Async initialization for DateFormat

pkg/intl/lib/date_format_internal.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/**
+ * This contains internal implementation details of the date formatting code
+ * which are exposed as public functions because they must be called by other
+ * libraries in order to configure the source for the locale data. We don't want
+ * them exposed as public API functions in the date formatting library, so they
+ * are put in a separate library here. These are for internal use only. User
+ * code should import one of the date_symbol_data... libraries and call the
+ * initializeDateFormatting method exposed there.
+ */
+
+#library('date_format_internal');
+#import('intl_helpers.dart');
+
+/**
+ * This holds the symbols to be used for date/time formatting, indexed
+ * by locale. Note that it will be set differently during initialization,
+ * depending on what implementation we are using. By default, it is initialized
+ * to an instance of UninitializedLocaleData, so any attempt to use it will
+ * result in an informative error message.
+ */
+var dateTimeSymbols =
+    const UninitializedLocaleData('initializeDateFormatting(<locale>)');
+
+/**
+ * This holds the patterns used for date/time formatting, indexed
+ * by locale. Note that it will be set differently during initialization,
+ * depending on what implementation we are using. By default, it is initialized
+ * to an instance of UninitializedLocaleData, so any attempt to use it will
+ * result in an informative error message.
+ */
+var dateTimePatterns =
+    const UninitializedLocaleData('initializeDateFormatting(<locale>)');
+
+/**
+ * Initialize the symbols dictionary. This should be passed a function that
+ * creates and returns the symbol data. We take a function so that if
+ * initializing the data is an expensive operation it need only be done once,
+ * no matter how many times this method is called.
+ */
+void initializeDateSymbols(Function symbols) {
+  if (dateTimeSymbols is UninitializedLocaleData) {
+    dateTimeSymbols = symbols();
+  }
+}
+
+/**
+ * Initialize the patterns dictionary. This should be passed a function that
+ * creates and returns the pattern data. We take a function so that if
+ * initializing the data is an expensive operation it need only be done once,
+ * no matter how many times this method is called.
+ */
+void initializeDatePatterns(Function patterns) {
+  if (dateTimePatterns is UninitializedLocaleData) {
+    dateTimePatterns = patterns();
+  }
+}
+
+Future initializeIndividualLocaleDateFormatting(Function init) {
+  return init(dateTimeSymbols, dateTimePatterns);
+}