src/docs/webui_build_configuration.md

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# WebUI Build Configuration

[TOC]

## WebUI BUILD.gn files
WebUI builds are configured using BUILD.gn files, which live in the top level
directory for a WebUI. For example, the BUILD.gn file for the Downloads page
is located at chrome/browser/resources/downloads/BUILD.gn.

These files specify the build steps necessary to convert the checked-in code to
the code that should be served at runtime.

## WebUI build rules

### **html_to_wrapper, css_to_wrapper**
```
These rules are used to inline HTML or CSS into a TypeScript file which can be
compiled by TS compiler and then imported with JS imports at runtime. This is
necessary when writing Web Components, which need to return their HTML in the
`template()` getter method.

By default, these rules accept input files from within the current directory.

By default, they output the wrapped `.html.ts` and `.css.ts` files to the target
generated directory (|target_gen_dir|).
```

#### **Arguments**
```
in_files: specifies the list of files to process with respect to the
          |in_folder|.
template: html_to_wrapper only. Defaults to "polymer", set to "native" if using
          the rule to wrap the HTML of a non-Polymer Web Component.
in_folder: Specifies the input folder where files are located. If not specified,
           the current directory (of the BUILD.gn file) is used.
out_folder: Specifies the location to write the wrapped files. If not specified,
            |target_gen_dir| is used.
minify: Whether to minify HTML/CSS with
        third_party/node/node_modules/html-minifier. Defaults to false.
use_js: Whether to output .js files instead of .ts files. Defaults to false.
scheme: One of ['chrome', 'relative']. Defaults to 'chrome'. Specifies whether
        dependencies of the generated wrapper file should be imported with
        "chrome://resources" or scheme-relative "//resources" URLs.
```

#### **Example**
```
import("//tools/polymer/html_to_wrapper.gni")
import("//tools/polymer/css_to_wrapper.gni")

# Generates "my_web_component.html.ts" in |target_gen_dir|.
html_to_wrapper("html_wrapper_files") {
  in_files = [ "my_web_component.html" ]
}

# Generates "my_style_module.css.ts" in |target_gen_dir|.
css_to_wrapper("css_wrapper_files") {
  in_files = [ "my_style_module.css" ]
}
```

### **preprocess_if_expr**
```
This rule is used to preprocess files containing `<if expr="*">`. These
expressions are most frequently used to enable code to only run on certain
platforms.

By default, reads input files from within the current directory and saves output
in |target_gen_dir|.
```

#### **Arguments**
```
in_folder: specifies the input folder, where all input files are located.
           Defaults to the folder where the BUILD.gn file resides.
out_folder: specifies the folder that the preprocessed files should be placed
            in. This must be a generated directory. Defaults to
            |target_gen_dir|.
in_files: specifies the list of input files to preprocess with respect to the
          |in_folder|.
out_manifest: Specifies a file where the list of output files and their
              directory should be written. This is most useful when the
              preprocessed files need to be listed in a grd file. Since
              preprocessed files are instead passed to ts_library in WebUIs
              that have been migrated to TypeScript, this is only used by
              WebUIs that have not been migrated to TypeScript yet.
```
#### **Example:**
```
import("//tools/grit/preprocess_if_expr.gni")

# Preprocesses "my_web_component.html.ts" and my_style_module.css.ts in
# |target_gen_dir|, into "$target_gen_dir/preprocessed".
preprocess_if_expr("preprocess_generated") {
  # Depend on the targets that generates these files.
  deps = [
    ":css_wrapper_files",
    ":html_wrapper_files",
  ]
  in_folder = target_gen_dir
  in_files = [
    "my_style_module.css.ts",
    "my_web_component.html.ts",
  ]
  out_folder = "$target_gen_dir/$preprocess_folder"
}

# Preprocess "my_web_component.ts" and "my_webui.ts" in the src dir into
# "$target_gen_dir/preprocessed".
preprocess_if_expr("preprocess_src") {
  in_folder = "."
  in_files = [
    "my_web_component.ts",
    "my_webui.ts",
  ]
  out_folder = "$target_gen_dir/$preprocess_folder"
}
```

### **ts_library**
```
This rule is used to compile TypeScript code to JavaScript. It outputs JS files
corresponding to each TS file passed as an input into the designated output
directory, and generates a manifest listing all the files it has output named
$target_name.manifest in the target generated directory.
```

#### **Input File notes**
```
All input files must be valid TypeScript. This means, among other things, that
files shouldn't contain any `<if expr>`, i.e. they should already have been
preprocessed, if necessary. It also means that e.g. HTML or image files
shouldn't be passed to ts_library.

All files that aren't imported with an absolute path (e.g.
`import {assert} from 'chrome://resources/js/assert_ts.js'; `) need to exist
inside the TypeScript root directory, at the expected location. For example,
if foo.ts in the top level directory contains the import statement
`import {Baz} from './bar/baz.js';`, then the folder structure when ts_library
is invoked should look like this:
  root_dir/
    foo.ts
    bar/
      baz.ts
```
#### **Arguments**
```
root_dir: This is the root directory where all TypeScript files to compile
          must reside (see note above).
out_dir: The directory where ts_library will write the compiled JavaScript
         files.
composite: Set this to "true" if the output needs to be used as a library by
           a different ts_library target (this frequently occurs when tests are
           compiled as a ts_library).
in_files: The input file paths to be compiled, with respect to the |in_folder|.
definitions: TypeScript definitions files used by the input TypeScript files,
             for example chrome.send or extension API definitions.
tsconfig_base: Specifies the tsconfig_base.json file to use. Necessary only
               if specifying configuration options for TS compiler that differ
               from the defaults in tools/typescript/tsconfig_base.json
deps: Specifies all other ts_library targets generating libraries that this
      target's library needs, for example the shared resources library at
      //ui/webui/resources:library.
extra_deps: Used to specify build targets generating the input files for TS
            compiler.
path_mappings: Additional non-default path mappings for absolute imports. The
               absolute 'chrome://resources/' paths are already mapped by
               default.
manifest_excludes: List of input files to exclude from the output
                   the manifest file.
```

#### **Example**

```
import("//tools/typescript/ts_library.gni")

# Compiles and outputs my_webui.js, my_web_component.js and
# my_web_component.html.js, in the "$target_gen_dir/tsc" folder.
ts_library("build_ts") {
  root_dir = "$target_gen_dir/preprocessed"
  out_dir = "$target_gen_dir/tsc"
  in_files = [
    "my_webui.ts",
    "my_style_module.css.ts",
    "my_web_component.html.ts",
    "my_web_component.ts",
  ]
  # List other ts_library targets for libraries the UI needs here
  deps = [
    "//third_party/polymer/v3_0:library",
    "//ui/webui/resources:library",
  ]
  definitions = [
    "//tools/typescript/definitions/chrome_send.d.ts",
  ]
  extra_deps = [
    ":copy_file",
    ":preprocess_src",
    ":preprocess_generated",
  ]
}
```

### **optimize_webui**
```
This rule is used to bundle larger user-facing WebUIs for improved performance.
It is generally not needed for debug UIs or UIs that have very few files to
import.
```

#### **Arguments**
```
host: The WebUI host. If specified without a scheme, the assumed root location
      will be "chrome://<host>". If specified with a scheme (e.g.
      "chrome-extension://aaaaaaaaa") the full <host> argument will be the root.
input: The location of the input files to be bundled.
js_module_in_files: The names of the root files to bundle. These files should
                    import all other dependencies (directly or indirectly).
                    These should be specified with respect to |input|.
js_out_files: The names of the final bundled files to output. If only one
              |js_module_in_files| is provided, there should be 1 file name
              specified. If 2 |js_module_in_files| are provided, there should
              be 3 file names (1 for each input, plus 1 for the shared bundle,
              which should be the last output file name).
out_manifest: File location to write the manifest of all output files created
              by optimize_webui(). Useful for generating grds.
deps: Targets generating any files being bundled. Note that this should include
      targets generating shared resources that are expected to be bundled in
      the UI, e.g. //ui/webui/resources:preprocess.
excludes: Paths of files that are not bundled. Often used for large mojo files
          that would otherwise be in many bundles, and for cr.m.js which relies
          on global variables.
```

#### **Example**
```
import("//chrome/browser/resources/tools/optimize_webui.gni")
import ("//chrome/common/features.gni")

# optimize_webui should generally only be called when the optimize_webui
# feature flag is enabled.
if (optimize_webui) {
  optimize_webui("build") {
    host = "mywebui"
    js_module_in_files = [ "my_webui.js" ]
    input = rebase_path("$target_gen_dir/tsc", root_build_dir)
    js_out_files = [ "my_webui.rollup.js" ]
    out_manifest = "$target_gen_dir/build_manifest.json"
    # Assumes the JS files were generated by a ts_library target called
    # build_ts.
    deps = [
      ":build_ts",
      "//ui/webui/resources:preprocess",
    ]
    excludes = [
      "chrome://resources/js/cr.m.js",
      "chrome://resources/mojo/mojo/public/js/bindings.js",
    ]
  }
}
```

### **generate_grd**
```
This rule is used to list the WebUI resources that need to be served at runtime
in a grd file.
```

#### **Arguments**
```
input_files: Input file paths to list in the grd file.
input_files_base_dir: Directory in which |input_files| can be found.
deps: Lists any targets responsible for generating |input_files|,
      |grdp files|, or |manifest_files|.
manifest_files: List of manifest files listing additional files that should be
                added to the grd.
grdp_files: List of .grdp files that should be included in the grd. Generally
            such files are also created with generate_grd().
grd_prefix: The prefix to use for the grd resource IDs. Resources will be named
            with the following pattern: IDR_GRD_PREFIX_INPUT_FILE_PATH
out_grd: The output grd file to write.
resource_path_rewrites: Paths to rewrite. In general, the path in input_files,
                        or the path listed in a manifest, will be used as the
                        resource path, i.e. "foo/bar.js" will have that path
                        relative to the root "chrome://mywebui/" at runtime.
                        By specifying
                        resource_path_rewrites = [ "foo/bar.js|foo_bar.js" ],
                        the path will instead be "foo_bar.js". Usually only
                        needed in somewhat complicated cases.
```

#### **Example**
```
import("//ui/webui/resources/tools/generate_grd.gni")

generate_grd("build_grd") {
  input_files = [ "my_webui.html" ]
  input_files_base_dir = rebase_path(".", "//")
  deps = [ ":build_ts" ]
  manifest_files = filter_include(get_target_outputs(":build_ts"), [ "*.manifest" ])
  # Or, configure statically the manifest file name:
  # manifest_files = [ "$target_gen_dir/build_ts.manifest" ]
  grd_prefix = "my_webui"
  out_grd = "$target_gen_dir/${grd_prefix}_resources.grd"
}
```

### **grit**
```
This rule is used to create a pak file from the grd file that contains all the
listed resources, so that they can be included in the binary and served at
runtime. Without creating a grit rule (and hooking up the target to the build),
the WebUI will not load since the resources will not exist. This rule also
generates C++ header files that are used to add the files to a specific
WebUIController.
```

#### **Arguments**
```
defines: The defines that grit should use when making the pak file. Generally
         should include chrome_grit_defines.
enable_input_discovery_for_gn_analyze: If using generated grd files, this needs
                                       to be set to false.
source: The source grd file.
outputs: List of files to output. Should always include the name of the pak
         file, and should generally also list the C++ header file and C++ header
         and .cc files for the resources map.
output_dir: Location to put the pak file. Often "$root_gen_dir/chrome".
deps: Names of any targets generating the grd file, and names of any targets
      generating the files to be packed, if they are not already dependencies
      of the target generating the grd file (as is often, but not always, the
      case).
```

#### **Example**
```
import("//tools/grit/grit_rule.gni")

grit("resources") {
  defines = chrome_grit_defines

  enable_input_discovery_for_gn_analyze = false
  source = "$target_gen_dir/resources.grd"
  deps = [ ":build_grd" ]

  outputs = [
    "grit/my_webui_resources.h",
    "grit/my_webui_resources_map.cc",
    "grit/my_webui_resources_map.h",
    "my_webui_resources.pak",
  ]
  output_dir = "$root_gen_dir/chrome"
}
```

### **build_webui**

<!-- TODO(crbug.com/1340376): Elevate build_webui() to the top of this document
      after it has been deployed to a few places. -->

See the [go/build-webui-pipeline](http://go/build-webui-pipeline) design doc for
more info (internal only).

The flow diagram below shows a high level view of how a typical modern user-facing
WebUI is being built.

![WebUI build pipeline flow diagram](images/webui_build_pipeline.png)

```
This umbrella rule captures the most common WebUI build pipeline configuration
and aims to hide the complexity of having to define all previously described
rules directly. Using build_webui() is the recommended approach for most modern
user-facing WebUIs that use WebComponents+TypeScript+Mojo.

The parameters passed to build_webui() are forwarded as needed to the other GN
rules described earlier.

Under the cover, build_webui() defines the following targets

preprocess_if_expr("preprocess")
html_to_wrapper("html_wrapper_files")
css_to_wrapper("css_wrapper_files")
copy("copy_mojo")
ts_library("build_ts")
optimize_webui("build_bundle")
generate_grd("build_grd")
grit("resources")

Some targets are only conditionally defined based on build_webui() input
parameters.

Only the ":build_ts" and ":resources" targets are public and can be referred to
from other parts of the build.
```

#### **Arguments**
```

List of files params:
static_files: Required parameter. List of
              1) non Web Component HTML/CSS files (don't confuse with
                 |css_files| below). These are passed to preprocess_if_expr()
              2) JPG/PNG/SVG files. These are included in the build verbatim
                 without any preprocessing.

web_component_files:  List of TS files that hold Web Component definitions with
                      equivalent HTML template files. These can be either native
                      or Polymer Web Components. Optional parameter.

non_web_component_files:  List of TS files that are not Web Components, or
                          Web Component files that don't have a corresponding
                          HTML template (rare case). Optional parameter.

icons_html_files: List of HTML files that hold Polymer iron-iconset-svg
                  instances. Optional parameter.

css_files: List of CSS files that hold Polymer style modules, or CSS variable
           definitions. These are passed css_to_wrapper(). Optional parameter.

mojo_files: List of Mojo JS generated files. These will be copied to a temporary
            location so that they can be passed to ts_library() along with other
            files. Optional parameter.

mojo_files_deps: List of Mojo targets that generate |mojo_files|. Must be
                 defined if |mojo_files| is defined.

TypeScript (ts_library()) related params:
ts_composite: See |composite| in ts_library(). Defaults to false, optional.
ts_definitions: See |definitions| in ts_library(). Optional parameter.
ts_deps: See |deps| in ts_library(). Optional parameter.
ts_extra_deps: See |extra_deps| in ts_library(). Optional parameter.
ts_path_mappings: See |path_mappings| in ts_library(). Optional parameter.
ts_use_local_config: Whether to pass a local "tsconfig_base.json" file as the
                     |tsconfig_base| to ts_library(). Optional, defaults to true.
                     If false, the default //tools/typescript/tsconfig_base.json
                     will be used.

HTML/CSS/JS optimization related params:
optimize: Specifies whether any optimization steps will be used, defaults to
          false. When true, html_to_wrapper() and css_to_wrapper() will be
          invoked with the |minify| flag on, to minify HTML/CSS code.
          optimize_webui() will be invoked to bundle+minify JS code (using
          Rollup and Terser). All other |optimize_*| parameters below must be
          specified if |optimize| is true.
optimize_webui_excludes: See |excludes| in optimize_webui().
optimize_webui_host: See |host| in optimize_webui().
optimize_webui_in_files: See |in_files| in optimize_webui().
optimize_webui_out_files: See |out_files| in optimize_webui().

Other params:
grd_prefix: See |grd_prefix| in generate_grd(). Required parameter.
html_to_wrapper_template: See |template| in html_to_wrapper().
extra_grdp_deps: List of external generate_grd() targets that generate .grdp
                 files. These will be included in the final generated
                 resources.grd file. Optional parameter.
extra_grdp_files: Output .grdp files of external generate_grd() targets. Must be
                  defined if |extra_grdp_deps| is defined.
grit_output_dir: See |output_dir| in grit(). Optional parameter, defaults to
                 "$root_gen_dir/chrome"
```

#### **Example**
```
import("//chrome/browser/resources/tools/build_webui.gni")

build_webui("build") {
  grd_prefix = "dummy-webui"

  static_files = [
    "index.html",
    "index.css",
  ]

  # Files holding a CustomElement element definition AND have an equivalent
  # .html template file.
  web_component_files = [
    "app.ts",
    "bar_view.ts",
    "foo_view.ts",
  ]

  # Files not holding a CustomElement element definition, or the CustomElement
  # does not have a corresponding HTML template.
  non_web_component_files = [
    "app_proxy.ts",
    "bar_proxy.ts",
    "foo_proxy.ts",
  ]

  # Files that are passed as input to css_to_wrapper().
  css_files = [
    "shared_style.css",
    "shared_vars.css",
  ]

  ts_definitions = [
    "//tools/typescript/definitions/chrome_send.d.ts",
    "//tools/typescript/definitions/metrics_private.d.ts",
  ]

  ts_deps = [
    "//third_party/polymer/v3_0:library",
    "//ui/webui/resources:library",
  ]
}

```

## Example build configurations

### **Simple UI with no web components**

```
This UI has only a single checked in TypeScript file, my_debug_page.ts, and a
checked in HTML file my_debug_page_index.html that imports the compiled JS
file using a script tag with `src="my_debug_page.js"` Neither of these files
use any `<if expr>`.
```

#### **BUILD.gn file**
```
import("//chrome/common/features.gni")
import("//tools/grit/grit_rule.gni")
import("//tools/typescript/ts_library.gni")
import("//ui/webui/resources/tools/generate_grd.gni")

# This UI has only a single TypeScript input file that does not define a Web
# Component. TS compiler will write out "$target_gen_dir/tsc/my_debug_page.js".
ts_library("build_ts") {
  root_dir = "."
  out_dir = "$target_gen_dir/tsc"
  in_files = [ "my_debug_page.ts" ]
  deps = [ "//ui/webui/resources:library" ]
}

# Both the HTML file and the JS file created by ts_library need to be listed in
# the grd. The JS file is already listed in the build_ts.manifest generated by
# ts_library(). Pass the HTML file via |input_files|.
generate_grd("build_grd") {
  deps = [ ":build_ts" ]
  grd_prefix= "my_debug_page"
  out_grd = "$target_gen_dir/resources.grd"
  input_files = [ "my_debug_page_index.html" ]
  input_files_base_dir = rebase_path(".", "//")
  manifest_files = filter_include(get_target_outputs(":build_ts"), [ "*.manifest" ])
}

# Create the pak, header, and resource map files.
grit("resources") {
  defines = chrome_grit_defines

  # This arguments is needed since the grd is generated at build time.
  enable_input_discovery_for_gn_analyze = false
  source = "$target_gen_dir/resources.grd"
  deps = [ ":build_grd" ]
  outputs = [
    "grit/my_debug_page_resources.h",
    "grit/my_debug_page_resources_map.cc",
    "grit/my_debug_page_resources_map.h",
    "my_debug_page_resources.pak",
  ]
  output_dir = "$root_gen_dir/chrome"
}
```

### **Non-Polymer UI with Web Components**
```
This UI has a top level TypeScript file that imports a single Web Component,
my_debug_app.ts. It has a corresponding HTML file for the app and a HTML file
for the top level index. None of these files use any `<if expr>`.
```

#### **BUILD.gn file**
```
import("//chrome/common/features.gni")
import("//tools/grit/grit_rule.gni")
import("//tools/typescript/ts_library.gni")
import("//ui/webui/resources/tools/generate_grd.gni")
import("//tools/polymer/html_to_wrapper.gni")

# Generate the wrapper file. This UI isn't using Polymer, so it needs to set
# the |template| argument.
html_to_wrapper("html_wrapper_files") {
  in_files = [ "my_debug_app.html" ]
  template = "native"
}

# Copy the checked-in files to the same directory. This is needed so that the
# TypeScript compiler can find all the files in the expected location.
copy("copy_files") {
  sources = [
    "my_debug_app.ts",
    "my_debug_page.ts",
  ]
  outputs = [ "$target_gen_dir/{{source_file_part}}" ]
}

# This UI has 3 TypeScript input files. One is the generated
# my_debug_app.html.ts, two are checked in: my_debug_page.ts and
# my_debug_app.ts. TS compiler will write out:
# "$target_gen_dir/tsc/my_debug_app.html.js",
# "$target_gen_dir/tsc/my_debug_app.js", and
# "$target_gen_dir/tsc/my_debug_page.js".
ts_library("build_ts") {
  root_dir = target_gen_dir
  out_dir = "$target_gen_dir/tsc"
  in_files = [
    "my_debug_app.ts",
    "my_debug_app.html.ts",
    "my_debug_page.ts",
  ]
  deps = [ "//ui/webui/resources:library" ]
  extra_deps = [
    ":copy_files",
    ":html_wrapper_files",
  ]
}

# The HTML file and the JS files created by ts_library need to be listed in
# the grd. The JS files are already listed in the build_ts.manifest generated by
# ts_library(). Pass the HTML file via |input_files|.
generate_grd("build_grd") {
  deps = [ ":build_ts" ]
  grd_prefix= "my_debug_page"
  out_grd = "$target_gen_dir/resources.grd"
  input_files = [ "my_debug_page_index.html" ]
  input_files_base_dir = rebase_path(".", "//")
  manifest_files = filter_include(get_target_outputs(":build_ts"), [ "*.manifest" ])
}

# Create the pak, header, and resource map files.
grit("resources") {
  defines = chrome_grit_defines

  # This arguments is needed since the grd is generated at build time.
  enable_input_discovery_for_gn_analyze = false
  source = "$target_gen_dir/resources.grd"
  deps = [ ":build_grd" ]
  outputs = [
    "grit/my_debug_page_resources.h",
    "grit/my_debug_page_resources_map.cc",
    "grit/my_debug_page_resources_map.h",
    "my_debug_page_resources.pak",
  ]
  output_dir = "$root_gen_dir/chrome"
}
```