Cache x y to deal with resizing canvas.

reticule.py

@@ -29,6 +29,12 @@ class Reticule:
         self.color = color
         self.item_ids = []
         self.init_reticule()
+        self.xy_cache = 0, 0
+        self.canvas.bind(
+            '<Configure>',
+            self.handle_canvas_resize,
+            add='+',
+            )
 
     def get_reticule_coords(self, x, y):
         '''
@@ -36,11 +42,14 @@ class Reticule:
         rectangle yield coords for four lines and an oval suitable for
         passing to canvas.coords() method.
         '''
-        width, height = int(self.canvas['width']), int(self.canvas['height'])
+        width = int(self.canvas['width'])
+        height = int(self.canvas['height'])
+
         top = y - self.radius_y
         left = x - self.radius_x
         bottom = y + self.radius_y
         right = x + self.radius_x
+
         yield 0, y, left, y
         yield right, y, width - 1, y
         yield x, 0, x, top
@@ -51,6 +60,7 @@ class Reticule:
         append = self.item_ids.append
         w, h = int(self.canvas['width']), int(self.canvas['height'])
         coords = self.get_reticule_coords(w >> 1, h >> 1)  # Center.
+
         append(canvas.create_line(*next(coords), fill=self.color))
         append(canvas.create_line(*next(coords), fill=self.color))
         append(canvas.create_line(*next(coords), fill=self.color))
@@ -58,6 +68,7 @@ class Reticule:
         append(canvas.create_oval(*next(coords), outline=self.color))
 
     def set_reticule(self, x, y):
+        self.xy_cache = x, y
         coords = self.get_reticule_coords(x, y)
         for item_id, item_coords in zip(self.item_ids, coords):
             self.canvas.coords(item_id, *item_coords)
@@ -65,6 +76,12 @@ class Reticule:
     def handle_set_event(self, event):
         self.set_reticule(event.x, event.y)
 
+    def handle_canvas_resize(self, _event):
+        # We're going to assume that some other callback has updated the
+        # canvas values.  Otherwise we could get the new width and height
+        # from the Configure event.
+        self.set_reticule(*self.xy_cache)
+
 
 if __name__ == '__main__':
     from tkinter import *
@@ -77,7 +94,6 @@ if __name__ == '__main__':
         # If the canvas window has enlarged then some of the lines will have
         # to be extended.
 
-
     canvas = Canvas(bg='black')
 
     canvas.bind('<Configure>', handle_canvas_resize)