[ Widget Preview ] Add --machine mode (#173654)

Currently only outputs a single event with information about where the
widget preview environment is served from:


`[{"event":"widget_preview.started","params":{"url":"http://localhost:61383"}}]`

Fixes https://github.com/flutter/flutter/issues/173545

packages/flutter_tools/lib/executable.dart

@@ -88,10 +88,7 @@ Future<void> main(List<String> args) async {
   final bool daemon = args.contains('daemon');
   final bool runMachine =
       (args.contains('--machine') && args.contains('run')) ||
-      (args.contains('--machine') && args.contains('attach')) ||
-      // `flutter widget-preview start` starts an application that requires a logger
-      // to be setup for machine mode.
-      (args.contains('widget-preview') && args.contains('start'));
+      (args.contains('--machine') && args.contains('attach'));
 
   // Cache.flutterRoot must be set early because other features use it (e.g.
   // enginePath's initializer uses it). This can only work with the real

packages/flutter_tools/lib/src/commands/widget_preview.dart

@@ -16,9 +16,11 @@ import '../base/logger.dart';
 import '../base/os.dart';
 import '../base/platform.dart';
 import '../base/process.dart';
+import '../base/terminal.dart';
 import '../build_info.dart';
 import '../bundle.dart' as bundle;
 import '../cache.dart';
+import '../convert.dart';
 import '../device.dart';
 import '../globals.dart' as globals;
 import '../isolated/resident_web_runner.dart';
@@ -116,7 +118,7 @@ abstract base class WidgetPreviewSubCommandBase extends FlutterCommand {
 final class WidgetPreviewStartCommand extends WidgetPreviewSubCommandBase with CreateBase {
   WidgetPreviewStartCommand({
     this.verbose = false,
-    required this.logger,
+    required Logger logger,
     required this.fs,
     required this.projectFactory,
     required this.cache,
@@ -126,11 +128,12 @@ final class WidgetPreviewStartCommand extends WidgetPreviewSubCommandBase with C
     required this.processManager,
     required this.artifacts,
     @visibleForTesting WidgetPreviewDtdServices? dtdServicesOverride,
-  }) {
+  }) : logger = WidgetPreviewMachineAwareLogger(logger) {
     if (dtdServicesOverride != null) {
       _dtdService = dtdServicesOverride;
     }
     addPubOptions();
+    addMachineOutputFlag(verboseHelp: verbose);
     argParser
       ..addFlag(
         kWebServer,
@@ -189,7 +192,7 @@ final class WidgetPreviewStartCommand extends WidgetPreviewSubCommandBase with C
   final FileSystem fs;
 
   @override
-  final Logger logger;
+  final WidgetPreviewMachineAwareLogger logger;
 
   @override
   final FlutterProjectFactory projectFactory;
@@ -255,6 +258,9 @@ final class WidgetPreviewStartCommand extends WidgetPreviewSubCommandBase with C
         ? fs.directory(customPreviewScaffoldOutput)
         : rootProject.widgetPreviewScaffold;
 
+    final bool machine = boolArg(FlutterGlobalOptions.kMachineFlag);
+    logger.machine = machine;
+
     // Check to see if a preview scaffold has already been generated. If not,
     // generate one.
     final bool generateScaffoldProject =
@@ -444,6 +450,7 @@ final class WidgetPreviewStartCommand extends WidgetPreviewSubCommandBase with C
         );
         unawaited(_widgetPreviewApp!.run(appStartedCompleter: appStarted));
         await appStarted.future;
+        logger.sendEvent('started', {'url': flutterDevice.devFS!.baseUri.toString()});
       }
     } on Exception catch (error) {
       throwToolExit(error.toString());
@@ -491,3 +498,145 @@ final class WidgetPreviewCleanCommand extends WidgetPreviewSubCommandBase {
     return FlutterCommandResult.success();
   }
 }
+
+/// A custom logger for the widget-preview commands that disables non-event output to stdio when
+/// machine mode is enabled.
+final class WidgetPreviewMachineAwareLogger extends DelegatingLogger {
+  WidgetPreviewMachineAwareLogger(super.delegate);
+
+  var machine = false;
+
+  @override
+  void printError(
+    String message, {
+    StackTrace? stackTrace,
+    bool? emphasis,
+    TerminalColor? color,
+    int? indent,
+    int? hangingIndent,
+    bool? wrap,
+  }) {
+    if (machine) {
+      return;
+    }
+    super.printError(
+      message,
+      stackTrace: stackTrace,
+      emphasis: emphasis,
+      color: color,
+      indent: indent,
+      hangingIndent: hangingIndent,
+      wrap: wrap,
+    );
+  }
+
+  @override
+  void printWarning(
+    String message, {
+    bool? emphasis,
+    TerminalColor? color,
+    int? indent,
+    int? hangingIndent,
+    bool? wrap,
+    bool fatal = true,
+  }) {
+    if (machine) {
+      return;
+    }
+    super.printWarning(
+      message,
+      emphasis: emphasis,
+      color: color,
+      indent: indent,
+      hangingIndent: hangingIndent,
+      wrap: wrap,
+      fatal: fatal,
+    );
+  }
+
+  @override
+  void printStatus(
+    String message, {
+    bool? emphasis,
+    TerminalColor? color,
+    bool? newline,
+    int? indent,
+    int? hangingIndent,
+    bool? wrap,
+  }) {
+    if (machine) {
+      return;
+    }
+    super.printStatus(
+      message,
+      emphasis: emphasis,
+      color: color,
+      newline: newline,
+      indent: indent,
+      hangingIndent: hangingIndent,
+      wrap: wrap,
+    );
+  }
+
+  @override
+  void printBox(String message, {String? title}) {
+    if (machine) {
+      return;
+    }
+    super.printBox(message, title: title);
+  }
+
+  @override
+  void printTrace(String message) {
+    if (machine) {
+      return;
+    }
+    super.printTrace(message);
+  }
+
+  @override
+  void sendEvent(String name, [Map<String, dynamic>? args]) {
+    if (!machine) {
+      return;
+    }
+    super.printStatus(
+      json.encode([
+        {'event': 'widget_preview.$name', 'params': ?args},
+      ]),
+    );
+  }
+
+  @override
+  Status startProgress(
+    String message, {
+    String? progressId,
+    int progressIndicatorPadding = kDefaultStatusPadding,
+  }) {
+    if (machine) {
+      return SilentStatus(stopwatch: Stopwatch());
+    }
+    return super.startProgress(
+      message,
+      progressId: progressId,
+      progressIndicatorPadding: progressIndicatorPadding,
+    );
+  }
+
+  @override
+  Status startSpinner({
+    VoidCallback? onFinish,
+    Duration? timeout,
+    SlowWarningCallback? slowWarningCallback,
+    TerminalColor? warningColor,
+  }) {
+    if (machine) {
+      return SilentStatus(stopwatch: Stopwatch());
+    }
+    return super.startSpinner(
+      onFinish: onFinish,
+      timeout: timeout,
+      slowWarningCallback: slowWarningCallback,
+      warningColor: warningColor,
+    );
+  }
+}

packages/flutter_tools/test/integration.shard/widget_preview_machine_test.dart

@@ -0,0 +1,123 @@
+// 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 'dart:convert';
+
+import 'package:file/file.dart';
+import 'package:flutter_tools/src/base/io.dart';
+import 'package:flutter_tools/src/commands/widget_preview.dart';
+import 'package:flutter_tools/src/widget_preview/dtd_services.dart';
+import 'package:http/http.dart';
+import 'package:process/process.dart';
+
+import '../src/common.dart';
+import 'test_data/basic_project.dart';
+import 'test_utils.dart';
+
+typedef ExpectedEvent = ({String event, FutureOr<void> Function(Map<String, Object?>)? validator});
+
+final launchEvents = <ExpectedEvent>[
+  (
+    event: 'widget_preview.started',
+    validator: (Map<String, Object?> params) async {
+      if (params case {'uri': final String uri}) {
+        try {
+          final Response response = await get(Uri.parse(uri));
+          expect(response.statusCode, HttpStatus.ok, reason: 'Failed to retrieve widget previewer');
+        } catch (e) {
+          fail('Failed to access widget previewer: $e');
+        }
+      }
+    },
+  ),
+];
+
+void main() {
+  late Directory tempDir;
+  Process? process;
+  DtdLauncher? dtdLauncher;
+  final project = BasicProject();
+  const ProcessManager processManager = LocalProcessManager();
+
+  setUp(() async {
+    tempDir = createResolvedTempDirectorySync('widget_preview_test.');
+    await project.setUpIn(tempDir);
+  });
+
+  tearDown(() async {
+    process?.kill();
+    process = null;
+    await dtdLauncher?.dispose();
+    dtdLauncher = null;
+    tryToDelete(tempDir);
+  });
+
+  Future<void> runWidgetPreviewMachineMode({
+    required List<ExpectedEvent> expectedEvents,
+    bool useWebServer = false,
+  }) async {
+    expect(expectedEvents, isNotEmpty);
+    process = await processManager.start(<String>[
+      flutterBin,
+      'widget-preview',
+      'start',
+      '--machine',
+      '--${WidgetPreviewStartCommand.kHeadless}',
+      if (useWebServer) '--${WidgetPreviewStartCommand.kWebServer}',
+    ], workingDirectory: tempDir.path);
+
+    final completer = Completer<void>();
+    var nextExpectationIndex = 0;
+    process!.stdout.transform(utf8.decoder).transform(const LineSplitter()).listen((
+      String message,
+    ) async {
+      printOnFailure('STDOUT: $message');
+      if (completer.isCompleted) {
+        return;
+      }
+      try {
+        final Object? event = json.decode(message);
+        if (event case [final Map<String, Object?> eventObject]) {
+          final ExpectedEvent expectation = expectedEvents[nextExpectationIndex];
+          if (expectation.event == eventObject['event']) {
+            await expectation.validator?.call(eventObject);
+            ++nextExpectationIndex;
+          }
+        }
+        if (nextExpectationIndex == expectedEvents.length) {
+          completer.complete();
+        }
+      } on FormatException {
+        // Do nothing.
+      }
+    });
+
+    process!.stderr.transform(utf8.decoder).transform(const LineSplitter()).listen((String msg) {
+      printOnFailure('STDERR: $msg');
+    });
+
+    unawaited(
+      process!.exitCode.then((int exitCode) {
+        if (completer.isCompleted) {
+          return;
+        }
+        completer.completeError(
+          TestFailure('The widget previewer exited unexpectedly (exit code: $exitCode)'),
+        );
+      }),
+    );
+    await completer.future;
+  }
+
+  group('flutter widget-preview start --machine', () {
+    testWithoutContext('launches in browser', () async {
+      await runWidgetPreviewMachineMode(expectedEvents: launchEvents);
+    });
+
+    testWithoutContext('launches web server', () async {
+      await runWidgetPreviewMachineMode(expectedEvents: launchEvents, useWebServer: true);
+    });
+  });
+}