Add Flags class. (#12268)

This class lives in the Context and allows callers to "inject"
flag values, where flag values are first extracted from the
command arguments, then from the global arguments as a fallback.

packages/flutter_tools/lib/src/base/flags.dart

@@ -0,0 +1,67 @@
+// Copyright 2017 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.
+
+import 'package:args/args.dart';
+
+import 'context.dart';
+
+/// command-line flags and options that were specified during the invocation of
+/// the Flutter tool.
+Flags get flags => context?.getVariable(Flags) ?? const _EmptyFlags();
+
+/// Encapsulation of the command-line flags and options that were specified
+/// during the invocation of the Flutter tool.
+///
+/// An instance of this class is set into the [AppContext] upon invocation of
+/// the Flutter tool (immediately after the arguments have been parsed in
+/// [FlutterCommandRunner]) and is available via the [flags] global property.
+class Flags {
+  Flags(this._globalResults) {
+    assert(_globalResults != null);
+  }
+
+  final ArgResults _globalResults;
+
+  /// Gets the value of the specified command-line flag/option that was set
+  /// during the invocation of the Flutter tool.
+  ///
+  /// This will first search for flags that are specific to the command and will
+  /// fall back to global flags.
+  ///
+  /// If a flag has a default value and the user did not explicitly specify a
+  /// value on the command-line, this will return the default value.
+  ///
+  /// If the specified flag is not defined or was not specified and had no
+  /// default, then this will return `null`.
+  dynamic operator [](String key) {
+    final ArgResults commandResults = _globalResults.command;
+    final Iterable<String> options = commandResults?.options;
+    if (options != null && options.contains(key))
+      return commandResults[key];
+    else if (_globalResults.options.contains(key))
+      return _globalResults[key];
+    return null;
+  }
+
+  /// `true` iff the given flag/option was either explicitly specified by the
+  /// user at  the command-line or it was defined to have a default value.
+  bool contains(String key) {
+    final ArgResults commandResults = _globalResults.command;
+    final Iterable<String> options = commandResults?.options;
+    return (options != null && options.contains(key)) || _globalResults.options.contains(key);
+  }
+}
+
+class _EmptyFlags implements Flags {
+  const _EmptyFlags();
+
+  @override
+  ArgResults get _globalResults => null;
+
+  @override
+  String operator [](String key) => null;
+
+  @override
+  bool contains(String key) => false;
+}

packages/flutter_tools/lib/src/runner/flutter_command_runner.dart

@@ -13,6 +13,7 @@ import '../artifacts.dart';
 import '../base/common.dart';
 import '../base/context.dart';
 import '../base/file_system.dart';
+import '../base/flags.dart';
 import '../base/logger.dart';
 import '../base/os.dart';
 import '../base/platform.dart';
@@ -163,6 +164,8 @@ class FlutterCommandRunner extends CommandRunner<Null> {
 
   @override
   Future<Null> runCommand(ArgResults globalResults) async {
+    context.setVariable(Flags, new Flags(globalResults));
+
     // Check for verbose.
     if (globalResults['verbose']) {
       // Override the logger.

packages/flutter_tools/test/base/flags_test.dart

@@ -0,0 +1,99 @@
+// Copyright 2017 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.
+
+import 'dart:async';
+
+import 'package:flutter_tools/src/base/flags.dart';
+import 'package:flutter_tools/src/cache.dart';
+import 'package:flutter_tools/src/runner/flutter_command.dart';
+import 'package:test/test.dart';
+
+import '../src/common.dart';
+import '../src/context.dart';
+
+typedef FutureOr<Null> _TestMethod();
+
+void main() {
+  Cache.disableLocking();
+
+  Future<Null> runCommand(Iterable<String> flags, _TestMethod testMethod) async {
+    final List<String> args = <String>['test']..addAll(flags);
+    final _TestCommand command = new _TestCommand(testMethod);
+    await createTestCommandRunner(command).run(args);
+  }
+
+  testUsingContext('runCommand works as expected', () async {
+    bool testRan = false;
+    await runCommand(<String>[], () {
+      testRan = true;
+    });
+    expect(testRan, isTrue);
+  });
+
+  group('flags', () {
+    test('returns no-op flags when not inside Flutter runner', () {
+      expect(flags, isNotNull);
+      expect(flags['foo'], isNull);
+    });
+
+    testUsingContext('returns null for undefined flags', () async {
+      await runCommand(<String>[], () {
+        expect(flags['undefined-flag'], isNull);
+      });
+    });
+
+    testUsingContext('picks up default values', () async {
+      await runCommand(<String>[], () {
+        expect(flags['verbose'], isFalse);
+        expect(flags['flag-defaults-to-false'], isFalse);
+        expect(flags['flag-defaults-to-true'], isTrue);
+        expect(flags['option-defaults-to-foo'], 'foo');
+      });
+    });
+
+    testUsingContext('returns null for flags with no default values', () async {
+      await runCommand(<String>[], () {
+        expect(flags['device-id'], isNull);
+        expect(flags['option-no-default'], isNull);
+      });
+    });
+
+    testUsingContext('picks up explicit values', () async {
+      await runCommand(<String>[
+        '--verbose',
+        '--flag-defaults-to-false',
+        '--option-no-default=explicit',
+        '--option-defaults-to-foo=qux',
+      ], () {
+        expect(flags['verbose'], isTrue);
+        expect(flags['flag-defaults-to-false'], isTrue);
+        expect(flags['option-no-default'], 'explicit');
+        expect(flags['option-defaults-to-foo'], 'qux');
+      });
+    });
+  });
+}
+
+class _TestCommand extends FlutterCommand {
+  _TestCommand(this.testMethod) {
+    argParser.addFlag('flag-defaults-to-false', defaultsTo: false);
+    argParser.addFlag('flag-defaults-to-true', defaultsTo: true);
+    argParser.addOption('option-no-default');
+    argParser.addOption('option-defaults-to-foo', defaultsTo: 'foo');
+  }
+
+  final _TestMethod testMethod;
+
+  @override
+  String get name => 'test';
+
+  @override
+  String get description => 'runs a test method';
+
+  @override
+  Future<FlutterCommandResult> runCommand() async {
+    await testMethod();
+    return null;
+  }
+}