From 15431fa2017d7997481fb393471884d3fbdeac50 Mon Sep 17 00:00:00 2001
From: Eric Brown <pdcxs@outlook.com>
Date: Fri, 2 Oct 2020 16:33:01 +0800
Subject: [PATCH] Fix bug using z_index in VGroup

Solve the problem in https://github.com/ManimCommunity/manim/issues/327
---
 manim/mobject/mobject.py | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/manim/mobject/mobject.py b/manim/mobject/mobject.py
index b93f0d40b0..03e193aacc 100644
--- a/manim/mobject/mobject.py
+++ b/manim/mobject/mobject.py
@@ -132,6 +132,14 @@ def add(self, *mobjects):
         if self in mobjects:
             raise ValueError("Mobject cannot contain self")
         self.submobjects = list_update(self.submobjects, mobjects)
+        
+        # This will introduce another use_z_index flag
+        # There may be better solutions
+        # But for now, this is the easiest solution
+        if self.use_z_index:
+            self.submobjects = sorted(self.submobjects,
+                key=lambda m: m.z_index)
+        
         return self
 
     def add_to_back(self, *mobjects):