Simplifications to ToolManager.{add,remove}_tool.

On the addition side, inline ToolManager._tool_added_event.  It's used
at a single place, and other events emitted by ToolManager are not split
out into their own helper methods.

On the removal side, deprecate the Tool.destroy handle: it's unused, and
if a tool really wants to know when it's getting removed, it can just
connect to its own tool_removed_event, which can also be used by other
callers.

Author: anntzer

doc/api/next_api_changes/deprecations/22509-AL.rst

@@ -0,0 +1,4 @@
+``ToolBase.destroy``
+~~~~~~~~~~~~~~~~~~~~
+... is deprecated.  To run code upon tool removal, connect to the
+``tool_removed_event`` event.

lib/matplotlib/backend_managers.py

@@ -206,17 +206,18 @@ def remove_tool(self, name):
         """
 
         tool = self.get_tool(name)
-        tool.destroy()
+        _api.deprecate_method_override(
+            backend_tools.ToolBase.destroy, tool, since="3.6",
+            alternative="tool_removed_event")()
 
         # If it's a toggle tool and toggled, untoggle
         if getattr(tool, 'toggled', False):
             self.trigger_tool(tool, 'toolmanager')
 
         self._remove_keys(name)
 
-        s = 'tool_removed_event'
-        event = ToolEvent(s, self, tool)
-        self._callbacks.process(s, event)
+        event = ToolEvent('tool_removed_event', self, tool)
+        self._callbacks.process(event.name, event)
 
         del self._tools[name]
 
@@ -284,13 +285,10 @@ def add_tool(self, name, tool, *args, **kwargs):
                 self._handle_toggle(tool_obj, None, None, None)
         tool_obj.set_figure(self.figure)
 
-        self._tool_added_event(tool_obj)
-        return tool_obj
+        event = ToolEvent('tool_added_event', self, tool_obj)
+        self._callbacks.process(event.name, event)
 
-    def _tool_added_event(self, tool):
-        s = 'tool_added_event'
-        event = ToolEvent(s, self, tool)
-        self._callbacks.process(s, event)
+        return tool_obj
 
     def _handle_toggle(self, tool, sender, canvasevent, data):
         """

lib/matplotlib/backend_tools.py

@@ -155,6 +155,7 @@ def trigger(self, sender, event, data=None):
         """
         pass
 
+    @_api.deprecated("3.6", alternative="tool_removed_event")
     def destroy(self):
         """
         Destroy the tool.