Refactor `external_ui` without making any name changes (I think) (#142192)

Work towards https://github.com/flutter/flutter/issues/142178.

---

This PR makes no _behavioral_ changes to executed code, and instead
focuses on organization and naming:

1. Extended the README to explain the intent of the test, as well as how
to run it
1. Renamed `main.dart` and `main_test.dart` to `frame_rate_main.dart`
and `frame_rate_test.dart` (we'll add more)
1. Did some refactoring of the test to make it more obvious what is
being asserted (i.e. `widgetBuilds` and friends)

dev/devicelab/bin/tasks/external_ui_integration_test.dart

@@ -8,5 +8,5 @@ import 'package:flutter_devicelab/tasks/integration_tests.dart';
 
 Future<void> main() async {
   deviceOperatingSystem = DeviceOperatingSystem.android;
-  await task(createExternalUiIntegrationTest());
+  await task(createExternalUiFrameRateIntegrationTest());
 }

dev/devicelab/bin/tasks/external_ui_integration_test_ios.dart

@@ -8,5 +8,5 @@ import 'package:flutter_devicelab/tasks/integration_tests.dart';
 
 Future<void> main() async {
   deviceOperatingSystem = DeviceOperatingSystem.ios;
-  await task(createExternalUiIntegrationTest());
+  await task(createExternalUiFrameRateIntegrationTest());
 }

dev/devicelab/lib/tasks/integration_tests.dart

@@ -40,10 +40,10 @@ TaskFunction createIntegrationTestFlavorsTest({Map<String, String>? environment}
   ).call;
 }
 
-TaskFunction createExternalUiIntegrationTest() {
+TaskFunction createExternalUiFrameRateIntegrationTest() {
   return DriverTest(
     '${flutterDirectory.path}/dev/integration_tests/external_ui',
-    'lib/main.dart',
+    'lib/frame_rate_main.dart',
   ).call;
 }
 

dev/integration_tests/external_ui/README.md

-# external_ui
+# external_textures
 
-A Flutter project for testing external texture rendering.
+Tests external texture rendering between a native[^1] platform and Flutter.
+
+Part of Flutter's API for [plugins](https://flutter.dev/docs/development/packages-and-plugins/developing-packages#plugin) includes passing _external textures_, or textures
+created outside of Flutter, to Flutter, typically using the [`Texture`][texture]
+widget. This is useful for plugins that render video, or for plugins that
+interact with the camera.
+
+For example:
+
+- [`packages/camera`][camera]
+- [`packages/video_player`][video_player]
+
+[texture]: https://api.flutter.dev/flutter/widgets/Texture-class.html
+[camera]: https://github.com/flutter/packages/tree/8255fbed74465425a1ec06a1804225e705e29f52/packages/camera
+[video_player]: https://github.com/flutter/packages/tree/8255fbed74465425a1ec06a1804225e705e29f52/packages/video_player
+
+Because external textures are created outside of Flutter, there is often subtle
+translation that needs to happen between the native platform and Flutter, which
+is hard to observe. These integration tests are designed to help catch these
+subtle translation issues.
+
+## How it works
+
+- Each `lib/*_main.dart` file is a Flutter app instrumenting a test case.
+- There is a cooresponding `test_driver/*_test.dart` that runs assertions.
+
+To run the test cases locally, use `flutter drive`[^2]:
+
+```shell
+flutter drive lib/frame_rate_main.dart --driver test_driver/frame_rate_test.dart
+```
+
+> [!TIP]
+> On CI, the test cases are run within our [device lab](../../devicelab/README.md).
+>
+> See [`devicelab/lib/tasks/integration_tests.dart`](../../devicelab/lib/tasks/integration_tests.dart)
+> and search for `createExternalUiFrameRateIntegrationTest`.
+>
+> The actual tests are run by task runners:
+>
+> - [Android](../../devicelab/bin/tasks/external_ui_integration_test.dart)
+> - [iOS](../../devicelab/bin/tasks/external_ui_integration_test_ios.dart)
+
+[^1]: Only iOS and Android.
+[^2]: Unfortunately documentation is quite limited. See [#142021](https://github.com/flutter/flutter/issues/142021).

dev/integration_tests/external_ui/test_driver/frame_rate_main_test.dart

+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:async';
+
+import 'package:flutter_driver/flutter_driver.dart';
+import 'package:test/test.dart' hide TypeMatcher, isInstanceOf;
+
+final RegExp _calibrationRegExp = RegExp('Flutter frame rate is (.*)fps');
+final RegExp _statsRegExp = RegExp('Produced: (.*)fps\nConsumed: (.*)fps\nWidget builds: (.*)');
+const Duration _samplingTime = Duration(seconds: 8);
+
+Future<void> main() async {
+  late final FlutterDriver driver;
+
+  setUpAll(() async {
+    driver = await FlutterDriver.connect();
+  });
+
+  tearDownAll(() async {
+    await driver.close();
+  });
+
+  // Verifies we consume texture frames at a rate close to the minimum of the
+  // rate at which they are produced and Flutter's frame rate. In addition,
+  // it verifies that widget builds are not triggered by external texture
+  // frames.
+  test('renders frames from the device at a rate similar to the frames produced', () async {
+    final SerializableFinder fab = find.byValueKey('fab');
+    final SerializableFinder summary = find.byValueKey('summary');
+
+    // Wait for calibration to complete and fab to appear.
+    await driver.waitFor(fab);
+
+    final String calibrationResult = await driver.getText(summary);
+    final Match? matchCalibration = _calibrationRegExp.matchAsPrefix(calibrationResult);
+    expect(matchCalibration, isNotNull);
+    final double flutterFrameRate = double.parse(matchCalibration?.group(1) ?? '0');
+
+    // Texture frame stats at 0.5x Flutter frame rate
+    await driver.tap(fab);
+    await Future<void>.delayed(_samplingTime);
+    await driver.tap(fab);
+
+    final String statsSlow = await driver.getText(summary);
+    final Match matchSlow = _statsRegExp.matchAsPrefix(statsSlow)!;
+    expect(matchSlow, isNotNull);
+
+    double framesProduced = double.parse(matchSlow.group(1)!);
+    expect(framesProduced, closeTo(flutterFrameRate / 2.0, 5.0));
+    double framesConsumed = double.parse(matchSlow.group(2)!);
+    expect(framesConsumed, closeTo(flutterFrameRate / 2.0, 5.0));
+    int widgetBuilds = int.parse(matchSlow.group(3)!);
+    expect(widgetBuilds, 1);
+
+    // Texture frame stats at 2.0x Flutter frame rate
+    await driver.tap(fab);
+    await Future<void>.delayed(_samplingTime);
+    await driver.tap(fab);
+
+    final String statsFast = await driver.getText(summary);
+    final Match matchFast = _statsRegExp.matchAsPrefix(statsFast)!;
+    expect(matchFast, isNotNull);
+
+    framesProduced = double.parse(matchFast.group(1)!);
+    expect(framesProduced, closeTo(flutterFrameRate * 2.0, 5.0));
+    framesConsumed = double.parse(matchFast.group(2)!);
+    expect(framesConsumed, closeTo(flutterFrameRate, 10.0));
+    widgetBuilds = int.parse(matchSlow.group(3)!);
+    expect(widgetBuilds, 1);
+  }, timeout: Timeout.none);
+}

dev/integration_tests/external_ui/test_driver/main_test.dart

-// Copyright 2014 The Flutter Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-import 'dart:async';
-
-import 'package:flutter_driver/flutter_driver.dart';
-import 'package:test/test.dart' hide TypeMatcher, isInstanceOf;
-
-final RegExp calibrationRegExp = RegExp('Flutter frame rate is (.*)fps');
-final RegExp statsRegExp = RegExp('Produced: (.*)fps\nConsumed: (.*)fps\nWidget builds: (.*)');
-const Duration samplingTime = Duration(seconds: 8);
-
-Future<void> main() async {
-  group('texture suite', () {
-    late FlutterDriver driver;
-
-    setUpAll(() async {
-      driver = await FlutterDriver.connect();
-    });
-
-    // This test verifies that we can consume texture frames at a rate
-    // close to the minimum of the rate at which they are produced
-    // and Flutter's frame rate. It also verifies that we do not rebuild the
-    // Widget tree during texture consumption. The test starts by measuring
-    // Flutter's frame rate.
-    test('texture rendering', () async {
-      final SerializableFinder fab = find.byValueKey('fab');
-      final SerializableFinder summary = find.byValueKey('summary');
-
-      // Wait for calibration to complete and fab to appear.
-      await driver.waitFor(fab);
-
-      final String calibrationResult = await driver.getText(summary);
-      final Match? matchCalibration = calibrationRegExp.matchAsPrefix(calibrationResult);
-      expect(matchCalibration, isNotNull);
-      final double flutterFrameRate = double.parse(matchCalibration?.group(1) ?? '0');
-
-      // Texture frame stats at 0.5x Flutter frame rate
-      await driver.tap(fab);
-      await Future<void>.delayed(samplingTime);
-      await driver.tap(fab);
-
-      final String statsSlow = await driver.getText(summary);
-      final Match matchSlow = statsRegExp.matchAsPrefix(statsSlow)!;
-      expect(matchSlow, isNotNull);
-      expect(double.parse(matchSlow.group(1)!), closeTo(flutterFrameRate / 2.0, 5.0));
-      expect(double.parse(matchSlow.group(2)!), closeTo(flutterFrameRate / 2.0, 5.0));
-      expect(int.parse(matchSlow.group(3)!), 1);
-
-      // Texture frame stats at 2.0x Flutter frame rate
-      await driver.tap(fab);
-      await Future<void>.delayed(samplingTime);
-      await driver.tap(fab);
-
-      final String statsFast = await driver.getText(summary);
-      final Match matchFast = statsRegExp.matchAsPrefix(statsFast)!;
-      expect(matchFast, isNotNull);
-      expect(double.parse(matchFast.group(1)!), closeTo(flutterFrameRate * 2.0, 5.0));
-      expect(double.parse(matchFast.group(2)!), closeTo(flutterFrameRate, 10.0));
-      expect(int.parse(matchFast.group(3)!), 1);
-    }, timeout: Timeout.none);
-
-    tearDownAll(() async {
-      driver.close();
-    });
-  });
-}