implement SkPath.computeMetrics (#18667)

* implement SkPath.computeMetrics

lib/web_ui/build.canvaskit.yaml

+# Build configuration used by CanvasKit tests. When building for CanvasKit
+# we must pass FLUTTER_WEB_USE_SKIA=true to dart2js.
+#
+# See also `build.html.yaml`.
+targets:
+  $default:
+    builders:
+      build_web_compilers|entrypoint:
+        options:
+          compiler: dart2js
+          dart2js_args:
+            - --no-minify
+            - --enable-asserts
+            - -DFLUTTER_WEB_USE_SKIA=true
+        generate_for:
+          include:
+            - test/canvaskit/**.dart

lib/web_ui/build.yaml → lib/web_ui/build.html.yaml

+# Build configuration used by HTML (non-CanvasKit) tests.
+#
+# See also `build.canvaskit.yaml`.
 targets:
   $default:
     builders:
@@ -12,4 +15,4 @@ targets:
             - test/**.dart
           exclude:
             - test/**vm_test.dart
-
+            - test/canvaskit/**.dart

lib/web_ui/dev/environment.dart

@@ -13,6 +13,7 @@ Environment get environment {
   _environment ??= Environment();
   return _environment;
 }
+
 Environment _environment;
 
 /// Contains various environment variables, such as common file paths and command-line options.
@@ -20,14 +21,26 @@ class Environment {
   factory Environment() {
     final io.File self = io.File.fromUri(io.Platform.script);
     final io.Directory engineSrcDir = self.parent.parent.parent.parent.parent;
-    final io.Directory engineToolsDir = io.Directory(pathlib.join(engineSrcDir.path, 'flutter', 'tools'));
-    final io.Directory outDir = io.Directory(pathlib.join(engineSrcDir.path, 'out'));
-    final io.Directory hostDebugUnoptDir = io.Directory(pathlib.join(outDir.path, 'host_debug_unopt'));
-    final io.Directory dartSdkDir = io.Directory(pathlib.join(hostDebugUnoptDir.path, 'dart-sdk'));
-    final io.Directory webUiRootDir = io.Directory(pathlib.join(engineSrcDir.path, 'flutter', 'lib', 'web_ui'));
-    final io.Directory integrationTestsDir = io.Directory(pathlib.join(engineSrcDir.path, 'flutter', 'e2etests', 'web'));
-
-    for (io.Directory expectedDirectory in <io.Directory>[engineSrcDir, outDir, hostDebugUnoptDir, dartSdkDir, webUiRootDir]) {
+    final io.Directory engineToolsDir =
+        io.Directory(pathlib.join(engineSrcDir.path, 'flutter', 'tools'));
+    final io.Directory outDir =
+        io.Directory(pathlib.join(engineSrcDir.path, 'out'));
+    final io.Directory hostDebugUnoptDir =
+        io.Directory(pathlib.join(outDir.path, 'host_debug_unopt'));
+    final io.Directory dartSdkDir =
+        io.Directory(pathlib.join(hostDebugUnoptDir.path, 'dart-sdk'));
+    final io.Directory webUiRootDir = io.Directory(
+        pathlib.join(engineSrcDir.path, 'flutter', 'lib', 'web_ui'));
+    final io.Directory integrationTestsDir = io.Directory(
+        pathlib.join(engineSrcDir.path, 'flutter', 'e2etests', 'web'));
+
+    for (io.Directory expectedDirectory in <io.Directory>[
+      engineSrcDir,
+      outDir,
+      hostDebugUnoptDir,
+      dartSdkDir,
+      webUiRootDir
+    ]) {
       if (!expectedDirectory.existsSync()) {
         throw ToolException('$expectedDirectory does not exist.');
       }
@@ -89,63 +102,71 @@ class Environment {
   String get pubExecutable => pathlib.join(dartSdkDir.path, 'bin', 'pub');
 
   /// The "dart2js" executable file.
-  String get dart2jsExecutable => pathlib.join(dartSdkDir.path, 'bin', 'dart2js');
+  String get dart2jsExecutable =>
+      pathlib.join(dartSdkDir.path, 'bin', 'dart2js');
 
   /// Path to where github.com/flutter/engine is checked out inside the engine workspace.
-  io.Directory get flutterDirectory => io.Directory(pathlib.join(engineSrcDir.path, 'flutter'));
+  io.Directory get flutterDirectory =>
+      io.Directory(pathlib.join(engineSrcDir.path, 'flutter'));
   io.Directory get webSdkRootDir => io.Directory(pathlib.join(
-    flutterDirectory.path,
-    'web_sdk',
-  ));
+        flutterDirectory.path,
+        'web_sdk',
+      ));
 
   /// Path to the "web_engine_tester" package.
   io.Directory get webEngineTesterRootDir => io.Directory(pathlib.join(
-    webSdkRootDir.path,
-    'web_engine_tester',
-  ));
+        webSdkRootDir.path,
+        'web_engine_tester',
+      ));
 
   /// Path to the "build" directory, generated by "package:build_runner".
   ///
   /// This is where compiled output goes.
   io.Directory get webUiBuildDir => io.Directory(pathlib.join(
-    webUiRootDir.path,
-    'build',
-  ));
+        webUiRootDir.path,
+        'build',
+      ));
 
   /// Path to the ".dart_tool" directory, generated by various Dart tools.
   io.Directory get webUiDartToolDir => io.Directory(pathlib.join(
-    webUiRootDir.path,
-    '.dart_tool',
-  ));
-
-    /// Path to the ".dart_tool" directory living under `engine/src/flutter`.
-    ///
-    /// This is a designated area for tool downloads which can be used by
-    /// multiple platforms. For exampe: Flutter repo for e2e tests.
+        webUiRootDir.path,
+        '.dart_tool',
+      ));
+
+  /// Path to the ".dart_tool" directory living under `engine/src/flutter`.
+  ///
+  /// This is a designated area for tool downloads which can be used by
+  /// multiple platforms. For exampe: Flutter repo for e2e tests.
   io.Directory get engineDartToolDir => io.Directory(pathlib.join(
-    engineSrcDir.path,
-    'flutter',
-    '.dart_tool',
-  ));
+        engineSrcDir.path,
+        'flutter',
+        '.dart_tool',
+      ));
 
   /// Path to the "dev" directory containing engine developer tools and
   /// configuration files.
   io.Directory get webUiDevDir => io.Directory(pathlib.join(
-    webUiRootDir.path,
-    'dev',
-  ));
+        webUiRootDir.path,
+        'dev',
+      ));
+
+  /// Path to the "test" directory containing web engine tests.
+  io.Directory get webUiTestDir => io.Directory(pathlib.join(
+        webUiRootDir.path,
+        'test',
+      ));
 
   /// Path to the clone of the flutter/goldens repository.
   io.Directory get webUiGoldensRepositoryDirectory => io.Directory(pathlib.join(
-    webUiDartToolDir.path,
-    'goldens',
-  ));
+        webUiDartToolDir.path,
+        'goldens',
+      ));
 
   /// Path to the script that clones the Flutter repo.
   io.File get cloneFlutterScript => io.File(pathlib.join(
-    engineToolsDir.path,
-    'clone_flutter.sh',
-  ));
+        engineToolsDir.path,
+        'clone_flutter.sh',
+      ));
 
   /// Path to flutter.
   ///
@@ -153,10 +174,9 @@ class Environment {
   ///
   /// Only use [cloneFlutterScript] to clone flutter to the engine build.
   io.File get flutterCommand => io.File(pathlib.join(
-    engineDartToolDir.path,
-    'flutter',
-    'bin',
-    'flutter',
-  ));
-
+        engineDartToolDir.path,
+        'flutter',
+        'bin',
+        'flutter',
+      ));
 }

lib/web_ui/dev/test_runner.dart

@@ -57,17 +57,17 @@ class TestCommand extends Command<bool> with ArgUtils {
             'tests.',
       )
       ..addFlag('use-system-flutter',
-        defaultsTo: false,
-        help: 'integration tests are using flutter repository for various tasks'
-        ', such as flutter drive, flutter pub get. If this flag is set, felt '
-        'will use flutter command without cloning the repository. This flag '
-        'can save internet bandwidth. However use with caution. Note that '
-        'since flutter repo is always synced to youngest commit older than '
-        'the engine commit for the tests running in CI, the tests results '
-        'won\'t be consistent with CIs when this flag is set. flutter '
-        'command should be set in the PATH for this flag to be useful.'
-        'This flag can also be used to test local Flutter changes.'
-      )
+          defaultsTo: false,
+          help:
+              'integration tests are using flutter repository for various tasks'
+              ', such as flutter drive, flutter pub get. If this flag is set, felt '
+              'will use flutter command without cloning the repository. This flag '
+              'can save internet bandwidth. However use with caution. Note that '
+              'since flutter repo is always synced to youngest commit older than '
+              'the engine commit for the tests running in CI, the tests results '
+              'won\'t be consistent with CIs when this flag is set. flutter '
+              'command should be set in the PATH for this flag to be useful.'
+              'This flag can also be used to test local Flutter changes.')
       ..addFlag(
         'update-screenshot-goldens',
         defaultsTo: false,
@@ -159,8 +159,6 @@ class TestCommand extends Command<bool> with ArgUtils {
       await _runPubGet();
     }
 
-    await _buildTests(targets: targetFiles);
-
     // Many tabs will be left open after Safari runs, quit Safari during
     // cleanup.
     if (browser == 'safari') {
@@ -180,14 +178,57 @@ class TestCommand extends Command<bool> with ArgUtils {
       });
     }
 
+    await _buildTargets();
+
     if (runAllTests) {
-      await _runAllTests();
+      await _runAllTestsForCurrentPlatform();
     } else {
-      await _runTargetTests(targetFiles);
+      await _runSpecificTests(targetFiles);
     }
     return true;
   }
 
+  /// Builds all test targets that will be run.
+  Future<void> _buildTargets() async {
+    final Stopwatch stopwatch = Stopwatch()..start();
+    List<FilePath> allTargets;
+    if (runAllTests) {
+      allTargets = environment.webUiTestDir
+          .listSync(recursive: true)
+          .whereType<io.File>()
+          .where((io.File f) => f.path.endsWith('_test.dart'))
+          .map<FilePath>((io.File f) => FilePath.fromWebUi(
+              path.relative(f.path, from: environment.webUiRootDir.path)))
+          .toList();
+    } else {
+      allTargets = targetFiles;
+    }
+
+    // Separate HTML targets from CanvasKit targets because the two use
+    // different dart2js options (and different build.*.yaml files).
+    final List<FilePath> htmlTargets = <FilePath>[];
+    final List<FilePath> canvasKitTargets = <FilePath>[];
+    final String canvasKitTestDirectory =
+        path.join(environment.webUiTestDir.path, 'canvaskit');
+    for (FilePath target in allTargets) {
+      if (path.isWithin(canvasKitTestDirectory, target.absolute)) {
+        canvasKitTargets.add(target);
+      } else {
+        htmlTargets.add(target);
+      }
+    }
+
+    if (htmlTargets.isNotEmpty) {
+      await _buildTests(targets: htmlTargets, forCanvasKit: false);
+    }
+
+    if (canvasKitTargets.isNotEmpty) {
+      await _buildTests(targets: canvasKitTargets, forCanvasKit: true);
+    }
+    stopwatch.stop();
+    print('The build took ${stopwatch.elapsedMilliseconds ~/ 1000} seconds.');
+  }
+
   /// Whether to start the browser in debug mode.
   ///
   /// In this mode the browser pauses before running the test to allow
@@ -222,12 +263,18 @@ class TestCommand extends Command<bool> with ArgUtils {
   /// ".dart_tool/goldens".
   bool get doUpdateScreenshotGoldens => boolArg('update-screenshot-goldens');
 
-  Future<void> _runTargetTests(List<FilePath> targets) async {
+  /// Runs all tests specified in [targets].
+  ///
+  /// Unlike [_runAllTestsForCurrentPlatform], this does not filter targets
+  /// by platform/browser capabilites, and instead attempts to run all of
+  /// them.
+  Future<void> _runSpecificTests(List<FilePath> targets) async {
     await _runTestBatch(targets, concurrency: 1, expectFailure: false);
     _checkExitCode();
   }
 
-  Future<void> _runAllTests() async {
+  /// Runs as many tests as possible on the current OS/browser combination.
+  Future<void> _runAllTestsForCurrentPlatform() async {
     final io.Directory testDir = io.Directory(path.join(
       environment.webUiRootDir.path,
       'test',
@@ -373,21 +420,34 @@ class TestCommand extends Command<bool> with ArgUtils {
     timestampFile.writeAsStringSync(timestamp);
   }
 
-  Future<void> _buildTests({List<FilePath> targets}) async {
+  /// Builds the specific test [targets].
+  ///
+  /// [targets] must not be null.
+  ///
+  /// When building for CanvasKit we have to use a separate `build.canvaskit.yaml`
+  /// config file. Otherwise, `build.html.yaml` is used. Because `build_runner`
+  /// overwrites the output directories, we redirect the CanvasKit output to a
+  /// separate directory, then copy the files back to `build/test`.
+  Future<void> _buildTests({List<FilePath> targets, bool forCanvasKit}) async {
+    print(
+        'Building ${targets.length} targets for ${forCanvasKit ? 'CanvasKit' : 'HTML'}');
+    final String canvasKitOutputRelativePath =
+        path.join('.dart_tool', 'canvaskit_tests');
     List<String> arguments = <String>[
       'run',
       'build_runner',
       'build',
       'test',
       '-o',
-      'build',
-      if (targets != null)
-        for (FilePath path in targets) ...[
-          '--build-filter=${path.relativeToWebUi}.js',
-          '--build-filter=${path.relativeToWebUi}.browser_test.dart.js',
-        ],
+      forCanvasKit ? canvasKitOutputRelativePath : 'build',
+      '--config',
+      // CanvasKit uses `build.canvaskit.yaml`, which HTML Uses `build.html.yaml`.
+      forCanvasKit ? 'canvaskit' : 'html',
+      for (FilePath path in targets) ...[
+        '--build-filter=${path.relativeToWebUi}.js',
+        '--build-filter=${path.relativeToWebUi}.browser_test.dart.js',
+      ],
     ];
-    final Stopwatch stopwatch = Stopwatch()..start();
 
     final int exitCode = await runProcess(
       environment.pubExecutable,
@@ -401,16 +461,28 @@ class TestCommand extends Command<bool> with ArgUtils {
         // In a testing on a 32-core 132GB workstation increasing this number to
         // 32 sped up the build from ~4min to ~1.5min.
         if (io.Platform.environment.containsKey('BUILD_MAX_WORKERS_PER_TASK'))
-          'BUILD_MAX_WORKERS_PER_TASK': io.Platform.environment['BUILD_MAX_WORKERS_PER_TASK'],
+          'BUILD_MAX_WORKERS_PER_TASK':
+              io.Platform.environment['BUILD_MAX_WORKERS_PER_TASK'],
       },
     );
-    stopwatch.stop();
-    print('The build took ${stopwatch.elapsedMilliseconds ~/ 1000} seconds.');
 
     if (exitCode != 0) {
       throw ToolException(
           'Failed to compile tests. Compiler exited with exit code $exitCode');
     }
+
+    if (forCanvasKit) {
+      final io.Directory canvasKitTemporaryOutputDirectory = io.Directory(
+          path.join(environment.webUiRootDir.path, canvasKitOutputRelativePath,
+              'test', 'canvaskit'));
+      final io.Directory canvasKitOutputDirectory = io.Directory(
+          path.join(environment.webUiBuildDir.path, 'test', 'canvaskit'));
+      if (await canvasKitOutputDirectory.exists()) {
+        await canvasKitOutputDirectory.delete(recursive: true);
+      }
+      await canvasKitTemporaryOutputDirectory
+          .rename(canvasKitOutputDirectory.path);
+    }
   }
 
   /// Runs a batch of tests.

lib/web_ui/lib/src/engine/compositor/path.dart

@@ -11,13 +11,17 @@ part of engine;
 class SkPath implements ui.Path {
   js.JsObject _skPath;
 
+  /// Cached constructor function for `SkPath`, so we don't have to look it up
+  /// every time we construct a new path.
+  static final js.JsFunction _skPathConstructor = canvasKit['SkPath'];
+
   SkPath() {
-    _skPath = js.JsObject(canvasKit['SkPath']);
+    _skPath = js.JsObject(_skPathConstructor);
     fillType = ui.PathFillType.nonZero;
   }
 
   SkPath.from(SkPath other) {
-    _skPath = js.JsObject(canvasKit['SkPath'], <js.JsObject>[other._skPath]);
+    _skPath = js.JsObject(_skPathConstructor, <js.JsObject>[other._skPath]);
     fillType = other.fillType;
   }
 
@@ -323,4 +327,9 @@ class SkPath implements ui.Path {
   String toSvgString() {
     return _skPath.callMethod('toSVGString');
   }
+
+  /// Return `true` if this path contains no segments.
+  bool get isEmpty {
+    return _skPath.callMethod('isEmpty');
+  }
 }

lib/web_ui/lib/src/engine/compositor/path_metrics.dart

@@ -7,120 +7,97 @@ part of engine;
 
 class SkPathMetrics extends IterableBase<ui.PathMetric>
     implements ui.PathMetrics {
-  SkPathMetrics(SkPath path, bool forceClosed)
-      : _iterator = SkPathMetricIterator._(_SkPathMeasure(path, forceClosed));
+  SkPathMetrics(this._path, this._forceClosed);
 
-  final Iterator<ui.PathMetric> _iterator;
+  final SkPath _path;
+  final bool _forceClosed;
 
+  /// The [SkPath.isEmpty] case is special-cased to avoid booting the WASM machinery just to find out there are no contours.
   @override
-  Iterator<ui.PathMetric> get iterator => _iterator;
+  Iterator<ui.PathMetric> get iterator => _path.isEmpty ? const SkPathMetricIteratorEmpty._() : SkContourMeasureIter(_path, _forceClosed);
 }
 
-class SkPathMetricIterator implements Iterator<ui.PathMetric> {
-  SkPathMetricIterator._(this._pathMeasure) : assert(_pathMeasure != null);
+class SkContourMeasureIter implements Iterator<ui.PathMetric> {
+  /// Cached constructor function for `SkContourMeasureIter`, so we don't have to look it
+  /// up every time we're constructing a new instance.
+  static final js.JsFunction _skContourMeasureIterConstructor = canvasKit['SkContourMeasureIter'];
 
-  _SkPathMetric _pathMetric;
-  _SkPathMeasure _pathMeasure;
+  SkContourMeasureIter(SkPath path, bool forceClosed)
+    : _skObject = js.JsObject(_skContourMeasureIterConstructor, <dynamic>[
+        path._skPath,
+        forceClosed,
+        1,
+      ]);
+
+  /// The JavaScript `SkContourMeasureIter` object.
+  final js.JsObject _skObject;
+
+  /// A monotonically increasing counter used to generate [ui.PathMetric.contourIndex].
+  ///
+  /// CanvasKit does not supply the contour index. We have to add it ourselves.
+  int _contourIndexCounter = 0;
 
   @override
-  ui.PathMetric get current => _pathMetric;
+  ui.PathMetric get current => _current;
+  SkContourMeasure _current;
 
   @override
   bool moveNext() {
-    if (_pathMeasure._nextContour()) {
-      _pathMetric = _SkPathMetric._(_pathMeasure);
-      return true;
+    final js.JsObject skContourMeasure = _skObject.callMethod('next');
+    if (skContourMeasure == null) {
+      _current = null;
+      return false;
     }
-    _pathMetric = null;
-    return false;
+
+    _current = SkContourMeasure(_contourIndexCounter, skContourMeasure);
+    _contourIndexCounter += 1;
+    return true;
   }
 }
 
-class _SkPathMetric implements ui.PathMetric {
-  _SkPathMetric._(this._measure)
-      : assert(_measure != null),
-        length = _measure.length(_measure.currentContourIndex),
-        isClosed = _measure.isClosed(_measure.currentContourIndex),
-        contourIndex = _measure.currentContourIndex;
+class SkContourMeasure implements ui.PathMetric {
+  SkContourMeasure(this.contourIndex, this._skObject);
 
-  @override
-  final double length;
-
-  @override
-  final bool isClosed;
+  final js.JsObject _skObject;
 
   @override
   final int contourIndex;
 
-  final _SkPathMeasure _measure;
-
-  @override
-  ui.Tangent getTangentForOffset(double distance) {
-    return _measure.getTangentForOffset(contourIndex, distance);
-  }
-
   @override
   ui.Path extractPath(double start, double end, {bool startWithMoveTo = true}) {
-    return _measure.extractPath(contourIndex, start, end,
-        startWithMoveTo: startWithMoveTo);
+    final js.JsObject skPath = _skObject
+        .callMethod('getSegment', <dynamic>[start, end, startWithMoveTo]);
+    return SkPath._fromSkPath(skPath);
   }
 
   @override
-  String toString() => 'PathMetric{length: $length, isClosed: $isClosed, '
-      'contourIndex: $contourIndex}';
-}
-
-class _SkPathMeasure {
-  _SkPathMeasure(SkPath path, bool forceClosed) {
-    currentContourIndex = -1;
-    pathMeasure = js.JsObject(canvasKit['SkPathMeasure'], <dynamic>[
-      path._skPath,
-      forceClosed,
-      1,
-    ]);
-  }
-
-  js.JsObject pathMeasure;
-
-  double length(int contourIndex) {
-    assert(contourIndex == currentContourIndex,
-        'PathMetrics are invalid if it is not the current contour.');
-    return pathMeasure.callMethod('getLength');
-  }
-
-  ui.Tangent getTangentForOffset(int contourIndex, double distance) {
-    assert(contourIndex == currentContourIndex,
-        'PathMetrics are invalid if it is not the current contour.');
-    final js.JsObject posTan =
-        pathMeasure.callMethod('getPosTan', <double>[distance]);
+  ui.Tangent getTangentForOffset(double distance) {
+    final js.JsObject posTan = _skObject.callMethod('getPosTan', <double>[distance]);
     return ui.Tangent(
       ui.Offset(posTan[0], posTan[1]),
       ui.Offset(posTan[2], posTan[3]),
     );
   }
 
-  ui.Path extractPath(int contourIndex, double start, double end,
-      {bool startWithMoveTo = true}) {
-    assert(contourIndex == currentContourIndex,
-        'PathMetrics are invalid if it is not the current contour.');
-    final js.JsObject skPath = pathMeasure
-        .callMethod('getSegment', <dynamic>[start, end, startWithMoveTo]);
-    return SkPath._fromSkPath(skPath);
+  @override
+  bool get isClosed {
+    return _skObject.callMethod('isClosed');
   }
 
-  bool isClosed(int contourIndex) {
-    assert(contourIndex == currentContourIndex,
-        'PathMetrics are invalid if it is not the current contour.');
-    return pathMeasure.callMethod('isClosed');
+  @override
+  double get length {
+    return _skObject.callMethod('length');
   }
+}
 
-  bool _nextContour() {
-    final bool next = pathMeasure.callMethod('nextContour');
-    if (next) {
-      currentContourIndex++;
-    }
-    return next;
-  }
+class SkPathMetricIteratorEmpty implements Iterator<ui.PathMetric> {
+  const SkPathMetricIteratorEmpty._();
+
+  @override
+  ui.PathMetric get current => null;
 
-  int currentContourIndex;
+  @override
+  bool moveNext() {
+    return false;
+  }
 }

lib/web_ui/pubspec.yaml

@@ -17,6 +17,8 @@ dev_dependencies:
   build_runner: 1.7.2
   build_test: 1.0.0
   build_web_compilers: 2.7.1
+  # TODO(yjbanov): the Dart SDK complains about nonVirtual for some reason.
+  petitparser: 3.0.2
   yaml: 2.2.0
   watcher: 0.9.7+12
   web_engine_tester:

lib/web_ui/test/canvaskit/path_metrics_test.dart

+// Copyright 2013 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.
+
+// @dart = 2.6
+import 'package:test/test.dart';
+
+import 'package:ui/src/engine.dart';
+import 'package:ui/ui.dart' as ui;
+
+void main() {
+  setUpAll(() async {
+    await ui.webOnlyInitializePlatform();
+  });
+
+  test('Using CanvasKit', () {
+    expect(experimentalUseSkia, true);
+  });
+
+  test(SkPathMetrics, () {
+    final ui.Path path = ui.Path();
+    expect(path, isA<SkPath>());
+    expect(path.computeMetrics().length, 0);
+
+    path.addRect(ui.Rect.fromLTRB(0, 0, 10, 10));
+    final ui.PathMetric metric = path.computeMetrics().single;
+    expect(metric.contourIndex, 0);
+    expect(metric.extractPath(0, 0.5).computeMetrics().length, 1);
+
+    final ui.Tangent tangent1 = metric.getTangentForOffset(5);
+    expect(tangent1.position, ui.Offset(5, 0));
+    expect(tangent1.vector, ui.Offset(1, 0));
+
+    final ui.Tangent tangent2 = metric.getTangentForOffset(15);
+    expect(tangent2.position, ui.Offset(10, 5));
+    expect(tangent2.vector, ui.Offset(0, 1));
+
+    expect(metric.isClosed, true);
+
+    path.addOval(ui.Rect.fromLTRB(10, 10, 100, 100));
+    expect(path.computeMetrics().length, 2);
+
+    // Path metrics can be iterated over multiple times.
+    final ui.PathMetrics metrics = path.computeMetrics();
+    expect(metrics.toList().length, 2);
+    expect(metrics.toList().length, 2);
+    expect(metrics.toList().length, 2);
+
+    // Can simultaneously iterate over multiple metrics from the same path.
+    final ui.PathMetrics metrics1 = path.computeMetrics();
+    final ui.PathMetrics metrics2 = path.computeMetrics();
+    final Iterator<ui.PathMetric> iter1 = metrics1.iterator;
+    final Iterator<ui.PathMetric> iter2 = metrics2.iterator;
+    expect(iter1.moveNext(), true);
+    expect(iter2.moveNext(), true);
+    expect(iter1.current, isNotNull);
+    expect(iter2.current, isNotNull);
+    expect(iter1.moveNext(), true);
+    expect(iter2.moveNext(), true);
+    expect(iter1.current, isNotNull);
+    expect(iter2.current, isNotNull);
+    expect(iter1.moveNext(), false);
+    expect(iter2.moveNext(), false);
+    expect(iter1.current, isNull);
+    expect(iter2.current, isNull);
+  });
+}