[tool] Add optional swift-format support (#5204)

Adds support for swift-format in the `format` command. For now this is optional, so is only triggered if the flag is explictly passed. In the future, once we have CI support for swift-format, we will likely want to make it on-by-default as with the other formatters.

Part of https://github.com/flutter/flutter/issues/41129

script/tool/lib/src/format_command.dart

@@ -31,6 +31,7 @@ const int _exitFlutterFormatFailed = 4;
 const int _exitJavaFormatFailed = 5;
 const int _exitGitFailed = 6;
 const int _exitDependencyMissing = 7;
+const int _exitSwiftFormatFailed = 8;
 
 final Uri _googleFormatterUrl = Uri.https('github.com',
     '/google/google-java-format/releases/download/google-java-format-1.3/google-java-format-1.3-all-deps.jar');
@@ -44,18 +45,25 @@ class FormatCommand extends PackageCommand {
     super.platform,
   }) {
     argParser.addFlag('fail-on-change', hide: true);
-    argParser.addOption('clang-format',
+    argParser.addOption(_clangFormatArg,
         defaultsTo: 'clang-format', help: 'Path to "clang-format" executable.');
-    argParser.addOption('java',
+    argParser.addOption(_javaArg,
         defaultsTo: 'java', help: 'Path to "java" executable.');
+    argParser.addOption(_swiftFormatArg,
+        help: 'Path to "swift-format" executable.');
   }
 
+  static const String _clangFormatArg = 'clang-format';
+  static const String _javaArg = 'java';
+  static const String _swiftFormatArg = 'swift-format';
+
   @override
   final String name = 'format';
 
   @override
   final String description =
-      'Formats the code of all packages (Java, Objective-C, C++, and Dart).\n\n'
+      'Formats the code of all packages (Java, Objective-C, C++, Dart, and '
+      'optionally Swift).\n\n'
       'This command requires "git", "flutter" and "clang-format" v5 to be in '
       'your path.';
 
@@ -71,6 +79,10 @@ class FormatCommand extends PackageCommand {
     await _formatDart(files);
     await _formatJava(files, googleFormatterPath);
     await _formatCppAndObjectiveC(files);
+    final String? swiftFormat = getNullableStringArg(_swiftFormatArg);
+    if (swiftFormat != null) {
+      await _formatSwift(swiftFormat, files);
+    }
 
     if (getBoolArg('fail-on-change')) {
       final bool modified = await _didModifyAnything();
@@ -141,10 +153,24 @@ class FormatCommand extends PackageCommand {
     }
   }
 
+  Future<void> _formatSwift(String swiftFormat, Iterable<String> files) async {
+    final Iterable<String> swiftFiles =
+        _getPathsWithExtensions(files, <String>{'.swift'});
+    if (swiftFiles.isNotEmpty) {
+      print('Formatting .swift files...');
+      final int exitCode =
+          await _runBatched(swiftFormat, <String>['-i'], files: swiftFiles);
+      if (exitCode != 0) {
+        printError('Failed to format Swift files: exit code $exitCode.');
+        throw ToolExit(_exitSwiftFormatFailed);
+      }
+    }
+  }
+
   Future<String> _findValidClangFormat() async {
-    final String clangFormatArg = getStringArg('clang-format');
-    if (await _hasDependency(clangFormatArg)) {
-      return clangFormatArg;
+    final String clangFormat = getStringArg(_clangFormatArg);
+    if (await _hasDependency(clangFormat)) {
+      return clangFormat;
     }
 
     // There is a known issue where "chromium/depot_tools/clang-format"

script/tool/test/format_command_test.dart

@@ -428,6 +428,71 @@ void main() {
         ]));
   });
 
+  group('swift-format', () {
+    test('formats Swift if --swift-format flag is provided', () async {
+      const List<String> files = <String>[
+        'macos/foo.swift',
+      ];
+      final RepositoryPackage plugin = createFakePlugin(
+        'a_plugin',
+        packagesDir,
+        extraFiles: files,
+      );
+
+      await runCapturingPrint(
+          runner, <String>['format', '--swift-format=/path/to/swift-format']);
+
+      expect(
+          processRunner.recordedCalls,
+          orderedEquals(<ProcessCall>[
+            ProcessCall(
+                '/path/to/swift-format',
+                <String>['-i', ...getPackagesDirRelativePaths(plugin, files)],
+                packagesDir.path),
+          ]));
+    });
+
+    test('skips Swift if --swift-format flag is not provided', () async {
+      const List<String> files = <String>[
+        'macos/foo.swift',
+      ];
+      createFakePlugin(
+        'a_plugin',
+        packagesDir,
+        extraFiles: files,
+      );
+
+      await runCapturingPrint(runner, <String>['format']);
+
+      expect(processRunner.recordedCalls, orderedEquals(<ProcessCall>[]));
+    });
+
+    test('fails if swift-format fails', () async {
+      const List<String> files = <String>[
+        'macos/foo.swift',
+      ];
+      createFakePlugin('a_plugin', packagesDir, extraFiles: files);
+
+      processRunner.mockProcessesForExecutable['swift-format'] =
+          <FakeProcessInfo>[
+        FakeProcessInfo(MockProcess(exitCode: 1), <String>['-i']),
+      ];
+      Error? commandError;
+      final List<String> output = await runCapturingPrint(
+          runner, <String>['format', '--swift-format=swift-format'],
+          errorHandler: (Error e) {
+        commandError = e;
+      });
+
+      expect(commandError, isA<ToolExit>());
+      expect(
+          output,
+          containsAllInOrder(<Matcher>[
+            contains('Failed to format Swift files: exit code 1.'),
+          ]));
+    });
+  });
+
   test('skips known non-repo files', () async {
     const List<String> skipFiles = <String>[
       '/example/build/SomeFramework.framework/Headers/SomeFramework.h',