docs/webui/webui_in_chrome.md - chromium/src.git - Git at Google

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 # Creating WebUI Interfaces
 [TOC]

 A WebUI page is made of a single-page application, which communicates
 with a C++ UI controller, as explained [here](webui_explainer.md).

 For WebUIs that are not served on iOS, the frontend resources (TS/HTML/CSS)
 should be placed in `chrome/browser/resources` and the backend code
 (WebUIController, WebUIConfig, and any handlers) should be placed next to the
 controller logic in `chrome/browser/ui/<feature>` or `chrome/browser/<feature>`.
 In the event that the WebUI is simple and there is no non-WebUI controller
 logic, then this guidance does not apply. For legacy reasons, many features
 still have the C++ code in `chrome/browser/ui/webui/<feature>`. For more
 guidance on overall directory and features structure, see [Design
 Principles](../chrome_browser_design_principles.md).

 WebUIs that are available on iOS need to have 2 separate backend
 implementations: one for iOS in ios/, and one for all other platforms in
 `chrome/browser/ui/<feature>`. If the backend or parts of it can be shared, that
 code will live in `components/<feature>`. To allow both implementations to
 access the frontend resources and other shared code (e.g., mojo interfaces),
 frontend resources and shared code for such WebUIs should be placed in
 `components/` instead of `chrome/`. Note: some legacy WebUIs are located in
 other folders, such as `content/`. This is discouraged for new WebUIs since code
 in `content/` and other folders may not be allowed to depend on WebUI shared
 infrastructure and utilities.

 In this example, we can start by creating folders for the new page in
 `chrome/browser/resources/hello_world` and `chrome/browser/hello_world`. When
 creating WebUI resources, follow the
 [Web Development Style Guide](../../styleguide/web/web.md).

 ## Making a basic WebUI page

 For a sample WebUI page you could start with the following files:

 `chrome/browser/resources/hello_world/hello_world.html`
 ```html
 <!DOCTYPE HTML>
 <html>
   <meta charset="utf-8">
   <link rel="stylesheet" href="hello_world.css">
   <hello-world-app></hello-world-app>
   <script type="module" src="app.js"></script>
 </html>
 ```

 `chrome/browser/resources/hello_world/hello_world.css`
 ```css
 body {
   margin: 0;
 }
 ```

 `chrome/browser/resources/hello_world/app.css`
 ```css
 /* Copyright 2024 The Chromium Authors
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file. */

 /* #css_wrapper_metadata_start
  * #type=style-lit
  * #scheme=relative
  * #css_wrapper_metadata_end */

 #example-div {
   color: blue;
 }
 ```

 `chrome/browser/resources/hello_world/app.html.ts`
 ```ts
 // Copyright 2024 The Chromium Authors
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.

 import {html} from '//resources/lit/v3_0/lit.rollup.js';
 import type {HelloWorldAppElement} from './app.js';

 export function getHtml(this: HelloWorldAppElement) {
   return html`
 <h1>Hello World</h1>
 <div id="example-div">${this.message_}</div>`;
 }
 ```

 `chrome/browser/resources/hello_world/app.ts`
 ```ts
 import '/strings.m.js';

 import {loadTimeData} from 'chrome://resources/js/load_time_data.js';
 import {CrLitElement} from '//resources/lit/v3_0/lit.rollup.js';

 import {getCss} from './app.css.js';
 import {getHtml} from './app.html.js';

 export class HelloWorldAppElement extends CrLitElement {
   static get is() {
     return 'hello-world-app';
   }

   static override get styles() {
     return getCss();
   }

   override render() {
     return getHtml.bind(this)();
   }

   static override get properties() {
     return {
       message_: {type: String},
     };
   }

   protected accessor message_: string = loadTimeData.getString('message');
 }

 declare global {
   interface HTMLElementTagNameMap {
     'hello-world-app': HelloWorldAppElement;
   }
 }

 customElements.define(HelloWorldAppElement.is, HelloWorldAppElement);
 ```

 Add a `BUILD.gn` file to get TypeScript compilation and to generate the JS file
 from which the template will be imported.

 `chrome/browser/resources/hello_world/BUILD.gn`
 ```py
 import("//ui/webui/resources/tools/build_webui.gni")

 build_webui("build") {
   grd_prefix = "hello_world"

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

   ts_files = [ "app.ts", "app.html.ts" ]
   css_files = [ "app.css" ]

   # Enable the proper webui_context_type depending on whether implementing
   # a chrome:// or chrome-untrusted:// page.
   webui_context_type = "trusted"

   ts_deps = [
     "//third_party/lit/v3_0:build_ts",
     "//ui/webui/resources/js:build_ts",
   ]
 }
 ```

 > Note: See [the build config docs for more examples](webui_build_configuration.md#example-build-configurations)
 of how the build could be configured.

 Finally, create an `OWNERS` file for the new folder.

 ### Adding the resources

 The `build_webui` target in `BUILD.gn` autogenerates some targets and files
 that need to be linked from the binary-wide resource targets:

 Add the new resource target to `chrome/browser/resources/BUILD.gn`

 ```py
 group("resources") {
   public_deps += [
     ...
     "hello_world:resources"
     ...
   ]
 }
 ```

 Add an entry to resource_ids.spec

 This file is for automatically generating resource ids. Ensure that your entry
 has a unique ID and preserves numerical ordering. If you see an error like "ValueError: Cannot jump to unvisited", please check the numeric order of your resource ids.

 `tools/gritsettings/resource_ids.spec`

 ```
   # START chrome/ WebUI resources section
   ... (lots)
   "<(SHARED_INTERMEDIATE_DIR)/chrome/browser/resources/hello_world/resources.grd": {
     "META": {"sizes": {"includes": [5]}},
     "includes": [2085],
   },
 ```

 Also add to `chrome/chrome_paks.gni`

 ```py
 template("chrome_extra_paks") {
   ... (lots)
   sources += [
     ...
     "$root_gen_dir/chrome/hello_world_resources.pak",
     ...
   ]
   deps += [
     ...
     "//chrome/browser/resources/hello_world:resources",
     ...
   ]
 }
 ```

 ### Adding URL constants for the new chrome URL

 `chrome/common/webui_url_constants.cc:`
 ```c++
 const char kChromeUIHelloWorldURL[] = "chrome://hello-world/";
 const char kChromeUIHelloWorldHost[] = "hello-world";
 ```

 `chrome/common/webui_url_constants.h:`
 ```c++
 extern const char kChromeUIHelloWorldURL[];
 extern const char kChromeUIHelloWorldHost[];
 ```

 ### Adding a WebUI class for handling requests to the `chrome://hello-world/` URL

 Next we need a class to handle requests to this new resource URL. Typically this will subclass `WebUIController` (WebUI
 dialogs will also need another class which will subclass `WebDialogDelegate`, this is shown later).

 `chrome/browser/hello_world/hello_world_ui.h`
 ```c++
 #ifndef CHROME_BROWSER_HELLO_WORLD_HELLO_WORLD_UI_H_
 #define CHROME_BROWSER_HELLO_WORLD_HELLO_WORLD_UI_H_

 #include "content/public/browser/web_ui_controller.h"

 // The WebUI for chrome://hello-world
 class HelloWorldUI : public content::WebUIController {
  public:
   explicit HelloWorldUI(content::WebUI* web_ui);
   ~HelloWorldUI() override;
 };

 #endif // CHROME_BROWSER_HELLO_WORLD_HELLO_WORLD_UI_H_
 ```

 `chrome/browser/hello_world/hello_world_ui.cc`
 ```c++
 #include "chrome/browser/hello_world/hello_world_ui.h"

 #include "chrome/common/webui_url_constants.h"
 #include "content/public/browser/browser_context.h"
 #include "content/public/browser/web_contents.h"
 #include "chrome/grit/hello_world_resources.h"
 #include "chrome/grit/hello_world_resources_map.h"
 #include "content/public/browser/web_ui.h"
 #include "content/public/browser/web_ui_data_source.h"
 #include "ui/webui/webui_util.h"


 HelloWorldUI::HelloWorldUI(content::WebUI* web_ui)
     : content::WebUIController(web_ui) {
   // Set up the chrome://hello-world source.
   content::WebUIDataSource* source = content::WebUIDataSource::CreateAndAdd(
       web_ui->GetWebContents()->GetBrowserContext(),
       chrome::kChromeUIHelloWorldHost);

   // Add required resources.
   webui::SetupWebUIDataSource(source, kHelloWorldResources,
       IDR_HELLO_WORLD_HELLO_WORLD_HTML);

   // As a demonstration of passing a variable for JS to use we pass in some
   // a simple message.
   source->AddString("message", "Hello World!");
 }

 HelloWorldUI::~HelloWorldUI() = default;
 ```

 To ensure that your code actually gets compiled, you need to add a
 `BUILD.gn` file defining a target for it, and hook up this target in
 `chrome/browser/hellow_world/BUILD.gn`:

 `chrome/browser/hello_world/BUILD.gn`
 ```py
 source_set("hello_world") {
   sources = [
     "hello_world_ui.cc",
     "hello_world_ui.h",
   ]
   public_deps = [ "//content/public/browser" ]
   deps = [
     "//chrome/browser/resources/hello_world:resources",
     "//chrome/common",
     "//ui/webui",
   ]
 }
 ```

 You will also need to reference this from `//chrome/browser/BUILD.gn`

 `chrome/browser/BUILD.gn`
 ```py
 ...
   deps = [
     ... (lots)
     "//chrome/browser/hello_world:hello_world",
     ... (lots)
   ]
 ...
 ```

 ### Preferred method: Add a WebUIConfig class and put it in the WebUIConfigMap
 `WebUIConfig`s contain minimal information about the host and scheme served
 by the `WebUIController` subclass. It also can enable or disable the UI for
 different conditions (e.g. feature flag status). You can create a
 `WebUIConfig` subclass and register it in the `WebUIConfigMap` to ensure your
 request handler is instantiated and used to handle any requests to the desired
 scheme + host. If you don't need to pass any arguments to your controller
 class, inherit from `DefaultWebUIConfig` to reduce the amount of code required:

 `chrome/browser/hello_world/hello_world_ui.h`
 ```c++
 // Forward declaration so that config definition can come before controller.
 class HelloWorldUI;

 class HelloWorldUIConfig : public content::DefaultWebUIConfig<HelloWorldUI> {
  public:
   HelloWorldUIConfig()
       : DefaultWebUIConfig(content::kChromeUIScheme,
                            chrome::kChromeUIHelloWorldHost) {}
 };
 ```

 Register your config in `chrome_web_ui_configs.cc`, for trusted UIs, or
 `chrome_untrusted_web_ui_configs.cc` for untrusted UIs.

 `chrome/browser/ui/webui/chrome_web_ui_configs.cc`
 ```c++
 + #include "chrome/browser/ui/webui/hello_world/hello_world_ui.h"
 ...
 +map.AddWebUIConfig(std::make_unique<hello_world::HelloWorldUIConfig>());
 ```

 Hook up the configs target to the build target for your new UI:
 `chrome/browser/ui/webui/BUILD.gn`
 ```py
 ...
 source_set("configs") {
 ...
 # Add in platform-specific deps += section if not on all platforms.
 deps = [
   ...
   "//chrome/browser/ui/webui/hello_world",
   ...
 ]
 }
 ```

 ### Old method: Add your WebUI request handler to the Chrome WebUI factory

 The Chrome WebUI factory is another way to setup your new request handler. This
 is how many older WebUIs in Chrome are registered, since not all UIs have been
 migrated to use the newer `WebUIConfig` (see
 [migration bug](https://crbug.com/1317510)). Only use this method for a new UI
 if the approach above using `WebUIConfig` does not work, and notify WebUI
 `PLATFORM_OWNERS`.

 `chrome/browser/ui/webui/chrome_web_ui_controller_factory.cc:`
 ```c++
 + #include "chrome/browser/ui/webui/hello_world/hello_world_ui.h"
 ...
 + if (url.host() == chrome::kChromeUIHelloWorldHost)
 +   return &NewWebUI<HelloWorldUI>;
 ```

 ### Check everything works

 You're done! Assuming no errors (because everyone gets their code perfect the first time) you should be able to compile
 and run chrome and navigate to `chrome://hello-world/` and see your nifty welcome text!

 ### Registering the URL for Metrics
 To track overall usage, a core metric `WebUI.CreatedForUrl` is automatically
 recorded when a new WebUIController instance is created.

 To support this, you must add your new URL's hash to a central list located in
 [tools/metrics/histograms/metadata/ui/enums.xml][enums-xml].

 [WebUIUrlHashesBrowserTest][hashes-test] includes a check that ensures all known
 URLs are in this list. The easiest way to calculate the hash for your new WebUI
 page is to run this test and look at the error output.

 ```
 autoninja -C out/Default browser_tests && ./out/Default/browser_tests --gtest_filter="WebUIUrlHashesBrowserTest.UrlsInHistogram"
 ```

 The test is expected to fail. The error message will provide the exact line you
 need. Copy the line and add it to `<enum name="WebUIUrlHashes">`

 [enums-xml]: https://source.chromium.org/chromium/chromium/src/+/main:tools/metrics/histograms/metadata/ui/enums.xml;drc=d31ce80b7f6c4a57ee7964be72fcfe761588c776;l=579
 [hashes-test]: https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/ui/webui/webui_url_hashes_browsertest.cc
 ## Making a WebUI Dialog

 Instead of having a full page for your WebUI, you might want a dialog in order to have a fully independent window.  To
 do that, some small changes are needed to your code.  First, we need to add a new class which inherits from
 `ui::WebDialogDelegate`.  The easiest way to do that is to edit the `hello_world_ui.*` files


 `chrome/browser/hello_world/hello_world_ui.h`
 ```c++
  // Leave the old content, but add this new code
  class HelloWorldDialog : public ui::WebDialogDelegate {
  public:
   static void Show();
   ~HelloWorldDialog() override;
   HelloWorldDialog(const HelloWorldDialog&) = delete;
   HelloWorldDialog& operator=(const HelloWorldDialog&) = delete;

  private:
   HelloWorldDialog();
   // ui::WebDialogDelegate:
   ui::mojom::ModalType GetDialogModalType() const override;
   std::u16string GetDialogTitle() const override;
   GURL GetDialogContentURL() const override;
   void GetWebUIMessageHandlers(
       std::vector<content::WebUIMessageHandler*>* handlers) const override;
   void GetDialogSize(gfx::Size* size) const override;
   std::string GetDialogArgs() const override;
   void OnDialogShown(content::WebUI* webui) override;
   void OnDialogClosed(const std::string& json_retval) override;
   void OnCloseContents(content::WebContents* source,
                        bool* out_close_dialog) override;
   bool ShouldShowDialogTitle() const override;

   content::WebUI* webui_ = nullptr;
 };
 ```

 `chrome/browser/hello_world/hello_world_ui.cc`
 ```c++
  // Leave the old content, but add this new stuff

 HelloWorldDialog::HelloWorldDialog() = default;

 void HelloWorldDialog::Show() {
   // HelloWorldDialog is self-deleting via OnDialogClosed().
   chrome::ShowWebDialog(nullptr, ProfileManager::GetActiveUserProfile(),
                         new HelloWorldDialog());
 }

 ui::mojom::ModalType HelloWorldDialog::GetDialogModalType() const {
   return ui::mojom::ModalType::kNone;
 }

 std::u16string HelloWorldDialog::GetDialogTitle() const {
   return u"Hello world";
 }

 GURL HelloWorldDialog::GetDialogContentURL() const {
   return GURL(chrome::kChromeUIHelloWorldURL[);
 }

 void HelloWorldDialog::GetWebUIMessageHandlers(
     std::vector<content::WebUIMessageHandler*>* handlers) const {}

 void HelloWorldDialog::GetDialogSize(gfx::Size* size) const {
   const int kDefaultWidth = 544;
   const int kDefaultHeight = 628;
   size->SetSize(kDefaultWidth, kDefaultHeight);
 }

 std::string HelloWorldDialog::GetDialogArgs() const {
   return "";
 }

 void HelloWorldDialog::OnDialogShown(content::WebUI* webui) {
   webui_ = webui;
 }

 void HelloWorldDialog::OnDialogClosed(const std::string& json_retval) {
   delete this;
 }

 void HelloWorldDialog::OnCloseContents(content::WebContents* source,
                                         bool* out_close_dialog) {
   *out_close_dialog = true;
 }

 bool HelloWorldDialog::ShouldShowDialogTitle() const {
   return true;
 }

 HelloWorldDialog::~HelloWorldDialog() = default;
 ```

 Finally, you will need to do something to actually show your dialog, which can be done by calling `HelloWorldDialog::Show()`.

 ## More elaborate configurations

 ### Referencing resources from another webui page
 Any code that is located in `ui/webui/resources` and served from
 `chrome://resources` and `chrome-untrusted://resources` can be used from any
 WebUI page. If you want to share some additional code from another WebUI page
 that is not in the shared resources, first see
 [Sharing Code in WebUI](./webui_code_sharing.md) to determine the best approach.

 If you determine that the code should be narrowly shared, the following
 explains how to add the narrowly shared resources to your WebUI data source.

 In the snippet below:

 ```cpp
 //...
 #include "chrome/grit/hello_world_resources.h"
 #include "chrome/grit/hello_world_resources_map.h"

 HelloWorldUI::HelloWorldUI(content::WebUI* web_ui)
     : content::WebUIController(web_ui) {
   // ...
   webui::SetupWebUIDataSource(source, kHelloWorldResources,
       IDR_HELLO_WORLD_HELLO_WORLD_CONTAINER_HTML);
 }
 ```

 `kHelloWorldResources` comes from from the imported grit-generated
 files, as configured by the build target, and reference the files listed
 in it so they can be served out of the given host name.
 For example, they would contain values like:

 ```cpp
 const webui::ResourcePath kHelloWorldResources[] = {
   {"hello_world.html", IDR_CHROME_BROWSER_RESOURCES_HELLO_WORLD_HELLO_WORLD_HTML},
   {"hello_world.css", IDR_CHROME_BROWSER_RESOURCES_HELLO_WORLD_HELLO_WORLD_CSS},
 };
 ```

 Using `WebUIDataSource::AddResourcePaths()` we can add the resources from grit
 files that are generated by limited sharing `build_webui()` targets as follows:

 ```cpp
 #include "chrome/grit/foo_shared_resources.h"
 #include "chrome/grit/bar_shared_resources.h"
 // ...
 HelloWorldUI::HelloWorldUI(content::WebUI* web_ui) {
   // ...
   // Add selected resources from foo_shared
   static constexpr webui::ResourcePath kResources[] = {
       {"foo_shared/foo_shared.css.js", IDR_FOO_SHARED_FOO_SHARED_CSS_JS},
       {"foo_shared/foo_shared_vars.css.js",
        IDR_FOO_SHARED_FOO_SHARED_VARS_CSS_JS},
   };
   source->AddResourcePaths(kResources);

   // Add all shared resources from bar_shared
   source->AddResourcePaths(kBarSharedResources);
 }
 ```

 ## Development tips

 ### Turn off optimizations

 It is suggested to turn off any optimizations during WebUI development by adding
 the following in the args.gn file:

 ```py
 optimize_webui = false
 ```

 This makes the build faster, as well as skips any minification and bundling,
 making it easier to use DevTools to view the code or add any breakpoints. Unless
 you are working on a performance specific issue, prefer this configuration for
 day to day development.

 ### Load WebUIs straight from disk (experimental)

 WebUI files are normally packed in a single `.pak` file (a custom format used in
 Chromium) during the build and are read and served from this file at runtime.
 This means that **every modification to a WebUI file requires**

 * rebuilding the `chrome` target **and**
 * relaunching the browser

 A mechanism has been recently added that allows **loading WebUI resources
 straight from disk** (available on Win, Mac, Linux), such that developers can
 iterate faster by **not having to do the above steps**. In order to use this
 flow, follow the steps below:

 **Step 1:** Add the following in your args.gn file and build the `chrome` target.
 ```py
 optimize_webui = false # explained in previous section
 load_webui_from_disk = true
 ```
 **Step 2:** Launch Chrome with the `--load-webui-from-disk` command line flag.

 **Step 3:** Make a change in a `.ts`, `.css` or `.html` file in the src/
 folder and build only the corresponding `:build_ts` target, for example
 ```
 autoninja -C out/Default/ chrome/browser/resources/settings:build_ts
 ```
 **Step 4:** Refresh the WebUI page. **It should use the latest contents.**

 You can now repeat steps 3-4 to quickly iterate, as many times as needed.

 Notes:

 * The above requires that `build_webui()` is used to build the WebUI and
   `webui::AddResourcePaths()` or `webui::SetupWebUIDataSource()` is used to
   register the WebUI's resources C++ side.
 * Removing the runtime flag will revert back to normal loading from the
   `.pak` file (even without rebuilding `chrome`)
 * Future improvements on this flow might eliminate the need for building
   `:build_ts` by automatically monitoring for file modifications and triggering
   it automatically, as well as make it faster by leveraging TypeScript's
   `noCheck` option. Follow [crbug.com/384636724](https://issues.chromium.org/issues/384636724)
   for updates.
	<style>
	.note::before {
	content: 'Note: ';
	font-variant: small-caps;
	font-style: italic;
	}

	.doc h1 {
	margin: 0;
	}
	</style>

	# Creating WebUI Interfaces
	[TOC]

	A WebUI page is made of a single-page application, which communicates
	with a C++ UI controller, as explained [here](webui_explainer.md).

	For WebUIs that are not served on iOS, the frontend resources (TS/HTML/CSS)
	should be placed in `chrome/browser/resources` and the backend code
	(WebUIController, WebUIConfig, and any handlers) should be placed next to the
	controller logic in `chrome/browser/ui/<feature>` or `chrome/browser/<feature>`.
	In the event that the WebUI is simple and there is no non-WebUI controller
	logic, then this guidance does not apply. For legacy reasons, many features
	still have the C++ code in `chrome/browser/ui/webui/<feature>`. For more
	guidance on overall directory and features structure, see [Design
	Principles](../chrome_browser_design_principles.md).

	WebUIs that are available on iOS need to have 2 separate backend
	implementations: one for iOS in ios/, and one for all other platforms in
	`chrome/browser/ui/<feature>`. If the backend or parts of it can be shared, that
	code will live in `components/<feature>`. To allow both implementations to
	access the frontend resources and other shared code (e.g., mojo interfaces),
	frontend resources and shared code for such WebUIs should be placed in
	`components/` instead of `chrome/`. Note: some legacy WebUIs are located in
	other folders, such as `content/`. This is discouraged for new WebUIs since code
	in `content/` and other folders may not be allowed to depend on WebUI shared
	infrastructure and utilities.

	In this example, we can start by creating folders for the new page in
	`chrome/browser/resources/hello_world` and `chrome/browser/hello_world`. When
	creating WebUI resources, follow the
	[Web Development Style Guide](../../styleguide/web/web.md).

	## Making a basic WebUI page

	For a sample WebUI page you could start with the following files:

	`chrome/browser/resources/hello_world/hello_world.html`
	```html
	<!DOCTYPE HTML>
	<html>
	<meta charset="utf-8">
	<link rel="stylesheet" href="hello_world.css">
	<hello-world-app></hello-world-app>
	<script type="module" src="app.js"></script>
	</html>
	```

	`chrome/browser/resources/hello_world/hello_world.css`
	```css
	body {
	margin: 0;
	}
	```

	`chrome/browser/resources/hello_world/app.css`
	```css
	/* Copyright 2024 The Chromium Authors
	* Use of this source code is governed by a BSD-style license that can be
	* found in the LICENSE file. */

	/* #css_wrapper_metadata_start
	* #type=style-lit
	* #scheme=relative
	* #css_wrapper_metadata_end */

	#example-div {
	color: blue;
	}
	```

	`chrome/browser/resources/hello_world/app.html.ts`
	```ts
	// Copyright 2024 The Chromium Authors
	// Use of this source code is governed by a BSD-style license that can be
	// found in the LICENSE file.

	import {html} from '//resources/lit/v3_0/lit.rollup.js';
	import type {HelloWorldAppElement} from './app.js';

	export function getHtml(this: HelloWorldAppElement) {
	return html`
	<h1>Hello World</h1>
	<div id="example-div">${this.message_}</div>`;
	}
	```

	`chrome/browser/resources/hello_world/app.ts`
	```ts
	import '/strings.m.js';

	import {loadTimeData} from 'chrome://resources/js/load_time_data.js';
	import {CrLitElement} from '//resources/lit/v3_0/lit.rollup.js';

	import {getCss} from './app.css.js';
	import {getHtml} from './app.html.js';

	export class HelloWorldAppElement extends CrLitElement {
	static get is() {
	return 'hello-world-app';
	}

	static override get styles() {
	return getCss();
	}

	override render() {
	return getHtml.bind(this)();
	}

	static override get properties() {
	return {
	message_: {type: String},
	};
	}

	protected accessor message_: string = loadTimeData.getString('message');
	}

	declare global {
	interface HTMLElementTagNameMap {
	'hello-world-app': HelloWorldAppElement;
	}
	}

	customElements.define(HelloWorldAppElement.is, HelloWorldAppElement);
	```

	Add a `BUILD.gn` file to get TypeScript compilation and to generate the JS file
	from which the template will be imported.

	`chrome/browser/resources/hello_world/BUILD.gn`
	```py
	import("//ui/webui/resources/tools/build_webui.gni")

	build_webui("build") {
	grd_prefix = "hello_world"

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

	ts_files = [ "app.ts", "app.html.ts" ]
	css_files = [ "app.css" ]

	# Enable the proper webui_context_type depending on whether implementing
	# a chrome:// or chrome-untrusted:// page.
	webui_context_type = "trusted"

	ts_deps = [
	"//third_party/lit/v3_0:build_ts",
	"//ui/webui/resources/js:build_ts",
	]
	}
	```

	> Note: See [the build config docs for more examples](webui_build_configuration.md#example-build-configurations)
	of how the build could be configured.

	Finally, create an `OWNERS` file for the new folder.

	### Adding the resources

	The `build_webui` target in `BUILD.gn` autogenerates some targets and files
	that need to be linked from the binary-wide resource targets:

	Add the new resource target to `chrome/browser/resources/BUILD.gn`

	```py
	group("resources") {
	public_deps += [
	...
	"hello_world:resources"
	...
	]
	}
	```

	Add an entry to resource_ids.spec

	This file is for automatically generating resource ids. Ensure that your entry
	has a unique ID and preserves numerical ordering. If you see an error like "ValueError: Cannot jump to unvisited", please check the numeric order of your resource ids.

	`tools/gritsettings/resource_ids.spec`

	```
	# START chrome/ WebUI resources section
	... (lots)
	"<(SHARED_INTERMEDIATE_DIR)/chrome/browser/resources/hello_world/resources.grd": {
	"META": {"sizes": {"includes": [5]}},
	"includes": [2085],
	},
	```

	Also add to `chrome/chrome_paks.gni`

	```py
	template("chrome_extra_paks") {
	... (lots)
	sources += [
	...
	"$root_gen_dir/chrome/hello_world_resources.pak",
	...
	]
	deps += [
	...
	"//chrome/browser/resources/hello_world:resources",
	...
	]
	}
	```

	### Adding URL constants for the new chrome URL

	`chrome/common/webui_url_constants.cc:`
	```c++
	const char kChromeUIHelloWorldURL[] = "chrome://hello-world/";
	const char kChromeUIHelloWorldHost[] = "hello-world";
	```

	`chrome/common/webui_url_constants.h:`
	```c++
	extern const char kChromeUIHelloWorldURL[];
	extern const char kChromeUIHelloWorldHost[];
	```

	### Adding a WebUI class for handling requests to the `chrome://hello-world/` URL

	Next we need a class to handle requests to this new resource URL. Typically this will subclass `WebUIController` (WebUI
	dialogs will also need another class which will subclass `WebDialogDelegate`, this is shown later).

	`chrome/browser/hello_world/hello_world_ui.h`
	```c++
	#ifndef CHROME_BROWSER_HELLO_WORLD_HELLO_WORLD_UI_H_
	#define CHROME_BROWSER_HELLO_WORLD_HELLO_WORLD_UI_H_

	#include "content/public/browser/web_ui_controller.h"

	// The WebUI for chrome://hello-world
	class HelloWorldUI : public content::WebUIController {
	public:
	explicit HelloWorldUI(content::WebUI* web_ui);
	~HelloWorldUI() override;
	};

	#endif // CHROME_BROWSER_HELLO_WORLD_HELLO_WORLD_UI_H_
	```

	`chrome/browser/hello_world/hello_world_ui.cc`
	```c++
	#include "chrome/browser/hello_world/hello_world_ui.h"

	#include "chrome/common/webui_url_constants.h"
	#include "content/public/browser/browser_context.h"
	#include "content/public/browser/web_contents.h"
	#include "chrome/grit/hello_world_resources.h"
	#include "chrome/grit/hello_world_resources_map.h"
	#include "content/public/browser/web_ui.h"
	#include "content/public/browser/web_ui_data_source.h"
	#include "ui/webui/webui_util.h"


	HelloWorldUI::HelloWorldUI(content::WebUI* web_ui)
	: content::WebUIController(web_ui) {
	// Set up the chrome://hello-world source.
	content::WebUIDataSource* source = content::WebUIDataSource::CreateAndAdd(
	web_ui->GetWebContents()->GetBrowserContext(),
	chrome::kChromeUIHelloWorldHost);

	// Add required resources.
	webui::SetupWebUIDataSource(source, kHelloWorldResources,
	IDR_HELLO_WORLD_HELLO_WORLD_HTML);

	// As a demonstration of passing a variable for JS to use we pass in some
	// a simple message.
	source->AddString("message", "Hello World!");
	}

	HelloWorldUI::~HelloWorldUI() = default;
	```

	To ensure that your code actually gets compiled, you need to add a
	`BUILD.gn` file defining a target for it, and hook up this target in
	`chrome/browser/hellow_world/BUILD.gn`:

	`chrome/browser/hello_world/BUILD.gn`
	```py
	source_set("hello_world") {
	sources = [
	"hello_world_ui.cc",
	"hello_world_ui.h",
	]
	public_deps = [ "//content/public/browser" ]
	deps = [
	"//chrome/browser/resources/hello_world:resources",
	"//chrome/common",
	"//ui/webui",
	]
	}
	```

	You will also need to reference this from `//chrome/browser/BUILD.gn`

	`chrome/browser/BUILD.gn`
	```py
	...
	deps = [
	... (lots)
	"//chrome/browser/hello_world:hello_world",
	... (lots)
	]
	...
	```

	### Preferred method: Add a WebUIConfig class and put it in the WebUIConfigMap
	`WebUIConfig`s contain minimal information about the host and scheme served
	by the `WebUIController` subclass. It also can enable or disable the UI for
	different conditions (e.g. feature flag status). You can create a
	`WebUIConfig` subclass and register it in the `WebUIConfigMap` to ensure your
	request handler is instantiated and used to handle any requests to the desired
	scheme + host. If you don't need to pass any arguments to your controller
	class, inherit from `DefaultWebUIConfig` to reduce the amount of code required:

	`chrome/browser/hello_world/hello_world_ui.h`
	```c++
	// Forward declaration so that config definition can come before controller.
	class HelloWorldUI;

	class HelloWorldUIConfig : public content::DefaultWebUIConfig<HelloWorldUI> {
	public:
	HelloWorldUIConfig()
	: DefaultWebUIConfig(content::kChromeUIScheme,
	chrome::kChromeUIHelloWorldHost) {}
	};
	```

	Register your config in `chrome_web_ui_configs.cc`, for trusted UIs, or
	`chrome_untrusted_web_ui_configs.cc` for untrusted UIs.

	`chrome/browser/ui/webui/chrome_web_ui_configs.cc`
	```c++
	+ #include "chrome/browser/ui/webui/hello_world/hello_world_ui.h"
	...
	+map.AddWebUIConfig(std::make_unique<hello_world::HelloWorldUIConfig>());
	```

	Hook up the configs target to the build target for your new UI:
	`chrome/browser/ui/webui/BUILD.gn`
	```py
	...
	source_set("configs") {
	...
	# Add in platform-specific deps += section if not on all platforms.
	deps = [
	...
	"//chrome/browser/ui/webui/hello_world",
	...
	]
	}
	```

	### Old method: Add your WebUI request handler to the Chrome WebUI factory

	The Chrome WebUI factory is another way to setup your new request handler. This
	is how many older WebUIs in Chrome are registered, since not all UIs have been
	migrated to use the newer `WebUIConfig` (see
	[migration bug](https://crbug.com/1317510)). Only use this method for a new UI
	if the approach above using `WebUIConfig` does not work, and notify WebUI
	`PLATFORM_OWNERS`.

	`chrome/browser/ui/webui/chrome_web_ui_controller_factory.cc:`
	```c++
	+ #include "chrome/browser/ui/webui/hello_world/hello_world_ui.h"
	...
	+ if (url.host() == chrome::kChromeUIHelloWorldHost)
	+ return &NewWebUI<HelloWorldUI>;
	```

	### Check everything works

	You're done! Assuming no errors (because everyone gets their code perfect the first time) you should be able to compile
	and run chrome and navigate to `chrome://hello-world/` and see your nifty welcome text!

	### Registering the URL for Metrics
	To track overall usage, a core metric `WebUI.CreatedForUrl` is automatically
	recorded when a new WebUIController instance is created.

	To support this, you must add your new URL's hash to a central list located in
	[tools/metrics/histograms/metadata/ui/enums.xml][enums-xml].

	[WebUIUrlHashesBrowserTest][hashes-test] includes a check that ensures all known
	URLs are in this list. The easiest way to calculate the hash for your new WebUI
	page is to run this test and look at the error output.

	```
	autoninja -C out/Default browser_tests && ./out/Default/browser_tests --gtest_filter="WebUIUrlHashesBrowserTest.UrlsInHistogram"
	```

	The test is expected to fail. The error message will provide the exact line you
	need. Copy the line and add it to `<enum name="WebUIUrlHashes">`

	[enums-xml]: https://source.chromium.org/chromium/chromium/src/+/main:tools/metrics/histograms/metadata/ui/enums.xml;drc=d31ce80b7f6c4a57ee7964be72fcfe761588c776;l=579
	[hashes-test]: https://source.chromium.org/chromium/chromium/src/+/main:chrome/browser/ui/webui/webui_url_hashes_browsertest.cc
	## Making a WebUI Dialog

	Instead of having a full page for your WebUI, you might want a dialog in order to have a fully independent window. To
	do that, some small changes are needed to your code. First, we need to add a new class which inherits from
	`ui::WebDialogDelegate`. The easiest way to do that is to edit the `hello_world_ui.*` files


	`chrome/browser/hello_world/hello_world_ui.h`
	```c++
	// Leave the old content, but add this new code
	class HelloWorldDialog : public ui::WebDialogDelegate {
	public:
	static void Show();
	~HelloWorldDialog() override;
	HelloWorldDialog(const HelloWorldDialog&) = delete;
	HelloWorldDialog& operator=(const HelloWorldDialog&) = delete;

	private:
	HelloWorldDialog();
	// ui::WebDialogDelegate:
	ui::mojom::ModalType GetDialogModalType() const override;
	std::u16string GetDialogTitle() const override;
	GURL GetDialogContentURL() const override;
	void GetWebUIMessageHandlers(
	std::vector<content::WebUIMessageHandler> handlers) const override;
	void GetDialogSize(gfx::Size* size) const override;
	std::string GetDialogArgs() const override;
	void OnDialogShown(content::WebUI* webui) override;
	void OnDialogClosed(const std::string& json_retval) override;
	void OnCloseContents(content::WebContents* source,
	bool* out_close_dialog) override;
	bool ShouldShowDialogTitle() const override;

	content::WebUI* webui_ = nullptr;
	};
	```

	`chrome/browser/hello_world/hello_world_ui.cc`
	```c++
	// Leave the old content, but add this new stuff

	HelloWorldDialog::HelloWorldDialog() = default;

	void HelloWorldDialog::Show() {
	// HelloWorldDialog is self-deleting via OnDialogClosed().
	chrome::ShowWebDialog(nullptr, ProfileManager::GetActiveUserProfile(),
	new HelloWorldDialog());
	}

	ui::mojom::ModalType HelloWorldDialog::GetDialogModalType() const {
	return ui::mojom::ModalType::kNone;
	}

	std::u16string HelloWorldDialog::GetDialogTitle() const {
	return u"Hello world";
	}

	GURL HelloWorldDialog::GetDialogContentURL() const {
	return GURL(chrome::kChromeUIHelloWorldURL[);
	}

	void HelloWorldDialog::GetWebUIMessageHandlers(
	std::vector<content::WebUIMessageHandler> handlers) const {}

	void HelloWorldDialog::GetDialogSize(gfx::Size* size) const {
	const int kDefaultWidth = 544;
	const int kDefaultHeight = 628;
	size->SetSize(kDefaultWidth, kDefaultHeight);
	}

	std::string HelloWorldDialog::GetDialogArgs() const {
	return "";
	}

	void HelloWorldDialog::OnDialogShown(content::WebUI* webui) {
	webui_ = webui;
	}

	void HelloWorldDialog::OnDialogClosed(const std::string& json_retval) {
	delete this;
	}

	void HelloWorldDialog::OnCloseContents(content::WebContents* source,
	bool* out_close_dialog) {
	*out_close_dialog = true;
	}

	bool HelloWorldDialog::ShouldShowDialogTitle() const {
	return true;
	}

	HelloWorldDialog::~HelloWorldDialog() = default;
	```

	Finally, you will need to do something to actually show your dialog, which can be done by calling `HelloWorldDialog::Show()`.

	## More elaborate configurations

	### Referencing resources from another webui page
	Any code that is located in `ui/webui/resources` and served from
	`chrome://resources` and `chrome-untrusted://resources` can be used from any
	WebUI page. If you want to share some additional code from another WebUI page
	that is not in the shared resources, first see
	[Sharing Code in WebUI](./webui_code_sharing.md) to determine the best approach.

	If you determine that the code should be narrowly shared, the following
	explains how to add the narrowly shared resources to your WebUI data source.

	In the snippet below:

	```cpp
	//...
	#include "chrome/grit/hello_world_resources.h"
	#include "chrome/grit/hello_world_resources_map.h"

	HelloWorldUI::HelloWorldUI(content::WebUI* web_ui)
	: content::WebUIController(web_ui) {
	// ...
	webui::SetupWebUIDataSource(source, kHelloWorldResources,
	IDR_HELLO_WORLD_HELLO_WORLD_CONTAINER_HTML);
	}
	```

	`kHelloWorldResources` comes from from the imported grit-generated
	files, as configured by the build target, and reference the files listed
	in it so they can be served out of the given host name.
	For example, they would contain values like:

	```cpp
	const webui::ResourcePath kHelloWorldResources[] = {
	{"hello_world.html", IDR_CHROME_BROWSER_RESOURCES_HELLO_WORLD_HELLO_WORLD_HTML},
	{"hello_world.css", IDR_CHROME_BROWSER_RESOURCES_HELLO_WORLD_HELLO_WORLD_CSS},
	};
	```

	Using `WebUIDataSource::AddResourcePaths()` we can add the resources from grit
	files that are generated by limited sharing `build_webui()` targets as follows:

	```cpp
	#include "chrome/grit/foo_shared_resources.h"
	#include "chrome/grit/bar_shared_resources.h"
	// ...
	HelloWorldUI::HelloWorldUI(content::WebUI* web_ui) {
	// ...
	// Add selected resources from foo_shared
	static constexpr webui::ResourcePath kResources[] = {
	{"foo_shared/foo_shared.css.js", IDR_FOO_SHARED_FOO_SHARED_CSS_JS},
	{"foo_shared/foo_shared_vars.css.js",
	IDR_FOO_SHARED_FOO_SHARED_VARS_CSS_JS},
	};
	source->AddResourcePaths(kResources);

	// Add all shared resources from bar_shared
	source->AddResourcePaths(kBarSharedResources);
	}
	```

	## Development tips

	### Turn off optimizations

	It is suggested to turn off any optimizations during WebUI development by adding
	the following in the args.gn file:

	```py
	optimize_webui = false
	```

	This makes the build faster, as well as skips any minification and bundling,
	making it easier to use DevTools to view the code or add any breakpoints. Unless
	you are working on a performance specific issue, prefer this configuration for
	day to day development.

	### Load WebUIs straight from disk (experimental)

	WebUI files are normally packed in a single `.pak` file (a custom format used in
	Chromium) during the build and are read and served from this file at runtime.
	This means that every modification to a WebUI file requires

	* rebuilding the `chrome` target and
	* relaunching the browser

	A mechanism has been recently added that allows **loading WebUI resources
	straight from disk** (available on Win, Mac, Linux), such that developers can
	iterate faster by not having to do the above steps. In order to use this
	flow, follow the steps below:

	Step 1: Add the following in your args.gn file and build the `chrome` target.
	```py
	optimize_webui = false # explained in previous section
	load_webui_from_disk = true
	```
	Step 2: Launch Chrome with the `--load-webui-from-disk` command line flag.

	Step 3: Make a change in a `.ts`, `.css` or `.html` file in the src/
	folder and build only the corresponding `:build_ts` target, for example
	```
	autoninja -C out/Default/ chrome/browser/resources/settings:build_ts
	```
	Step 4: Refresh the WebUI page. It should use the latest contents.

	You can now repeat steps 3-4 to quickly iterate, as many times as needed.

	Notes:

	* The above requires that `build_webui()` is used to build the WebUI and
	`webui::AddResourcePaths()` or `webui::SetupWebUIDataSource()` is used to
	register the WebUI's resources C++ side.
	* Removing the runtime flag will revert back to normal loading from the
	`.pak` file (even without rebuilding `chrome`)
	* Future improvements on this flow might eliminate the need for building
	`:build_ts` by automatically monitoring for file modifications and triggering
	it automatically, as well as make it faster by leveraging TypeScript's
	`noCheck` option. Follow [crbug.com/384636724](https://issues.chromium.org/issues/384636724)
	for updates.