tools/gn/function_template.cc

   1 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
   2 // Use of this source code is governed by a BSD-style license that can be
   3 // found in the LICENSE file.
   4
   5 #include "tools/gn/functions.h"
   6
   7 #include "tools/gn/parse_tree.h"
   8 #include "tools/gn/scope.h"
   9 #include "tools/gn/template.h"
  10 #include "tools/gn/value.h"
  11
  12 namespace functions {
  13
  14 const char kTemplate[] = "template";
  15 const char kTemplate_HelpShort[] =
  16     "template: Define a template rule.";
  17 const char kTemplate_Help[] =
  18     "template: Define a template rule.\n"
  19     "\n"
  20     "  A template defines a custom name that acts like a function. It\n"
  21     "  provides a way to add to the built-in target types.\n"
  22     "\n"
  23     "  The template() function is used to declare a template. To invoke the\n"
  24     "  template, just use the name of the template like any other target\n"
  25     "  type.\n"
  26     "\n"
  27     "  Often you will want to declare your template in a special file that\n"
  28     "  other files will import (see \"gn help import\") so your template\n"
  29     "  rule can be shared across build files.\n"
  30     "\n"
  31     "Variables and templates:\n"
  32     "\n"
  33     "  When you call template() it creates a closure around all variables\n"
  34     "  currently in scope with the code in the template block. When the\n"
  35     "  template is invoked, the closure will be executed.\n"
  36     "\n"
  37     "  When the template is invoked, the code in the caller is executed and\n"
  38     "  passed to the template code as an implicit \"invoker\" variable. The\n"
  39     "  template uses this to read state out of the invoking code.\n"
  40     "\n"
  41     "  One thing explicitly excluded from the closure is the \"current\n"
  42     "  directory\" against which relative file names are resolved. The\n"
  43     "  current directory will be that of the invoking code, since typically\n"
  44     "  that code specifies the file names. This means all files internal\n"
  45     "  to the template should use absolute names.\n"
  46     "\n"
  47     "  A template will typically forward some or all variables from the\n"
  48     "  invoking scope to a target that it defines. Often, such variables\n"
  49     "  might be optional. Use the pattern:\n"
  50     "\n"
  51     "    if (defined(invoker.deps)) {\n"
  52     "      deps = invoker.deps\n"
  53     "    }\n"
  54     "\n"
  55     "  The function forward_variables_from() provides a shortcut to forward\n"
  56     "  one or more or possibly all variables in this manner:\n"
  57     "\n"
  58     "    forward_variables_from(invoker, [\"deps\", \"public_deps\"])\n"
  59     "\n"
  60     "Target naming:\n"
  61     "\n"
  62     "  Your template should almost always define a built-in target with the\n"
  63     "  name the template invoker specified. For example, if you have an IDL\n"
  64     "  template and somebody does:\n"
  65     "    idl(\"foo\") {...\n"
  66     "  you will normally want this to expand to something defining a\n"
  67     "  source_set or static_library named \"foo\" (among other things you may\n"
  68     "  need). This way, when another target specifies a dependency on\n"
  69     "  \"foo\", the static_library or source_set will be linked.\n"
  70     "\n"
  71     "  It is also important that any other targets your template expands to\n"
  72     "  have globally unique names, or you will get collisions.\n"
  73     "\n"
  74     "  Access the invoking name in your template via the implicit\n"
  75     "  \"target_name\" variable. This should also be the basis of how other\n"
  76     "  targets that a template expands to to ensure uniquness.\n"
  77     "\n"
  78     "  A typical example would be a template that defines an action to\n"
  79     "  generate some source files, and a source_set to compile that source.\n"
  80     "  Your template would name the source_set \"target_name\" because\n"
  81     "  that's what you want external targets to depend on to link your code.\n"
  82     "  And you would name the action something like \"${target_name}_action\"\n"
  83     "  to make it unique. The source set would have a dependency on the\n"
  84     "  action to make it run.\n"
  85     "\n"
  86     "Example of defining a template:\n"
  87     "\n"
  88     "  template(\"my_idl\") {\n"
  89     "    # Be nice and help callers debug problems by checking that the\n"
  90     "    # variables the template requires are defined. This gives a nice\n"
  91     "    # message rather than giving the user an error about an\n"
  92     "    # undefined variable in the file defining the template\n"
  93     "    #\n"
  94     "    # You can also use defined() to give default values to variables\n"
  95     "    # unspecified by the invoker.\n"
  96     "    assert(defined(invoker.sources),\n"
  97     "           \"Need sources in $target_name listing the idl files.\")\n"
  98     "\n"
  99     "    # Name of the intermediate target that does the code gen. This must\n"
 100     "    # incorporate the target name so it's unique across template\n"
 101     "    # instantiations.\n"
 102     "    code_gen_target_name = target_name + \"_code_gen\"\n"
 103     "\n"
 104     "    # Intermediate target to convert IDL to C source. Note that the name\n"
 105     "    # is based on the name the invoker of the template specified. This\n"
 106     "    # way, each time the template is invoked we get a unique\n"
 107     "    # intermediate action name (since all target names are in the global\n"
 108     "    # scope).\n"
 109     "    action_foreach(code_gen_target_name) {\n"
 110     "      # Access the scope defined by the invoker via the implicit\n"
 111     "      # \"invoker\" variable.\n"
 112     "      sources = invoker.sources\n"
 113     "\n"
 114     "      # Note that we need an absolute path for our script file name.\n"
 115     "      # The current directory when executing this code will be that of\n"
 116     "      # the invoker (this is why we can use the \"sources\" directly\n"
 117     "      # above without having to rebase all of the paths). But if we need\n"
 118     "      # to reference a script relative to the template file, we'll need\n"
 119     "      # to use an absolute path instead.\n"
 120     "      script = \"//tools/idl/idl_code_generator.py\"\n"
 121     "\n"
 122     "      # Tell GN how to expand output names given the sources.\n"
 123     "      # See \"gn help source_expansion\" for more.\n"
 124     "      outputs = [ \"$target_gen_dir/{{source_name_part}}.cc\",\n"
 125     "                  \"$target_gen_dir/{{source_name_part}}.h\" ]\n"
 126     "    }\n"
 127     "\n"
 128     "    # Name the source set the same as the template invocation so\n"
 129     "    # instancing this template produces something that other targets\n"
 130     "    # can link to in their deps.\n"
 131     "    source_set(target_name) {\n"
 132     "      # Generates the list of sources, we get these from the\n"
 133     "      # action_foreach above.\n"
 134     "      sources = get_target_outputs(\":$code_gen_target_name\")\n"
 135     "\n"
 136     "      # This target depends on the files produced by the above code gen\n"
 137     "      # target.\n"
 138     "      deps = [ \":$code_gen_target_name\" ]\n"
 139     "    }\n"
 140     "  }\n"
 141     "\n"
 142     "Example of invoking the resulting template:\n"
 143     "\n"
 144     "  # This calls the template code above, defining target_name to be\n"
 145     "  # \"foo_idl_files\" and \"invoker\" to be the set of stuff defined in\n"
 146     "  # the curly brackets.\n"
 147     "  my_idl(\"foo_idl_files\") {\n"
 148     "    # Goes into the template as \"invoker.sources\".\n"
 149     "    sources = [ \"foo.idl\", \"bar.idl\" ]\n"
 150     "  }\n"
 151     "\n"
 152     "  # Here is a target that depends on our template.\n"
 153     "  executable(\"my_exe\") {\n"
 154     "    # Depend on the name we gave the template call above. Internally,\n"
 155     "    # this will produce a dependency from executable to the source_set\n"
 156     "    # inside the template (since it has this name), which will in turn\n"
 157     "    # depend on the code gen action.\n"
 158     "    deps = [ \":foo_idl_files\" ]\n"
 159     "  }\n";
 160
 161 Value RunTemplate(Scope* scope,
 162                   const FunctionCallNode* function,
 163                   const std::vector<Value>& args,
 164                   BlockNode* block,
 165                   Err* err) {
 166   // TODO(brettw) determine if the function is built-in and throw an error if
 167   // it is.
 168   if (args.size() != 1) {
 169     *err = Err(function->function(),
 170                "Need exactly one string arg to template.");
 171     return Value();
 172   }
 173   if (!args[0].VerifyTypeIs(Value::STRING, err))
 174     return Value();
 175   std::string template_name = args[0].string_value();
 176
 177   const Template* existing_template = scope->GetTemplate(template_name);
 178   if (existing_template) {
 179     *err = Err(function, "Duplicate template definition.",
 180                "A template with this name was already defined.");
 181     err->AppendSubErr(Err(existing_template->GetDefinitionRange(),
 182                           "Previous definition."));
 183     return Value();
 184   }
 185
 186   scope->AddTemplate(template_name, new Template(scope, function));
 187   return Value();
 188 }
 189
 190 }  // namespace functions