Add view handler to serve files when Notebook 7 is installed (#287)

* serve files from nbclassic when notebook 7 is installed

* update handler comment

nbclassic/notebookapp.py

@@ -280,6 +280,7 @@ def initialize_handlers(self):
         handlers.extend(load_handlers('nbclassic.tree.handlers'))
         handlers.extend(load_handlers('nbclassic.notebook.handlers'))
         handlers.extend(load_handlers('nbclassic.edit.handlers'))
+        handlers.extend(load_handlers('nbclassic.view.handlers'))
         self.handlers.extend(handlers)
 
         # Wildcard routes

nbclassic/view/__init__.py

@@ -0,0 +1 @@
+"""Tornado handlers for viewing HTML files."""

nbclassic/view/handlers.py

@@ -0,0 +1,40 @@
+#encoding: utf-8
+"""Tornado handlers for viewing HTML files."""
+
+# Copyright (c) Jupyter Development Team.
+# Distributed under the terms of the Modified BSD License.
+
+from tornado import web, gen
+
+
+from jupyter_server.base.handlers import JupyterHandler, path_regex
+from jupyter_server.utils import url_escape, ensure_async, url_path_join
+
+from nbclassic import nbclassic_path
+
+
+class CustomViewHandler(JupyterHandler):
+    """Render HTML files within an iframe."""
+
+    @web.authenticated
+    @gen.coroutine
+    def get(self, path):
+        """Get a view on a given path."""
+
+        path = path.strip('/')
+        exists = yield ensure_async(self.contents_manager.file_exists(path))
+        if not exists:
+            raise web.HTTPError(404, u'File does not exist: %s' % path)
+
+        basename = path.rsplit('/', 1)[-1]
+        file_url = url_path_join(self.base_url, "files", url_escape(path))
+        self.write(self.render_template('view.html',
+            file_url=file_url,
+            page_title=basename,
+            )
+        )
+
+
+default_handlers = [
+    (r"{}/view{}".format(nbclassic_path(), path_regex), CustomViewHandler),
+]