBase:
 
     def __init__(self):
-        self._force_update = True
+        self._force_update = True  # Remove together with invalidate()/valid().
         self._old_limits = None
         super().__init__()
 
     def update_lim(self, axes):
         x1, x2 = axes.get_xlim()
         y1, y2 = axes.get_ylim()
-
         if self._force_update or self._old_limits != (x1, x2, y1, y2):
-            self._update(x1, x2, y1, y2)
+            self._update_grid(x1, y1, x2, y2)
             self._force_update = False
             self._old_limits = (x1, x2, y1, y2)
 
-    def _update(self, x1, x2, y1, y2):
-        pass
+    def _update_grid(self, x1, y1, x2, y2):
+        """Cache relevant computations when the axes limits have changed."""
 
+    @_api.deprecated("3.4")
     def invalidate(self):
         self._force_update = True
 
+    @_api.deprecated("3.4")
     def valid(self):
         return not self._force_update
 
@@ -555,6 +556,7 @@ def get_children(self):
         children.extend(super().get_children())
         return children
 
+    @_api.deprecated("3.4")
     def invalidate_grid_helper(self):
         self._grid_helper.invalidate()
 

lib/mpl_toolkits/axisartist/grid_helper_curvelinear.py

@@ -289,7 +289,6 @@ def __init__(self, aux_trans,
         """
         super().__init__()
         self.grid_info = None
-        self._old_values = None
         self._aux_trans = aux_trans
         self.grid_finder = GridFinder(aux_trans,
                                       extreme_finder,
@@ -302,16 +301,7 @@ def update_grid_finder(self, aux_trans=None, **kw):
         if aux_trans is not None:
             self.grid_finder.update_transform(aux_trans)
         self.grid_finder.update(**kw)
-        self.invalidate()
-
-    def _update(self, x1, x2, y1, y2):
-        """bbox in 0-based image coordinates"""
-        # update wcsgrid
-        if self.valid() and self._old_values == (x1, x2, y1, y2):
-            return
-        self._update_grid(x1, y1, x2, y2)
-        self._old_values = (x1, x2, y1, y2)
-        self._force_update = False
+        self._old_limits = None  # Force revalidation.
 
     def new_fixed_axis(self, loc,
                        nth_coord=None,