Add listen command and basic test, and don’t do unnecessary repeated work when listening or poking the android server.

packages/flutter_tools/bin/sky_tools.dart

@@ -13,6 +13,7 @@ import 'package:sky_tools/src/build.dart';
 import 'package:sky_tools/src/cache.dart';
 import 'package:sky_tools/src/init.dart';
 import 'package:sky_tools/src/install.dart';
+import 'package:sky_tools/src/listen.dart';
 import 'package:sky_tools/src/logs.dart';
 import 'package:sky_tools/src/run_mojo.dart';
 import 'package:sky_tools/src/start.dart';
@@ -149,6 +150,7 @@ void main(List<String> args) {
     ..addCommand(new CacheCommand())
     ..addCommand(new InitCommand())
     ..addCommand(new InstallCommand())
+    ..addCommand(new ListenCommand())
     ..addCommand(new LogsCommand())
     ..addCommand(new RunMojoCommand())
     ..addCommand(new StartCommand())

packages/flutter_tools/lib/src/device.dart

@@ -256,19 +256,21 @@ class AndroidDevice extends _Device {
       return false;
     }
 
-    // Set up port forwarding for observatory.
-    String observatoryPortString = 'tcp:$_observatoryPort';
-    runCheckedSync(
-        [adbPath, 'forward', observatoryPortString, observatoryPortString]);
+    if (!poke) {
+      // Set up port forwarding for observatory.
+      String observatoryPortString = 'tcp:$_observatoryPort';
+      runCheckedSync(
+          [adbPath, 'forward', observatoryPortString, observatoryPortString]);
 
-    // Actually start the server.
-    await Process.start('pub', ['run', 'sky_tools:sky_server', _serverPort],
-        workingDirectory: serverRoot, mode: ProcessStartMode.DETACHED);
+      // Actually start the server.
+      await Process.start('pub', ['run', 'sky_tools:sky_server', _serverPort],
+          workingDirectory: serverRoot, mode: ProcessStartMode.DETACHED);
 
-    // Set up reverse port-forwarding so that the Android app can reach the
-    // server running on localhost.
-    String serverPortString = 'tcp:$_serverPort';
-    runCheckedSync([adbPath, 'reverse', serverPortString, serverPortString]);
+      // Set up reverse port-forwarding so that the Android app can reach the
+      // server running on localhost.
+      String serverPortString = 'tcp:$_serverPort';
+      runCheckedSync([adbPath, 'reverse', serverPortString, serverPortString]);
+    }
 
     String relativeDartMain = path.relative(mainDart, from: serverRoot);
     String url = 'http://localhost:$_serverPort/$relativeDartMain';

packages/flutter_tools/lib/src/install.dart

@@ -15,7 +15,11 @@ class InstallCommand extends Command {
   final name = 'install';
   final description = 'Install your Flutter app on attached devices.';
   AndroidDevice android = null;
-  InstallCommand([this.android]);
+  InstallCommand([this.android]) {
+    if (android == null) {
+      android = new AndroidDevice();
+    }
+  }
 
   @override
   Future<int> run() async {
@@ -31,9 +35,6 @@ class InstallCommand extends Command {
 
     Map<BuildPlatform, ApplicationPackage> packages =
         ApplicationPackageFactory.getAvailableApplicationPackages();
-    if (android == null) {
-      android = new AndroidDevice();
-    }
     ApplicationPackage androidApp = packages[BuildPlatform.android];
     if (androidApp != null && android.isConnected()) {
       installedSomewhere = android.installApp(androidApp) || installedSomewhere;

packages/flutter_tools/lib/src/listen.dart

@@ -0,0 +1,118 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+library sky_tools.listen;
+
+import 'dart:async';
+import 'dart:io';
+
+import 'package:args/command_runner.dart';
+import 'package:logging/logging.dart';
+import 'package:sky_tools/src/application_package.dart';
+import 'package:sky_tools/src/device.dart';
+import 'package:sky_tools/src/process.dart';
+
+final Logger _logging = new Logger('sky_tools.listen');
+
+class ListenCommand extends Command {
+  final name = 'listen';
+  final description = 'Listen for changes to files and reload the running app '
+      'on all connected devices.';
+  AndroidDevice android = null;
+  List<String> watchCommand;
+
+  /// Only run once.  Used for testing.
+  bool singleRun;
+
+  ListenCommand({this.android, this.singleRun: false}) {
+    argParser.addFlag('checked',
+        negatable: true,
+        defaultsTo: true,
+        help: 'Toggle Dart\'s checked mode.');
+    argParser.addOption('target',
+        defaultsTo: '.',
+        abbr: 't',
+        help: 'Target app path or filename to start.');
+
+    if (android == null) {
+      android = new AndroidDevice();
+    }
+  }
+
+  @override
+  Future<int> run() async {
+    if (argResults.rest.length > 0) {
+      watchCommand = _initWatchCommand(argResults.rest);
+    } else {
+      watchCommand = _initWatchCommand(['.']);
+    }
+
+    Map<BuildPlatform, ApplicationPackage> packages =
+        ApplicationPackageFactory.getAvailableApplicationPackages();
+    ApplicationPackage androidApp = packages[BuildPlatform.android];
+
+    while (true) {
+      _logging.info('Updating running Sky apps...');
+
+      if (android.isConnected()) {
+        await android.startServer(
+            argResults['target'], true, argResults['checked'], androidApp);
+      }
+
+      if (singleRun || !watchDirectory()) {
+        break;
+      }
+    }
+
+    return 0;
+  }
+
+  List<String> _initWatchCommand(List<String> directories) {
+    if (Platform.isMacOS) {
+      try {
+        runCheckedSync(['which', 'fswatch']);
+      } catch (e) {
+        _logging.severe('"listen" command is only useful if you have installed '
+            'fswatch on Mac.  Run "brew install fswatch" to install it with '
+            'homebrew.');
+        return null;
+      }
+      return ['fswatch', '-r', '-v', '-1']..addAll(directories);
+    } else if (Platform.isLinux) {
+      try {
+        runCheckedSync(['which', 'inotifywait']);
+      } catch (e) {
+        _logging.severe('"listen" command is only useful if you have installed '
+            'inotifywait on Linux.  Run "apt-get install inotify-tools" or '
+            'equivalent to install it.');
+        return null;
+      }
+      return [
+        'inotifywait',
+        '-r',
+        '-e',
+        // Only listen for events that matter, to avoid triggering constantly
+        // from the editor watching files
+        'modify,close_write,move,create,delete',
+      ]..addAll(directories);
+    } else {
+      _logging.severe('"listen" command is only available on Mac and Linux.');
+    }
+    return null;
+  }
+
+  bool watchDirectory() {
+    if (watchCommand == null) {
+      return false;
+    }
+
+    try {
+      runCheckedSync(watchCommand);
+    } catch (e) {
+      _logging.warning('Watching directories failed.', e);
+      return false;
+    }
+    return true;
+  }
+}

packages/flutter_tools/test/listen_test.dart

@@ -0,0 +1,34 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+library stop_test;
+
+import 'package:args/command_runner.dart';
+import 'package:mockito/mockito.dart';
+import 'package:sky_tools/src/application_package.dart';
+import 'package:sky_tools/src/listen.dart';
+import 'package:test/test.dart';
+
+import 'src/common.dart';
+
+main() => defineTests();
+
+defineTests() {
+  group('listen', () {
+    test('returns 0 when no device is connected', () {
+      ApplicationPackageFactory.srcPath = './';
+      ApplicationPackageFactory.setBuildPath(
+          BuildType.prebuilt, BuildPlatform.android, './');
+
+      MockAndroidDevice android = new MockAndroidDevice();
+      when(android.isConnected()).thenReturn(false);
+      ListenCommand command =
+          new ListenCommand(android: android, singleRun: true);
+
+      CommandRunner runner = new CommandRunner('test_flutter', '')
+        ..addCommand(command);
+      runner.run(['listen']).then((int code) => expect(code, equals(0)));
+    });
+  });
+}