MD History: Lazily load element files which are not needed for first paint

chrome/browser/resources/vulcanize.py

 #!/usr/bin/env python
 # Copyright 2016 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import os
 import subprocess
 import sys
 import tempfile
 
@@ skipping 30 matching lines @@
 
   '--redirect', 'chrome://resources/cr_elements/|%s' % _CR_ELEMENTS_PATH,
   '--redirect', 'chrome://resources/css/|%s' % _CSS_RESOURCES_PATH,
   '--redirect', 'chrome://resources/html/|%s' % _HTML_RESOURCES_PATH,
   '--redirect', 'chrome://resources/js/|%s' % _JS_RESOURCES_PATH,
   '--redirect', 'chrome://resources/polymer/v1_0/|%s' % _POLYMER_PATH,
 
   '--strip-comments',
 ]
 
+def _run_cmd(cmd_parts, stdout=None):
+  cmd = "'" + "' '".join(cmd_parts) + "'"
+  process = subprocess.Popen(
+      cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=True)
+  stdout, stderr = process.communicate()
+
+  if stderr:
+    print >> sys.stderr, '%s failed: %s' % (cmd, stderr)
+    raise
+
+  return stdout
 
 def _vulcanize(directory, host, html_in_file, html_out_file='vulcanized.html',
                js_out_file='crisper.js', extra_args=None):
-  def _run_cmd(cmd_parts, stdout=None):
-    cmd = "'" + "' '".join(cmd_parts) + "'"
-    process = subprocess.Popen(
-        cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, shell=True)
-    stdout, stderr = process.communicate()
-
-    if stderr:
-      print >> sys.stderr, '%s failed: %s' % (cmd, stderr)
-      raise
-
-    return stdout
-
-  print 'Vulcanizing %s' % directory
+  print 'Vulcanizing %s/%s' % (directory, html_in_file)
 
   target_path = os.path.join(_HERE_PATH, directory)
   html_in_path = os.path.join(target_path, html_in_file)
   html_out_path = os.path.join(target_path, html_out_file)
   js_out_path = os.path.join(target_path, js_out_file)
   extra_args = extra_args or []
 
   output = _run_cmd(['vulcanize'] + _VULCANIZE_BASE_ARGS + extra_args +
                     ['--redirect', 'chrome://%s/|%s' % (host, target_path),
                      html_in_path])
 
   with tempfile.NamedTemporaryFile(mode='wt+', delete=False) as tmp:
     # Grit includes are not supported, use HTML imports instead.
     tmp.write(output.replace(
         '<include src="', '<include src-disabled="'))
 
   try:
     _run_cmd(['crisper', '--source', tmp.name,
                          '--script-in-head', 'false',
                          '--html', html_out_path,
                          '--js', js_out_path])
 
     # TODO(tsergeant): Remove when JS resources are minified by default:
     # crbug.com/619091.
     _run_cmd(['uglifyjs', js_out_path,
                          '--beautify', 'indent-level=2,quote_style=3',
                          '--comments', '/Copyright|license|LICENSE|\<\/?if/',
                          '--output', js_out_path])
-    _run_cmd(['polymer-css-build', html_out_path])
   finally:
     os.remove(tmp.name)
 
 
+def _css_build(directory, files):
+  target_path = os.path.join(_HERE_PATH, directory)
+  paths = map(lambda f: os.path.join(target_path, f), files)
+
+  _run_cmd(['polymer-css-build'] + paths)
+
+
 def main():
   _vulcanize(directory='md_downloads', host='downloads',
              html_in_file='downloads.html')
+  _css_build(directory='md_downloads', files=['vulcanized.html'])
 
   # Already loaded by history.html:
   history_extra_args = ['--exclude', 'chrome://resources/html/util.html',
                         '--exclude', 'chrome://history/constants.html']
   _vulcanize(directory='md_history', host='history', html_in_file='app.html',
              html_out_file='app.vulcanized.html', js_out_file='app.crisper.js',
              extra_args=history_extra_args)
 
+  # Ensures that no file transitively imported by app.vulcanized.html is
+  # imported by lazy_load.vulcanized.html.

[calamity, 2016/09/22 04:00:10]

This seems to just do this silently. Any way to warn if lazy-loading of an
element gets broken by someone adding an import to the critical path?

[tsergeant, 2016/09/22 04:27:58]

For the moment, this will have to be enforced by code review. It should be
pretty obvious if, say, paper-button moves from one vulcanized bundle to the
other just by eyeballing the files.

We could consider writing a script to figure out what elements are in each
bundle and reporting changes, but that's not a high priority right now.

+  lazy_load_extra_args = ['--exclude', 'chrome://history/app.html']
+  _vulcanize(directory='md_history', host='history',
+             html_in_file='lazy_load.html',
+             html_out_file='lazy_load.vulcanized.html',
+             js_out_file='lazy_load.crisper.js',
+             extra_args=history_extra_args + lazy_load_extra_args)
+  _css_build(directory='md_history', files=['app.vulcanized.html',
+                                            'lazy_load.vulcanized.html'])
 
 if __name__ == '__main__':
   main()