New build flag system, convert Google Now flag

build/build_header.gni

+# Copyright 2015 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.
+
+# Generates a header with preprocessor defines specified by the build file.

[Mark Mentovai, 2015/11/19 21:52:55]

The gypi version points to here, which is a good hint that the two need to be
kept in sync. This file should also point to the gypi.

+#
+# There are two categories of defines. The first are "flags" which are enable/
+# disable options that are either true or false. Specify these in the template
+# as a list of strings that encode key/value pairs like this:
+#
+#   flags = [ "ENABLE_FOO=1", ENABLE_BAR=$enable_bar" ]
+#
+# The value must either be 0/1 or true/false or an error will be thrown at
+# build time. To check the value of the flag in C code:
+#
+#   #include "path/to/here/header_file.h"
+#
+#   #if BUILDFLAG(ENABLE_FOO)
+#   ...
+#
+# The second category are "variables" where the value of the define is
+# important rather than just a true/false value. In this case, the value is
+# just emitted in the header so should be in C format.
+#
+#   variables = [ "FOO_PATH=\"/la/dee/da\" ]
+#
+# And then use like:
+#
+#   #include "path/to/here/header_file.h"
+#
+#   const char kMyPath[] = BUILDVAR(FOO_PATH)
+#
+#
+# Template parameters
+#
+#   header [required, string]
+#       File name for generated header. By default, this will go in the
+#       generated file directory for this target, and you would include it
+#       with:
+#         #include "<path_to_this_BUILD_file>/<header>"
+#
+#   header_dir [string, optional]
+#       Override the default location of the generated header. The string will
+#       be treated as a subdirectory of the root_gen_dir. For example:
+#         header_dir = "foo/bar"
+#       Then you can include the header as:
+#         #include "foo/bar/baz.h"
+#
+#   deps, public_deps, testonly, visibility
+#       Normal meaning.
+#
+#
+# Grit defines
+#
+# If one .grd file uses a flag, just add to the grit target:
+#
+#   defines = [
+#     "enable_doom_melon=$enable_doom_melon",
+#   ]
+#
+# If multiple .grd files use it, you'll want to put the defines in a .gni file
+# so it can be shared. Generally this .gni file should include all grit defines
+# for a given module (for some definition of "module"). Then do:
+#
+#   defines = ui_grit_defines
+#
+# If you forget to do this, the flag will be implicitly false in the .grd file
+# and those resources won't be compiled. You'll know because the resource
+# #define won't be generated and any code that uses it won't compile. If you
+# see a missing IDS_* string, this is probably the reason.
+#
+#
+# Example
+#
+#   build_header("foo_features") {
+#     header = "foo_features.h"
+#
+#     flags = [
+#       # This uses the GN build flag enable_doom_melon as the definition.
+#       "ENABLE_DOOM_MELON=$enable_doom_melon",
+#
+#       # This force-enables the flag.
+#       "ENABLE_SPACE_LASER=1",
+#     ]
+#
+#     variables = [
+#       "SPAM_SERVER_URL=\"http://www.example.com/",

[Mark Mentovai, 2015/11/19 21:52:55]

You probably want a \" on the end too.

+#     ]
+#   }
+template("build_header") {
+  action(target_name) {
+    script = "//build/write_build_header.py"
+
+    if (defined(invoker.header_dir)) {
+      header_dir = "$root_gen_dir/${invoker.header_dir}"
+      path_for_guard = "${invoker.header_dir}/${invoker.header}"
+    } else {
+      header_dir = target_gen_dir
+
+      # Compute the path from teh root to this file.

[Mark Mentovai, 2015/11/19 21:52:55]

teh

+      path_for_guard = get_path_info(rebase_path(".", "//"), "dir")
+      path_for_guard += "/${invoker.header}"
+    }
+
+    gen_header_file = "$header_dir/${invoker.header}"
+
+    outputs = [
+      gen_header_file,
+    ]
+
+    # Always write --flags and --variables to the file so it's not empty.
+    # Empty will confuse GN into thinking the response file isn't used.
+    response_file_contents = [ "--flags" ]
+    if (defined(invoker.flags)) {
+      response_file_contents += invoker.flags
+    }
+    response_file_contents += [ "--variables" ]
+    if (defined(invoker.variables)) {
+      response_file_contents += invoker.variables
+    }
+
+    args = [
+      "--output",
+      rebase_path(gen_header_file, root_build_dir),
+      "--path-for-guard",
+      path_for_guard,
+      "--definitions",
+      "{{response_file_name}}",
+    ]
+
+    forward_variables_from(invoker,
+                           [
+                             "deps",
+                             "public_deps",
+                             "testonly",
+                             "visibility",
+                           ])
+  }
+}