Make Material/CupertinoLocalizations non-nullable (flutter#68807)

Author: goderbauer

packages/flutter/lib/src/cupertino/bottom_tab_bar.dart

@@ -209,14 +209,7 @@ class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
 
   List<Widget> _buildTabItems(BuildContext context) {
     final List<Widget> result = <Widget>[];
-    final CupertinoLocalizations? localizations = CupertinoLocalizations.of(context);
-    assert(
-      localizations != null,
-      'CupertinoTabBar requires a Localizations parent in order to provide an '
-        'appropriate Semantics hint for tab indexing. A CupertinoApp will '
-        'provide the DefaultCupertinoLocalizations, or you can instantiate your '
-        'own Localizations.'
-    );
+    final CupertinoLocalizations localizations = CupertinoLocalizations.of(context);
 
     for (int index = 0; index < items.length; index += 1) {
       final bool active = index == currentIndex;
@@ -226,7 +219,7 @@ class CupertinoTabBar extends StatelessWidget implements PreferredSizeWidget {
           Expanded(
             child: Semantics(
               selected: active,
-              hint: localizations!.tabSemanticsLabel(
+              hint: localizations.tabSemanticsLabel(
                 tabIndex: index + 1,
                 tabCount: items.length,
               ),

packages/flutter/lib/src/cupertino/date_picker.dart

@@ -623,7 +623,7 @@ class _CupertinoDatePickerDateTimeState extends State<CupertinoDatePicker> {
     super.didChangeDependencies();
 
     textDirectionFactor = Directionality.of(context) == TextDirection.ltr ? 1 : -1;
-    localizations = CupertinoLocalizations.of(context)!;
+    localizations = CupertinoLocalizations.of(context);
 
     alignCenterLeft = textDirectionFactor == 1 ? Alignment.centerLeft : Alignment.centerRight;
     alignCenterRight = textDirectionFactor == 1 ? Alignment.centerRight : Alignment.centerLeft;
@@ -1113,7 +1113,7 @@ class _CupertinoDatePickerDateState extends State<CupertinoDatePicker> {
     super.didChangeDependencies();
 
     textDirectionFactor = Directionality.of(context) == TextDirection.ltr ? 1 : -1;
-    localizations = CupertinoLocalizations.of(context)!;
+    localizations = CupertinoLocalizations.of(context);
 
     alignCenterLeft = textDirectionFactor == 1 ? Alignment.centerLeft : Alignment.centerRight;
     alignCenterRight = textDirectionFactor == 1 ? Alignment.centerRight : Alignment.centerLeft;
@@ -1621,7 +1621,7 @@ class _CupertinoTimerPickerState extends State<CupertinoTimerPicker> {
     super.didChangeDependencies();
 
     textDirection = Directionality.of(context)!;
-    localizations = CupertinoLocalizations.of(context)!;
+    localizations = CupertinoLocalizations.of(context);
 
     _measureLabelMetrics();
   }

packages/flutter/lib/src/cupertino/debug.dart

@@ -0,0 +1,46 @@
+// Copyright 2014 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 'package:flutter/foundation.dart';
+import 'package:flutter/widgets.dart';
+
+import 'localizations.dart';
+
+/// Asserts that the given context has a [Localizations] ancestor that contains
+/// a [CupertinoLocalizations] delegate.
+///
+/// To call this function, use the following pattern, typically in the
+/// relevant Widget's build method:
+///
+/// ```dart
+/// assert(debugCheckHasCupertinoLocalizations(context));
+/// ```
+///
+/// Does nothing if asserts are disabled. Always returns true.
+bool debugCheckHasCupertinoLocalizations(BuildContext context) {
+  assert(() {
+    if (Localizations.of<CupertinoLocalizations>(context, CupertinoLocalizations) == null) {
+      throw FlutterError.fromParts(<DiagnosticsNode>[
+        ErrorSummary('No CupertinoLocalizations found.'),
+        ErrorDescription(
+          '${context.widget.runtimeType} widgets require CupertinoLocalizations '
+          'to be provided by a Localizations widget ancestor.'
+        ),
+        ErrorDescription(
+          'The cupertino library uses Localizations to generate messages, '
+          'labels, and abbreviations.'
+        ),
+        ErrorHint(
+          'To introduce a CupertinoLocalizations, either use a '
+          'CupertinoApp at the root of your application to include them '
+          'automatically, or add a Localization widget with a '
+          'CupertinoLocalizations delegate.'
+        ),
+        ...context.describeMissingAncestor(expectedAncestorType: CupertinoLocalizations)
+      ]);
+    }
+    return true;
+  }());
+  return true;
+}

packages/flutter/lib/src/cupertino/dialog.dart

@@ -226,7 +226,7 @@ class CupertinoAlertDialog extends StatelessWidget {
 
   @override
   Widget build(BuildContext context) {
-    final CupertinoLocalizations localizations = CupertinoLocalizations.of(context)!;
+    final CupertinoLocalizations localizations = CupertinoLocalizations.of(context);
     final bool isInAccessibilityMode = _isInAccessibilityMode(context);
     final double textScaleFactor = MediaQuery.of(context)!.textScaleFactor;
     return CupertinoUserInterfaceLevel(

packages/flutter/lib/src/cupertino/localizations.dart

@@ -5,7 +5,7 @@
 import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 
-import 'date_picker.dart';
+import 'debug.dart';
 
 /// Determines the order of the columns inside [CupertinoDatePicker] in
 /// time and date time mode.
@@ -242,17 +242,21 @@ abstract class CupertinoLocalizations {
   /// The `CupertinoLocalizations` from the closest [Localizations] instance
   /// that encloses the given context.
   ///
+  /// If no [CupertinoLocalizations] are available in the given `context`, this
+  /// method throws an exception.
+  ///
   /// This method is just a convenient shorthand for:
-  /// `Localizations.of<CupertinoLocalizations>(context, CupertinoLocalizations)`.
+  /// `Localizations.of<CupertinoLocalizations>(context, CupertinoLocalizations)!`.
   ///
   /// References to the localized resources defined by this class are typically
   /// written in terms of this method. For example:
   ///
   /// ```dart
   /// CupertinoLocalizations.of(context).anteMeridiemAbbreviation;
   /// ```
-  static CupertinoLocalizations? of(BuildContext context) {
-    return Localizations.of<CupertinoLocalizations>(context, CupertinoLocalizations);
+  static CupertinoLocalizations of(BuildContext context) {
+    debugCheckHasCupertinoLocalizations(context);
+    return Localizations.of<CupertinoLocalizations>(context, CupertinoLocalizations)!;
   }
 }
 

packages/flutter/lib/src/cupertino/route.dart

@@ -1146,7 +1146,7 @@ Future<T?> showCupertinoDialog<T>({
   return showGeneralDialog(
     context: context,
     barrierDismissible: barrierDismissible,
-    barrierLabel: CupertinoLocalizations.of(context)!.modalBarrierDismissLabel,
+    barrierLabel: CupertinoLocalizations.of(context).modalBarrierDismissLabel,
     barrierColor: CupertinoDynamicColor.resolve(_kModalBarrierColor, context)!,
     // This transition duration was eyeballed comparing with iOS
     transitionDuration: const Duration(milliseconds: 250),

packages/flutter/lib/src/cupertino/text_selection.dart

@@ -152,7 +152,7 @@ class _CupertinoTextSelectionToolbarWrapperState extends State<_CupertinoTextSel
     }
 
     final List<Widget> items = <Widget>[];
-    final CupertinoLocalizations? localizations = CupertinoLocalizations.of(context);
+    final CupertinoLocalizations localizations = CupertinoLocalizations.of(context);
     final EdgeInsets arrowPadding = widget.isArrowPointingDown
       ? EdgeInsets.only(bottom: _kToolbarArrowSize.height)
       : EdgeInsets.only(top: _kToolbarArrowSize.height);
@@ -183,17 +183,17 @@ class _CupertinoTextSelectionToolbarWrapperState extends State<_CupertinoTextSel
     }
 
     if (widget.handleCut != null) {
-      addToolbarButton(localizations!.cutButtonLabel, widget.handleCut!);
+      addToolbarButton(localizations.cutButtonLabel, widget.handleCut!);
     }
     if (widget.handleCopy != null) {
-      addToolbarButton(localizations!.copyButtonLabel, widget.handleCopy!);
+      addToolbarButton(localizations.copyButtonLabel, widget.handleCopy!);
     }
     if (widget.handlePaste != null
         && _clipboardStatus.value == ClipboardStatus.pasteable) {
-      addToolbarButton(localizations!.pasteButtonLabel, widget.handlePaste!);
+      addToolbarButton(localizations.pasteButtonLabel, widget.handlePaste!);
     }
     if (widget.handleSelectAll != null) {
-      addToolbarButton(localizations!.selectAllButtonLabel, widget.handleSelectAll!);
+      addToolbarButton(localizations.selectAllButtonLabel, widget.handleSelectAll!);
     }
 
     return CupertinoTextSelectionToolbar._(

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

@@ -201,7 +201,7 @@ class AboutListTile extends StatelessWidget {
     assert(debugCheckHasMaterialLocalizations(context));
     return ListTile(
       leading: icon,
-      title: child ?? Text(MaterialLocalizations.of(context)!.aboutListTileTitle(
+      title: child ?? Text(MaterialLocalizations.of(context).aboutListTileTitle(
         applicationName ?? _defaultApplicationName(context),
       )),
       dense: dense,
@@ -405,7 +405,7 @@ class AboutDialog extends StatelessWidget {
       ),
       actions: <Widget>[
         TextButton(
-          child: Text(MaterialLocalizations.of(context)!.viewLicensesButtonLabel),
+          child: Text(MaterialLocalizations.of(context).viewLicensesButtonLabel),
           onPressed: () {
             showLicensePage(
               context: context,
@@ -417,7 +417,7 @@ class AboutDialog extends StatelessWidget {
           },
         ),
         TextButton(
-          child: Text(MaterialLocalizations.of(context)!.closeButtonLabel),
+          child: Text(MaterialLocalizations.of(context).closeButtonLabel),
           onPressed: () {
             Navigator.pop(context);
           },
@@ -493,7 +493,7 @@ class _LicensePageState extends State<LicensePage> {
   Widget build(BuildContext context) {
     return _MasterDetailFlow(
       detailPageFABlessGutterWidth: _getGutterSize(context),
-      title: Text(MaterialLocalizations.of(context)!.licensesPageTitle),
+      title: Text(MaterialLocalizations.of(context).licensesPageTitle),
       detailPageBuilder: _packageLicensePage,
       masterViewBuilder: _packagesView,
     );
@@ -720,7 +720,7 @@ class _PackageListTile extends StatelessWidget {
       color: isSelected ? Theme.of(context)!.highlightColor : Theme.of(context)!.cardColor,
       child: ListTile(
         title: Text(packageName),
-        subtitle: Text(MaterialLocalizations.of(context)!.licensesPackageDetailText(numberLicenses)),
+        subtitle: Text(MaterialLocalizations.of(context).licensesPackageDetailText(numberLicenses)),
         selected: isSelected,
         onTap: onTap,
       ),
@@ -890,7 +890,7 @@ class _PackageLicensePageState extends State<_PackageLicensePage> {
   @override
   Widget build(BuildContext context) {
     assert(debugCheckHasMaterialLocalizations(context));
-    final MaterialLocalizations localizations = MaterialLocalizations.of(context)!;
+    final MaterialLocalizations localizations = MaterialLocalizations.of(context);
     final ThemeData? theme = Theme.of(context);
     final String title = widget.packageName;
     final String subtitle = localizations.licensesPackageDetailText(widget.licenseEntries.length);

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

@@ -543,7 +543,7 @@ class _AppBarState extends State<AppBar> {
         leading = IconButton(
           icon: const Icon(Icons.menu),
           onPressed: _handleDrawerButton,
-          tooltip: MaterialLocalizations.of(context)!.openAppDrawerTooltip,
+          tooltip: MaterialLocalizations.of(context).openAppDrawerTooltip,
         );
       } else {
         if (!hasEndDrawer && canPop)
@@ -616,7 +616,7 @@ class _AppBarState extends State<AppBar> {
       actions = IconButton(
         icon: const Icon(Icons.menu),
         onPressed: _handleDrawerButtonEnd,
-        tooltip: MaterialLocalizations.of(context)!.openAppDrawerTooltip,
+        tooltip: MaterialLocalizations.of(context).openAppDrawerTooltip,
       );
     }
 

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

@@ -98,7 +98,7 @@ class BackButton extends StatelessWidget {
     return IconButton(
       icon: const BackButtonIcon(),
       color: color,
-      tooltip: MaterialLocalizations.of(context)!.backButtonTooltip,
+      tooltip: MaterialLocalizations.of(context).backButtonTooltip,
       onPressed: () {
         if (onPressed != null) {
           onPressed!();
@@ -152,7 +152,7 @@ class CloseButton extends StatelessWidget {
     return IconButton(
       icon: const Icon(Icons.close),
       color: color,
-      tooltip: MaterialLocalizations.of(context)!.closeButtonTooltip,
+      tooltip: MaterialLocalizations.of(context).closeButtonTooltip,
       onPressed: () {
         if (onPressed != null) {
           onPressed!();