Add conditional feature to templates.

client/dom/scripts/templateloader.py

Index: client/dom/scripts/templateloader.py
diff --git a/client/dom/scripts/templateloader.py b/client/dom/scripts/templateloader.py
new file mode 100644
index 0000000000000000000000000000000000000000..64e4a8da0e0c8ce36e7391bd7f7d3f04cf4a5e90
--- /dev/null
+++ b/client/dom/scripts/templateloader.py
@@ -0,0 +1,113 @@
+#!/usr/bin/python
+# Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+# for details. All rights reserved. Use of this source code is governed by a
+# BSD-style license that can be found in the LICENSE file.
+
+# Template loader and preprocessor.
+#
+# Preprocessor language:
+#
+#   $if VAR
+#   $else
+#   $endif
+#
+# VAR must be defined in the conditions dictionary.
+
+import os
+
+class TemplateLoader(object):
+  """Loads template files from a path."""
+
+  def __init__(self, root, subpaths, conditions = {}):
+    """Initializes loader.
+
+    Args:
+      root - a string, the directory under which the templates are stored.
+      subpaths - a list of strings, subpaths of root in search order.
+      conditions - a dictionay from strings to booleans.  Any conditional
+        expression must be a key in the map.
+    """
+    self._root = root
+    self._subpaths = subpaths
+    self._conditions = conditions
+    self._cache = {}
+
+  def TryLoad(self, name):
+    """Returns content of template file as a string, or None of not found."""
+    if name in self._cache:
+      return self._cache[name]
+
+    for subpath in self._subpaths:
+      template_file = os.path.join(self._root, subpath, name)
+      if os.path.exists(template_file):
+        template = ''.join(open(template_file).readlines())
+        template = self._Preprocess(template, template_file)
+        self._cache[name] = template
+        return template
+
+    return None
+
+  def Load(self, name):
+    """Returns contents of template file as a string, or raises an exception."""
+    template = self.TryLoad(name)
+    if template is not None:  # Can be empty string
+      return template
+    raise Exception("Could not find template '%s' on %s / %s" % (
+        name, self._root, self._subpaths))
+
+  def _Preprocess(self, template, filename):
+    def error(lineno, message):
+      raise Exception('%s:%s: %s' % (filename, lineno, message))
+
+    lines = template.splitlines(True)
+    out = []
+
+    condition_stack = []
+    active = True
+    seen_else = False
+
+    for (lineno, full_line) in enumerate(lines):
+      line = full_line.strip()
+
+      if line.startswith('$'):
+        words = line.split()
+        directive = words[0]
+
+        if directive == '$if':
+          if len(words) != 2:
+            error(lineno, '$if does not have single variable')
+          variable = words[1]
+          if variable in self._conditions:
+            condition_stack.append((active, seen_else))
+            active = self._conditions[variable]
+            seen_else = False
+          else:
+            error(lineno, "Unknown $if variable '%s'" % variable)
+
+        elif directive == '$else':
+          if not condition_stack:
+            error(lineno, '$else without $if')
+          if seen_else:
+            raise error(lineno, 'Double $else')
+          seen_else = True
+          active = not active
+
+        elif directive == '$endif':
+          if not condition_stack:
+            error(lineno, '$endif without $if')
+          (active, seen_else) = condition_stack.pop()
+
+        else:
+          # Something else, like '$!MEMBERS'
+          if active:
+            out.append(full_line)
+
+      else:
+        if active:
+          out.append(full_line);
+        continue
+
+    if condition_stack:
+      error(len(lines), 'Unterminated $if')
+
+    return ''.join(out)