core(emulation): refactor emulation settings & CLI flags

clients/devtools-entry.js

@@ -9,7 +9,7 @@ const lighthouse = require('../lighthouse-core/index.js');
 const RawProtocol = require('../lighthouse-core/gather/connections/raw.js');
 const log = require('lighthouse-logger');
 const {registerLocaleData, lookupLocale} = require('../lighthouse-core/lib/i18n/i18n.js');
-const desktopDense4G = require('../lighthouse-core/config/constants.js').throttling.desktopDense4G;
+const constants = require('../lighthouse-core/config/constants.js');
 
 /** @typedef {import('../lighthouse-core/gather/connections/connection.js')} Connection */
 
@@ -28,9 +28,13 @@ function createConfig(categoryIDs, device) {
   /** @type {LH.SharedFlagsSettings} */
   const settings = {
     onlyCategories: categoryIDs,
+    // DevTools handles all the screen emulation
+    screenEmulation: {disabled: true},
   };
   if (device === 'desktop') {
-    settings.throttling = desktopDense4G;
+    settings.throttling = constants.throttling.desktopDense4G;
+    settings.emulatedUserAgent = constants.userAgents.desktop;
+    settings.formFactor = 'desktop';
   }
 
   return {

clients/test/lightrider-entry-test.js

@@ -80,7 +80,7 @@ describe('lightrider-entry', () => {
       const lrDevice = 'desktop';
       await lhBackground.runLighthouseInLR(mockConnection, url, {}, {lrDevice});
       const config = runStub.mock.calls[0][1].config;
-      assert.equal(config.settings.emulatedFormFactor, 'desktop');
+      assert.equal(config.settings.formFactor, 'desktop');
 
       runStub.mockRestore();
     });
@@ -94,7 +94,7 @@ describe('lightrider-entry', () => {
       const lrDevice = 'mobile';
       await lhBackground.runLighthouseInLR(mockConnection, url, {}, {lrDevice});
       const config = runStub.mock.calls[0][1].config;
-      assert.equal(config.settings.emulatedFormFactor, 'mobile');
+      assert.equal(config.settings.formFactor, 'mobile');
 
       runStub.mockRestore();
     });

docs/emulation.md

@@ -0,0 +1,32 @@
+
+# Emulation in Lighthouse
+
+In Lighthouse, "Emulation" refers to the screen/viewport emulation and UserAgent string spoofing.
+["Throttling"](./throttling.md) covers the similar topics around network and CPU throttling/simulation.
+
+With the default configuration, Lighthouse emulates a mobile device. There's [a `desktop` configuration](../lighthouse-core/config/desktop-config.js), available to CLI users with `--preset=desktop`, which applies a consistent desktop environment and scoring calibration. This is recommended as a replacement for `--emulated-form-factor=desktop`.
+
+### Advanced emulation setups
+
+Some products use Lighthouse in scenarios where emulation is applied outside of Lighthouse (e.g. by Puppeteer) or running against Chrome on real mobile devices.
+
+You must always set `formFactor`. It doesn't control emulation, but it determines how Lighthouse should interpret the run in regards to scoring performance metrics and skipping mobile-only tests in desktop.
+
+You can choose how `screenEmulation` is applied. It can accept an object of `{width: number, height: number, deviceScaleRatio: number, mobile: boolean, disabled: false}` to apply that screen emulation or an object of `{disabled: true}` if Lighthouse should avoid applying screen emulation. It's typically set to disabled if either emulation is applied outside of Lighthouse, or it's being run on a mobile device. The `mobile` boolean applies overlay scrollbars and a few other mobile-specific screen emulation characteristics.
+
+You can choose how to handle userAgent emulation. The `emulatedUserAgent` property accepts either a `string` to apply the provided userAgent or a `boolean` -- `true` if the default UA spoofing should be applied (default) or `false` if no UA spoofing should be applied. Typically `false` is used if UA spoofing is applied outside of Lighthouse or on a mobile device. You can also redundantly apply userAgent emulation with no risk.
+
+If you're using Lighthouse on a mobile device, you want to set `--screenEmulation.disabled` and `--throttling.cpuSlowdownMultiplier=1`. (`--formFactor=mobile` is the default already).
+
+### Changes made in v7
+
+In Lighthouse v7, most of the configuration regarding emulation changed to be more intuitive and clear. The [tracking issue](https://github.com/GoogleChrome/lighthouse/issues/10910
+) captures additional motivations.
+
+* Removed: The `emulatedFormFactor` property (which determined how emulation is applied).
+* Removed: The `TestedAsMobileDevice` artifact. Instead of being inferred, the  `formFactor` property is used.
+* Removed: The `internalDisableDeviceScreenEmulation` property. It's equivalent to the new `screenEmulation: false`.
+* Added: The `formFactor` property.
+* Added: The `screenEmulation` property.
+* Added: The `emulatedUserAgent` property.
+* (`throttling` and `throttlingMethod` remain unchanged)

docs/plugins.md

@@ -223,7 +223,6 @@ The following artifacts are available for use in the audits of Lighthouse plugin
 - `RuntimeExceptions`
 - `ScriptElements`
 - `Stacks`
-- `TestedAsMobileDevice`
 - `Timing`
 - `URL`
 - `ViewportDimensions`
@@ -371,6 +370,6 @@ Most artifacts will try to represent as truthfully as possible what was observed
 - [Field Performance](https://github.com/treosh/lighthouse-plugin-field-performance) - A plugin to gather and display Chrome UX Report field data
 - [AMP Plugin](https://github.com/ampproject/amp-toolbox/tree/main/packages/lighthouse-plugin-amp) - Runs pages through the AMP validator.
 - [Publisher Ads Audits](https://github.com/googleads/pub-ads-lighthouse-plugin) - a well-written, but complex, plugin
-- [Green Web Foundation](https://github.com/thegreenwebfoundation/lighthouse-plugin-greenhouse) - A plugin to see which domains run on renewable power. 
+- [Green Web Foundation](https://github.com/thegreenwebfoundation/lighthouse-plugin-greenhouse) - A plugin to see which domains run on renewable power.
 - [requests-content-md5](https://www.npmjs.com/package/lighthouse-plugin-md5) - Generates MD5 hashes from the content of network requests..
 

docs/readme.md

@@ -116,7 +116,7 @@ Alternatively, you can instruct Chrome to ignore the invalid certificate by addi
 
 Lighthouse can run against a real mobile device. You can follow the [Remote Debugging on Android (Legacy Workflow)](https://developer.chrome.com/devtools/docs/remote-debugging-legacy) up through step 3.3, but the TL;DR is install & run adb, enable USB debugging, then port forward 9222 from the device to the machine with Lighthouse.
 
-You'll likely want to use the CLI flags `--emulated-form-factor=none --throttling.cpuSlowdownMultiplier=1` to disable any additional emulation.
+You'll likely want to use the CLI flags `--screenEmulation.disabled --throttling.cpuSlowdownMultiplier=1` to disable any additional emulation.
 
 ```sh
 $ adb kill-server
@@ -128,7 +128,7 @@ $ adb devices -l
 
 $ adb forward tcp:9222 localabstract:chrome_devtools_remote
 
-$ lighthouse --port=9222 --emulated-form-factor=none --throttling.cpuSlowdownMultiplier=1 https://example.com
+$ lighthouse --port=9222 --screenEmulation.disabled --throttling.cpuSlowdownMultiplier=1 https://example.com
 ```
 
 ## Lighthouse as trace processor

docs/understanding-results.md

@@ -121,7 +121,7 @@ An object containing information about the configuration used by Lighthouse.
   },
   "gatherMode": false,
   "disableStorageReset": false,
-  "emulatedFormFactor": "mobile",
+  "formFactor": "mobile",
   "blockedUrlPatterns": null,
   "additionalTraceCategories": null,
   "extraHeaders": null,

lighthouse-cli/bin.js

@@ -96,12 +96,6 @@ async function begin() {
     cliFlags.outputPath = 'stdout';
   }
 
-  // @ts-expect-error - deprecation message for removed disableDeviceEmulation; can remove warning in v6.
-  if (cliFlags.disableDeviceEmulation) {
-    log.warn('config', 'The "--disable-device-emulation" has been removed in v5.' +
-        ' Please use "--emulated-form-factor=none" instead.');
-  }
-
   if (cliFlags.precomputedLanternDataPath) {
     const lanternDataStr = fs.readFileSync(cliFlags.precomputedLanternDataPath, 'utf8');
     /** @type {LH.PrecomputedLanternData} */

lighthouse-cli/cli-flags.js

@@ -32,8 +32,8 @@ function getFlags(manualArgv) {
           'lighthouse <url> --output=json --output-path=./report.json --save-assets',
           'Save trace, screenshots, and named JSON report.')
       .example(
-          'lighthouse <url> --emulated-form-factor=none --throttling-method=provided',
-          'Disable device emulation and all throttling')
+          'lighthouse <url> --screenEmulation.disabled --throttling-method=provided --no-emulatedUserAgent',
+          'Disable emulation and all throttling')
       .example(
           'lighthouse <url> --chrome-flags="--window-size=412,660"',
           'Launch Chrome with a specific window size')
@@ -137,9 +137,17 @@ function getFlags(manualArgv) {
           default: 'localhost',
           describe: 'The hostname to use for the debugging protocol.',
         },
-        'emulated-form-factor': {
+        'form-factor': {
           type: 'string',
-          describe: 'Controls the emulated device form factor (mobile vs. desktop) if not disabled',
+          describe: 'Determines how performance metrics are scored and if mobile-only audits are skipped. For desktop, --preset=desktop instead.',
+        },
+        'screenEmulation': {
+          describe: 'Sets screen emulation parameters. See also --preset. Use --screenEmulation.disabled to disable. Otherwise set these 4 parameters individually: --screenEmulation.mobile --screenEmulation.width=360 --screenEmulation.height=640 --screenEmulation.deviceScaleFactor=2',
+          coerce: coerceScreenEmulation,
+        },
+        'emulatedUserAgent': {
+          type: 'string',
+          describe: 'Sets useragent emulation',
         },
         'max-wait-for-load': {
           type: 'number',
@@ -185,7 +193,7 @@ function getFlags(manualArgv) {
       })
       .group([
         'save-assets', 'list-all-audits', 'list-trace-categories', 'print-config', 'additional-trace-categories',
-        'config-path', 'preset', 'chrome-flags', 'port', 'hostname', 'emulated-form-factor',
+        'config-path', 'preset', 'chrome-flags', 'port', 'hostname', 'form-factor', 'screenEmulation', 'emulatedUserAgent',
         'max-wait-for-load', 'enable-error-reporting', 'gather-mode', 'audit-mode',
         'only-audits', 'only-categories', 'skip-audits', 'budget-path',
       ], 'Configuration:')
@@ -278,9 +286,9 @@ function getFlags(manualArgv) {
       })
 
       // Choices added outside of `options()` and cast so tsc picks them up.
-      .choices('emulated-form-factor', /** @type {['mobile', 'desktop', 'none']} */ (['mobile', 'desktop', 'none']))
+      .choices('form-factor', /** @type {['mobile', 'desktop']} */ (['mobile', 'desktop']))
       .choices('throttling-method', /** @type {['devtools', 'provided', 'simulate']} */ (['devtools', 'provided', 'simulate']))
-      .choices('preset', /** @type {['perf', 'experimental']} */ (['perf', 'experimental']))
+      .choices('preset', /** @type {['perf', 'experimental', 'desktop']} */ (['perf', 'experimental', 'desktop']))
 
       .check(argv => {
         // Lighthouse doesn't need a URL if...
@@ -418,6 +426,48 @@ function coerceThrottling(value) {
   return throttlingSettings;
 }
 
+/**
+ * Take yarg's unchecked object value and ensure it is a proper LH.screenEmulationSettings.
+ * @param {unknown} value
+ * @return {LH.ScreenEmulationSettings}
+ */
+function coerceScreenEmulation(value) {
+  if (!isObjectOfUnknownValues(value)) {
+    throw new Error(`Invalid value: Argument 'screenEmulation' must be an object, specified per-property ('screenEmulation.width', 'screenEmulation.deviceScaleFactor', etc)`);
+  }
+
+  /** @type {Array<keyof LH.ScreenEmulationSettings>} */
+  const keys = ['width', 'height', 'deviceScaleFactor', 'mobile', 'disabled'];
+  /** @type {LH.ScreenEmulationSettings} */
+  const screenEmulationSettings = {};
+
+  for (const key of keys) {
+    const possibleSetting = value[key];
+    switch (key) {
+      case 'width':
+      case 'height':
+      case 'deviceScaleFactor':
+        if (possibleSetting !== undefined && typeof possibleSetting !== 'number') {
+          throw new Error(`Invalid value: 'screenEmulation.${key}' must be a number`);
+        }
+        screenEmulationSettings[key] = possibleSetting;
+
+        break;
+      case 'mobile':
+      case 'disabled':
+        if (possibleSetting !== undefined && typeof possibleSetting !== 'boolean') {
+          throw new Error(`Invalid value: 'screenEmulation.${key}' must be a boolean`);
+        }
+        screenEmulationSettings[key] = possibleSetting;
+        break;
+      default:
+        throw new Error(`Unrecognized screenEmulation option: ${key}`);
+    }
+  }
+
+  return screenEmulationSettings;
+}
+
 module.exports = {
   getFlags,
 };

lighthouse-cli/test/cli/__snapshots__/index-test.js.snap

@@ -1461,10 +1461,10 @@ Object {
     "budgets": null,
     "channel": "cli",
     "disableStorageReset": false,
-    "emulatedFormFactor": "mobile",
+    "emulatedUserAgent": "Mozilla/5.0 (Linux; Android 7.0; Moto G (4)) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/84.0.4143.7 Mobile Safari/537.36 Chrome-Lighthouse",
     "extraHeaders": null,
+    "formFactor": "mobile",
     "gatherMode": false,
-    "internalDisableDeviceScreenEmulation": false,
     "locale": "en-US",
     "maxWaitForFcp": 30000,
     "maxWaitForLoad": 45000,
@@ -1474,6 +1474,13 @@ Object {
       "html",
     ],
     "precomputedLanternData": null,
+    "screenEmulation": Object {
+      "deviceScaleFactor": 2.625,
+      "disabled": false,
+      "height": 640,
+      "mobile": true,
+      "width": 360,
+    },
     "skipAudits": null,
     "throttling": Object {
       "cpuSlowdownMultiplier": 4,
@@ -1607,10 +1614,10 @@ Object {
     "budgets": null,
     "channel": "cli",
     "disableStorageReset": false,
-    "emulatedFormFactor": "mobile",
+    "emulatedUserAgent": "Mozilla/5.0 (Linux; Android 7.0; Moto G (4)) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/84.0.4143.7 Mobile Safari/537.36 Chrome-Lighthouse",
     "extraHeaders": null,
+    "formFactor": "mobile",
     "gatherMode": false,
-    "internalDisableDeviceScreenEmulation": false,
     "locale": "en-US",
     "maxWaitForFcp": 30000,
     "maxWaitForLoad": 45000,
@@ -1622,6 +1629,13 @@ Object {
       "json",
     ],
     "precomputedLanternData": null,
+    "screenEmulation": Object {
+      "deviceScaleFactor": 2.625,
+      "disabled": false,
+      "height": 640,
+      "mobile": true,
+      "width": 360,
+    },
     "skipAudits": null,
     "throttling": Object {
       "cpuSlowdownMultiplier": 4,

lighthouse-cli/test/cli/index-test.js

@@ -95,4 +95,43 @@ describe('CLI Tests', function() {
       expect(config).toMatchSnapshot();
     });
   });
+
+  describe('preset', () => {
+    it('desktop should set appropriate config', () => {
+      const ret = spawnSync('node', [indexPath, '--print-config', '--preset=desktop'], {
+        encoding: 'utf8',
+      });
+
+      const config = JSON.parse(ret.stdout);
+      const {emulatedUserAgent, formFactor, screenEmulation, throttling, throttlingMethod} =
+        config.settings;
+      const emulationSettings =
+            {emulatedUserAgent, formFactor, screenEmulation, throttling, throttlingMethod};
+
+      /* eslint-disable max-len */
+      expect(emulationSettings).toMatchInlineSnapshot(`
+        Object {
+          "emulatedUserAgent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_14_6) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/84.0.4143.7 Safari/537.36 Chrome-Lighthouse",
+          "formFactor": "desktop",
+          "screenEmulation": Object {
+            "deviceScaleFactor": 1,
+            "disabled": false,
+            "height": 940,
+            "mobile": false,
+            "width": 1350,
+          },
+          "throttling": Object {
+            "cpuSlowdownMultiplier": 1,
+            "downloadThroughputKbps": 0,
+            "requestLatencyMs": 0,
+            "rttMs": 40,
+            "throughputKbps": 10240,
+            "uploadThroughputKbps": 0,
+          },
+          "throttlingMethod": "simulate",
+        }
+      `);
+      /* eslint-enable max-len */
+    });
+  });
 });

lighthouse-cli/test/smokehouse/test-definitions/dobetterweb/dbw-expectations.js

@@ -13,7 +13,6 @@ const expectations = [
   {
     artifacts: {
       HostFormFactor: 'desktop',
-      TestedAsMobileDevice: true,
       Stacks: [{
         id: 'jquery',
       }, {

lighthouse-cli/test/smokehouse/test-definitions/screenshot/screenshot-config.js

@@ -9,6 +9,6 @@
 module.exports = {
   extends: 'lighthouse:default',
   settings: {
-    emulatedFormFactor: 'desktop',
+    formFactor: 'desktop',
   },
 };

lighthouse-core/audits/content-width.js

@@ -38,21 +38,21 @@ class ContentWidth extends Audit {
       title: str_(UIStrings.title),
       failureTitle: str_(UIStrings.failureTitle),
       description: str_(UIStrings.description),
-      requiredArtifacts: ['ViewportDimensions', 'TestedAsMobileDevice'],
+      requiredArtifacts: ['ViewportDimensions'],
     };
   }
 
   /**
    * @param {LH.Artifacts} artifacts
+   * @param {LH.Audit.Context} context
    * @return {LH.Audit.Product}
    */
-  static audit(artifacts) {
-    const IsMobile = artifacts.TestedAsMobileDevice;
+  static audit(artifacts, context) {
     const viewportWidth = artifacts.ViewportDimensions.innerWidth;
     const windowWidth = artifacts.ViewportDimensions.outerWidth;
     const widthsMatch = viewportWidth === windowWidth;
 
-    if (!IsMobile) {
+    if (context.settings.formFactor === 'desktop') {
       return {
         score: 1,
         notApplicable: true,

lighthouse-core/audits/metrics/first-contentful-paint.js

@@ -27,7 +27,7 @@ class FirstContentfulPaint extends Audit {
       title: str_(i18n.UIStrings.firstContentfulPaintMetric),
       description: str_(UIStrings.description),
       scoreDisplayMode: Audit.SCORING_MODES.NUMERIC,
-      requiredArtifacts: ['traces', 'devtoolsLogs', 'TestedAsMobileDevice'],
+      requiredArtifacts: ['traces', 'devtoolsLogs'],
     };
   }
 
@@ -65,8 +65,7 @@ class FirstContentfulPaint extends Audit {
     const devtoolsLog = artifacts.devtoolsLogs[Audit.DEFAULT_PASS];
     const metricComputationData = {trace, devtoolsLog, settings: context.settings};
     const metricResult = await ComputedFcp.request(metricComputationData, context);
-    const isDesktop = artifacts.TestedAsMobileDevice === false;
-    const options = isDesktop ? context.options.desktop : context.options.mobile;
+    const options = context.options[context.settings.formFactor];
 
     return {
       score: Audit.computeLogNormalScore(