Date picker i18n (#12324)

* formatYear

* localize date picker

* tests

* clean-ups

* address comments

packages/flutter/lib/src/material/material_localizations.dart

@@ -7,6 +7,8 @@ import 'dart:async';
 import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 import 'package:intl/intl.dart' as intl;
+import 'package:intl/date_symbols.dart' as intl;
+import 'package:intl/date_symbol_data_local.dart' as intl_local_date_data;
 
 import 'i18n/localizations.dart';
 import 'time.dart';
@@ -121,6 +123,52 @@ abstract class MaterialLocalizations {
   /// Formats [timeOfDay] according to the value of [timeOfDayFormat].
   String formatTimeOfDay(TimeOfDay timeOfDay);
 
+  /// Full unabbreviated year format, e.g. 2017 rather than 17.
+  String formatYear(DateTime date);
+
+  /// Formats the date using a medium-width format.
+  ///
+  /// Abbreviates month and days of week. This appears in the header of the date
+  /// picker invoked using [showDatePicker].
+  ///
+  /// Examples:
+  ///
+  /// - US English: Wed, Sep 27
+  /// - Russian: ср, сент. 27
+  String formatMediumDate(DateTime date);
+
+  /// Formats the month and the year of the given [date].
+  ///
+  /// The returned string does not contain the day of the month. This appears
+  /// in the date picker invoked using [showDatePicker].
+  String formatMonthYear(DateTime date);
+
+  /// List of week day names in narrow format, usually 1- or 2-letter
+  /// abbreviations of full names.
+  ///
+  /// The list begins with the value corresponding to Sunday and ends with
+  /// Saturday. Use [firstDayOfWeekIndex] to find the first day of week in this
+  /// list.
+  ///
+  /// Examples:
+  ///
+  /// - US English: S, M, T, W, T, F, S
+  /// - Russian: вс, пн, вт, ср, чт, пт, сб - notice that the list begins with
+  ///   вс (Sunday) even though the first day of week for Russian is Monday.
+  List<String> get narrowWeekDays;
+
+  /// Index of the first day of week, where 0 points to Sunday, and 6 points to
+  /// Saturday.
+  ///
+  /// This getter is compatible with [narrowWeekDays]. For example:
+  ///
+  /// ```dart
+  /// var localizations = MaterialLocalizations.of(context);
+  /// // The name of the first day of week for the current locale.
+  /// var firstDayOfWeek = localizations.narrowWeekDays[localizations.firstDayOfWeekIndex];
+  /// ```
+  int get firstDayOfWeekIndex;
+
   /// The `MaterialLocalizations` from the closest [Localizations] instance
   /// that encloses the given context.
   ///
@@ -146,13 +194,30 @@ class DefaultMaterialLocalizations implements MaterialLocalizations {
   /// [LocalizationsDelegate] implementations typically call the static [load]
   /// function, rather than constructing this class directly.
   DefaultMaterialLocalizations(this.locale)
-      : this._localeName = _computeLocaleName(locale) {
-    assert(locale != null);
+      : assert(locale != null),
+        this._localeName = _computeLocaleName(locale) {
+    _loadDateIntlDataIfNotLoaded();
+
     if (localizations.containsKey(locale.languageCode))
       _nameToValue.addAll(localizations[locale.languageCode]);
     if (localizations.containsKey(_localeName))
       _nameToValue.addAll(localizations[_localeName]);
 
+    const String kMediumDatePattern = 'E, MMM\u00a0d';
+    if (intl.DateFormat.localeExists(_localeName)) {
+      _fullYearFormat = new intl.DateFormat.y(_localeName);
+      _mediumDateFormat = new intl.DateFormat(kMediumDatePattern, _localeName);
+      _yearMonthFormat = new intl.DateFormat('yMMMM', _localeName);
+    } else if (intl.DateFormat.localeExists(locale.languageCode)) {
+      _fullYearFormat = new intl.DateFormat.y(locale.languageCode);
+      _mediumDateFormat = new intl.DateFormat(kMediumDatePattern, locale.languageCode);
+      _yearMonthFormat = new intl.DateFormat('yMMMM', locale.languageCode);
+    } else {
+      _fullYearFormat = new intl.DateFormat.y();
+      _mediumDateFormat = new intl.DateFormat(kMediumDatePattern);
+      _yearMonthFormat = new intl.DateFormat('yMMMM');
+    }
+
     if (intl.NumberFormat.localeExists(_localeName)) {
       _decimalFormat = new intl.NumberFormat.decimalPattern(_localeName);
       _twoDigitZeroPaddedFormat = new intl.NumberFormat('00', _localeName);
@@ -183,6 +248,13 @@ class DefaultMaterialLocalizations implements MaterialLocalizations {
   /// If the number is less than 10, zero-pads it.
   intl.NumberFormat _twoDigitZeroPaddedFormat;
 
+  /// Full unabbreviated year format, e.g. 2017 rather than 17.
+  intl.DateFormat _fullYearFormat;
+
+  intl.DateFormat _mediumDateFormat;
+
+  intl.DateFormat _yearMonthFormat;
+
   static String _computeLocaleName(Locale locale) {
     final String localeName = locale.countryCode.isEmpty ? locale.languageCode : locale.toString();
     return intl.Intl.canonicalizedLocale(localeName);
@@ -225,6 +297,29 @@ class DefaultMaterialLocalizations implements MaterialLocalizations {
     return _twoDigitZeroPaddedFormat.format(timeOfDay.minute);
   }
 
+  @override
+  String formatYear(DateTime date) {
+    return _fullYearFormat.format(date);
+  }
+
+  @override
+  String formatMediumDate(DateTime date) {
+    return _mediumDateFormat.format(date);
+  }
+
+  @override
+  String formatMonthYear(DateTime date) {
+    return _yearMonthFormat.format(date);
+  }
+
+  @override
+  List<String> get narrowWeekDays {
+    return _fullYearFormat.dateSymbols.NARROWWEEKDAYS;
+  }
+
+  @override
+  int get firstDayOfWeekIndex => (_fullYearFormat.dateSymbols.FIRSTDAYOFWEEK + 1) % 7;
+
   /// Formats a [number] using local decimal number format.
   ///
   /// Inserts locale-appropriate thousands separator, if necessary.
@@ -415,3 +510,20 @@ const Map<String, TimeOfDayFormat> _icuTimeOfDayToEnum = const <String, TimeOfDa
   'a h:mm': TimeOfDayFormat.a_space_h_colon_mm,
   'ah:mm': TimeOfDayFormat.a_space_h_colon_mm,
 };
+
+/// Tracks if date i18n data has been loaded.
+bool _dateIntlDataInitialized = false;
+
+/// Loads i18n data for dates if it hasn't be loaded yet.
+///
+/// Only the first invocation of this function has the effect of loading the
+/// data. Subsequent invocations have no effect.
+void _loadDateIntlDataIfNotLoaded() {
+  if (!_dateIntlDataInitialized) {
+    // The returned Future is intentionally dropped on the floor. The
+    // function only returns it to be compatible with the async counterparts.
+    // The Future has no value otherwise.
+    intl_local_date_data.initializeDateFormatting();
+    _dateIntlDataInitialized = true;
+  }
+}

packages/flutter/test/widgets/date_picker_test.dart

-// 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.
-
-import 'package:flutter_test/flutter_test.dart';
-import 'package:flutter/material.dart';
-
-void main() {
-  testWidgets('Can select a day', (WidgetTester tester) async {
-    DateTime currentValue;
-
-    final Widget widget = new Directionality(
-      textDirection: TextDirection.ltr,
-      child: new Material(
-        child: new ListView(
-          children: <Widget>[
-            new MonthPicker(
-              selectedDate: new DateTime.utc(2015, 6, 9, 7, 12),
-              firstDate: new DateTime.utc(2013),
-              lastDate: new DateTime.utc(2018),
-              onChanged: (DateTime dateTime) {
-                currentValue = dateTime;
-              },
-            ),
-          ],
-        ),
-      ),
-    );
-
-    await tester.pumpWidget(widget);
-
-    expect(currentValue, isNull);
-    await tester.tap(find.text('2015'));
-    await tester.pumpWidget(widget);
-    await tester.tap(find.text('2014'));
-    await tester.pumpWidget(widget);
-    expect(currentValue, equals(new DateTime(2014, 6, 9)));
-    await tester.tap(find.text('30'));
-    expect(currentValue, equals(new DateTime(2013, 1, 30)));
-  }, skip: true);
-}