DRAFT: Refactor navbar generating function.

On top of  pydata#1771, should fix pydata#1773 (once fininished)

This try to split the function into two parts: one that wrangle the
data,  one that generate html.

The generating the data part should be cacheable, while the other can be
made more flexible to be reusable in different part of the theme
(typically generating or not dropdowns in the sidebar or navbar).

src/pydata_sphinx_theme/toctree.py

@@ -1,7 +1,9 @@
 """Methods to build the toctree used in the html pages."""
 
+from dataclasses import dataclass
 from functools import cache
 from itertools import count
+from textwrap import dedent
 from typing import Iterator, List, Tuple, Union
 from urllib.parse import urlparse
 
@@ -51,6 +53,16 @@ def _get_ancestor_pagename(app: Sphinx, pagename: str, startdepth: int) -> str:
     return out, toctree
 
 
+@dataclass
+class LinkInfo:
+    """Dataclass to generate toctree data."""
+
+    is_current: bool
+    href: str
+    title: str
+    is_external: bool
+
+
 def add_toctree_functions(
     app: Sphinx, pagename: str, templatename: str, context, doctree
 ) -> None:
@@ -129,14 +141,14 @@ def generate_header_nav_before_dropdown(
             # NOTE: `env.toctree_includes` is a dict mapping pagenames to any (possibly
             # hidden) TocTree directives on that page (i.e., the "child" pages nested
             # under `pagename`).
-            active_header_page = [
-                *_get_toctree_ancestors(app.env.toctree_includes, pagename)
-            ]
+            header_pages = [*_get_toctree_ancestors(app.env.toctree_includes, pagename)]
         else:
-            active_header_page = toctree.get_toctree_ancestors(pagename)
-        if active_header_page:
+            header_pages = toctree.get_toctree_ancestors(pagename)
+        if header_pages:
             # The final list item will be the top-most ancestor
-            active_header_page = active_header_page[-1]
+            active_header_page = header_pages[-1]
+        else:
+            active_header_page = None
 
         # NOTE: `env.tocs` is a dict mapping pagenames to hierarchical bullet-lists
         # ("nodetrees" in Sphinx parlance) of in-page headings (including `toctree::`
@@ -145,6 +157,8 @@ def generate_header_nav_before_dropdown(
         root_toc = app.env.tocs[app.config.root_doc]
 
         links_html = []
+        links_data = []
+
         # Iterate through each node in the root document toc.
         # Grab the toctree pages and find the relative link + title.
         for toc in traverse_or_findall(root_toc, TocTreeNodeClass):
@@ -175,28 +189,65 @@ def generate_header_nav_before_dropdown(
                 link_status = "nav-external" if is_absolute else "nav-internal"
                 link_href = page if is_absolute else context["pathto"](page)
 
+                links_data.append(
+                    LinkInfo(
+                        is_current=(page == active_header_page),
+                        href=link_href,
+                        title=title,
+                        is_external=is_absolute,
+                    )
+                )
+
                 # create the html output
                 links_html.append(
-                    f"""
+                    dedent(
+                        f"""
                     <li class="nav-item pst-header-nav-item {current}">
                       <a class="nav-link {link_status}" href="{link_href}">
                         {title}
                       </a>
                     </li>
                 """
+                    )
                 )
 
         # Add external links defined in configuration as sibling list items
         for external_link in context["theme_external_links"]:
+            links_data.append(
+                LinkInfo(
+                    is_current=False,
+                    href=external_link["url"],
+                    title=external_link["name"],
+                    is_external=True,
+                )
+            )
             links_html.append(
-                f"""
-                <li class="nav-item pst-header-nav-item">
+                dedent(
+                    f"""
+                <li class="nav-item pst-header-nav-item ">
                   <a class="nav-link nav-external" href="{ external_link["url"] }">
                     { external_link["name"] }
                   </a>
                 </li>
                 """
+                )
+            )
+        lhtml = []
+        for link in links_data:
+            lhtml.append(
+                dedent(
+                    f"""
+                <li class="nav-item pst-header-nav-item {"current active" if link.is_current else ""}">
+                  <a class="nav-link {"nav-external" if link.is_external else "nav-internal"}" href="{ link.href}">
+                    { link.title }
+                  </a>
+                </li>
+            """
+                )
             )
+        for a, b, d in zip(links_html, lhtml, links_data):
+            assert a == b, (a, b, d)
+        assert links_html == lhtml, (links_html, lhtml)
 
         # The first links will always be visible
         links_solo = links_html[:n_links_before_dropdown]