Add a build command to felt (#12303)

- The build command supports a `--watch` or `-w` flag to watch for
changes and rebuild.

Fixes https://github.com/flutter/flutter/issues/40392

lib/web_ui/dev/build.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.
+
+import 'dart:async';
+
+import 'package:args/command_runner.dart';
+import 'package:meta/meta.dart';
+import 'package:path/path.dart' as path;
+import 'package:watcher/watcher.dart';
+
+import 'environment.dart';
+import 'utils.dart';
+
+class BuildCommand extends Command<bool> {
+  BuildCommand() {
+    argParser
+      ..addFlag(
+        'watch',
+        abbr: 'w',
+        help: 'Run the build in watch mode so it rebuilds whenever a change'
+            'is made.',
+      );
+  }
+
+  @override
+  String get name => 'build';
+
+  @override
+  String get description => 'Build the Flutter web engine.';
+
+  bool get isWatchMode => argResults['watch'];
+
+  @override
+  FutureOr<bool> run() async {
+    final FilePath libPath = FilePath.fromWebUi('lib');
+    final Pipeline buildPipeline = Pipeline(steps: <PipelineStep>[
+      gn,
+      ninja,
+    ]);
+    await buildPipeline.start();
+
+    if (isWatchMode) {
+      print('Initial build done!');
+      print('Watching directory: ${libPath.relativeToCwd}/');
+      PipelineWatcher(
+        dir: libPath.absolute,
+        pipeline: buildPipeline,
+      ).start();
+      // Return a never-ending future.
+      return Completer<bool>().future;
+    } else {
+      return true;
+    }
+  }
+}
+
+Future<void> gn() {
+  print('Running gn...');
+  return runProcess(
+    path.join(environment.flutterDirectory.path, 'tools', 'gn'),
+    <String>[
+      '--unopt',
+      '--full-dart-sdk',
+    ],
+  );
+}
+
+// TODO(mdebbar): Make the ninja step interruptable in the pipeline.
+Future<void> ninja() {
+  print('Running ninja...');
+  return runProcess('ninja', <String>[
+    '-C',
+    environment.hostDebugUnoptDir.path,
+  ]);
+}
+
+enum PipelineStatus {
+  idle,
+  started,
+  stopping,
+  stopped,
+  error,
+  done,
+}
+
+typedef PipelineStep = Future<void> Function();
+
+class Pipeline {
+  Pipeline({@required this.steps});
+
+  final Iterable<PipelineStep> steps;
+
+  Future<dynamic> _currentStepFuture;
+
+  PipelineStatus status = PipelineStatus.idle;
+
+  Future<void> start() async {
+    status = PipelineStatus.started;
+    try {
+      for (PipelineStep step in steps) {
+        if (status != PipelineStatus.started) {
+          break;
+        }
+        _currentStepFuture = step();
+        await _currentStepFuture;
+      }
+      status = PipelineStatus.done;
+    } catch (_) {
+      status = PipelineStatus.error;
+    } finally {
+      _currentStepFuture = null;
+    }
+  }
+
+  Future<void> stop() {
+    status = PipelineStatus.stopping;
+    return (_currentStepFuture ?? Future<void>.value(null)).then((_) {
+      status = PipelineStatus.stopped;
+    });
+  }
+}
+
+class PipelineWatcher {
+  PipelineWatcher({
+    @required this.dir,
+    @required this.pipeline,
+  }) : watcher = DirectoryWatcher(dir);
+
+  /// The path of the directory to watch for changes.
+  final String dir;
+
+  /// The pipeline to be executed when an event is fired by the watcher.
+  final Pipeline pipeline;
+
+  /// Used to watch a directory for any file system changes.
+  final DirectoryWatcher watcher;
+
+  void start() {
+    watcher.events.listen(_onEvent);
+  }
+
+  int _pipelineRunCount = 0;
+  Timer _scheduledPipeline;
+
+  void _onEvent(WatchEvent event) {
+    final String relativePath = path.relative(event.path, from: dir);
+    print('- [${event.type}] ${relativePath}');
+
+    _pipelineRunCount++;
+    _scheduledPipeline?.cancel();
+    _scheduledPipeline = Timer(const Duration(milliseconds: 100), () {
+      _scheduledPipeline = null;
+      _runPipeline();
+    });
+  }
+
+  void _runPipeline() {
+    int runCount;
+    switch (pipeline.status) {
+      case PipelineStatus.started:
+        pipeline.stop().then((_) {
+          runCount = _pipelineRunCount;
+          pipeline.start().then((_) => _pipelineDone(runCount));
+        });
+        break;
+
+      case PipelineStatus.stopping:
+        // We are already trying to stop the pipeline. No need to do anything.
+        break;
+
+      default:
+        runCount = _pipelineRunCount;
+        pipeline.start().then((_) => _pipelineDone(runCount));
+        break;
+    }
+  }
+
+  void _pipelineDone(int pipelineRunCount) {
+    if (pipelineRunCount == _pipelineRunCount) {
+      print('*** Done! ***');
+    }
+  }
+}

lib/web_ui/dev/felt.dart

@@ -6,6 +6,7 @@ import 'dart:io' as io;
 
 import 'package:args/command_runner.dart';
 
+import 'build.dart';
 import 'licenses.dart';
 import 'test_runner.dart';
 
@@ -14,7 +15,8 @@ CommandRunner runner = CommandRunner<bool>(
   'Command-line utility for building and testing Flutter web engine.',
 )
   ..addCommand(LicensesCommand())
-  ..addCommand(TestsCommand());
+  ..addCommand(TestsCommand())
+  ..addCommand(BuildCommand());
 
 void main(List<String> args) async {
   if (args.isEmpty) {

lib/web_ui/pubspec.yaml

@@ -15,5 +15,6 @@ dev_dependencies:
   build_runner: 1.6.5
   build_test: 0.10.8
   build_web_compilers: 2.1.5
+  watcher: 0.9.7+12
   web_engine_tester:
     path: ../../web_sdk/web_engine_tester