GH-8: Download and cache the assets on run

setup.py

@@ -1,12 +1,4 @@
-from distutils import dir_util
-from os.path import dirname, join
-from urllib.request import urlretrieve
-
 from setuptools import setup
 
 if __name__ == "__main__":
-    cdn = "https://cdnjs.cloudflare.com/ajax/libs/mermaid/9.4.3/mermaid.js"
-    static = join(dirname(__file__), "src", "django_mermaid", "static")
-    dir_util.create_tree(static, ["mermaid.js"])
-    urlretrieve(cdn, join(static, "mermaid.js"))
     setup()

src/django_mermaid/apps.py

@@ -1,5 +1,17 @@
+from distutils import dir_util
+from os.path import dirname, join, exists
+from urllib.request import urlretrieve
+
 from django.apps import AppConfig
 
 
 class MermaidConfig(AppConfig):
     name = "django_mermaid"
+
+    def ready(self):
+        """Download mermaid.js from CDN if not already present"""
+        cdn = "https://cdnjs.cloudflare.com/ajax/libs/mermaid/9.4.3/mermaid.js"
+        static = join(dirname(__file__), "static")
+        if not exists(join(static, "mermaid.js")):
+            dir_util.create_tree(static, ["mermaid.js"])
+            urlretrieve(cdn, join(static, "mermaid.js"))