Validate Mbin community names when creating a community

lib/l10n/app_en.arb

@@ -537,6 +537,32 @@
   "create_link_invalid": "Url is invalid",
   "create_microblog": "Microblog",
   "create_community": "Community",
+  "community_nameMbinHelp": "2–25 characters. Letters, numbers and underscores only.",
+  "community_nameInvalidCharacters": "Only letters, numbers and underscores are allowed.",
+  "community_nameTooShort": "Name must be at least {count} characters.",
+  "@community_nameTooShort": {
+    "placeholders": {
+      "count": {
+        "type": "int"
+      }
+    }
+  },
+  "community_nameTooLong": "Name must be at most {count} characters.",
+  "@community_nameTooLong": {
+    "placeholders": {
+      "count": {
+        "type": "int"
+      }
+    }
+  },
+  "community_nameUseSuggestion": "Use “{name}”",
+  "@community_nameUseSuggestion": {
+    "placeholders": {
+      "name": {
+        "type": "String"
+      }
+    }
+  },
   "microblog_communityHelperText": "Defaults to the 'random' community for microblogs since that's where Mbin stores uncatagorized microblogs.",
   "selectCommunity": "Select a community ...",
   "title": "Title",

lib/src/screens/explore/community_owner_panel.dart

@@ -1,9 +1,11 @@
 import 'package:auto_route/auto_route.dart';
 import 'package:flutter/material.dart';
 import 'package:interstellar/src/controller/controller.dart';
+import 'package:interstellar/src/controller/server.dart';
 import 'package:interstellar/src/models/community.dart';
 import 'package:interstellar/src/models/user.dart';
 import 'package:interstellar/src/screens/explore/user_item.dart';
+import 'package:interstellar/src/utils/mbin_community_name.dart';
 import 'package:interstellar/src/utils/utils.dart';
 import 'package:interstellar/src/widgets/loading_button.dart';
 import 'package:interstellar/src/widgets/markdown/drafts_controller.dart';
@@ -112,23 +114,75 @@ class _CommunityOwnerPanelGeneralState
         widget.data?.isPostingRestrictedToMods ?? false;
   }
 
+  String? _mbinNameError(BuildContext context, MbinCommunityNameIssue? issue) {
+    switch (issue) {
+      case MbinCommunityNameIssue.invalidCharacters:
+        return l(context).community_nameInvalidCharacters;
+      case MbinCommunityNameIssue.tooShort:
+        return l(context).community_nameTooShort(mbinCommunityNameMinLength);
+      case MbinCommunityNameIssue.tooLong:
+        return l(context).community_nameTooLong(mbinCommunityNameMaxLength);
+      case null:
+        return null;
+    }
+  }
+
   @override
   Widget build(BuildContext context) {
     final descriptionDraftController = context.watch<DraftsController>().auto(
       'community:description${widget.data == null ? '' : ':${widget.data}'}',
     );
 
+    final isCreating = widget.data == null;
+    // Mbin is the only backend that rejects names outside of
+    // /^[a-zA-Z0-9_]{2,25}$/, so only validate/suggest for it. The name field
+    // itself is only shown while creating; edits never touch the name.
+    final enforceMbinName =
+        isCreating &&
+        context.watch<AppController>().serverSoftware == ServerSoftware.mbin;
+
+    final name = _nameController.text;
+    final mbinNameIssue = enforceMbinName ? mbinCommunityNameIssue(name) : null;
+    final mbinNameSuggestion = enforceMbinName
+        ? suggestMbinCommunityName(name)
+        : null;
+    final nameInvalidForMbin =
+        enforceMbinName && !isValidMbinCommunityName(name);
+    final mbinNameErrorText = _mbinNameError(context, mbinNameIssue);
+
     return ListView(
       padding: const EdgeInsets.all(16),
       children: [
-        if (widget.data == null)
+        if (isCreating)
           Padding(
             padding: const EdgeInsets.symmetric(vertical: 16),
-            child: TextEditor(
-              _nameController,
-              label: 'Name',
-              onChanged: (_) => setState(() {}),
-              maxLength: 25,
+            child: Column(
+              crossAxisAlignment: CrossAxisAlignment.stretch,
+              children: [
+                TextEditor(
+                  _nameController,
+                  label: 'Name',
+                  onChanged: (_) => setState(() {}),
+                  maxLength: enforceMbinName ? mbinCommunityNameMaxLength : 25,
+                  helperText: enforceMbinName
+                      ? l(context).community_nameMbinHelp
+                      : null,
+                  errorText: mbinNameErrorText,
+                ),
+                if (mbinNameSuggestion case final suggestion?)
+                  Align(
+                    alignment: AlignmentDirectional.centerStart,
+                    child: TextButton.icon(
+                      onPressed: () => setState(() {
+                        _nameController.text = suggestion;
+                      }),
+                      icon: const Icon(Symbols.auto_fix_high_rounded),
+                      label: Text(
+                        l(context).community_nameUseSuggestion(suggestion),
+                      ),
+                    ),
+                  ),
+              ],
             ),
           ),
         Padding(
@@ -180,6 +234,7 @@ class _CommunityOwnerPanelGeneralState
           child: LoadingFilledButton(
             onPressed:
                 _nameController.text.isEmpty ||
+                    nameInvalidForMbin ||
                     _titleController.text.isEmpty ||
                     (_titleController.text == widget.data?.title &&
                         _descriptionController.text ==

lib/src/utils/mbin_community_name.dart

@@ -0,0 +1,61 @@
+/// Helpers for validating and repairing Mbin magazine (community) names.
+///
+/// Mbin restricts magazine names to 2-25 characters consisting only of
+/// letters, digits and underscores (`RegPatterns::MAGAZINE_NAME` /
+/// `/^[a-zA-Z0-9_]{2,25}$/` upstream). Lemmy and PieFed use different rules,
+/// so callers should only apply these checks when talking to an Mbin server.
+library;
+
+const int mbinCommunityNameMinLength = 2;
+const int mbinCommunityNameMaxLength = 25;
+
+final RegExp _mbinCommunityNameRegExp = RegExp(r'^[a-zA-Z0-9_]{2,25}$');
+final RegExp _mbinCommunityNameInvalidChars = RegExp(r'[^a-zA-Z0-9_]');
+
+/// Whether [name] is a valid Mbin magazine name that can be submitted as-is.
+bool isValidMbinCommunityName(String name) =>
+    _mbinCommunityNameRegExp.hasMatch(name);
+
+/// The reason [name] is not a valid Mbin magazine name, or `null` when it is
+/// valid (or still empty, which is treated as "not entered yet").
+MbinCommunityNameIssue? mbinCommunityNameIssue(String name) {
+  if (name.isEmpty) return null;
+  if (_mbinCommunityNameInvalidChars.hasMatch(name)) {
+    return MbinCommunityNameIssue.invalidCharacters;
+  }
+  if (name.length < mbinCommunityNameMinLength) {
+    return MbinCommunityNameIssue.tooShort;
+  }
+  if (name.length > mbinCommunityNameMaxLength) {
+    return MbinCommunityNameIssue.tooLong;
+  }
+  return null;
+}
+
+enum MbinCommunityNameIssue { invalidCharacters, tooShort, tooLong }
+
+/// A best-effort valid name derived from [name], or `null` when nothing
+/// usable can be salvaged (e.g. the input has no letters/digits at all) or
+/// when [name] is already valid.
+String? suggestMbinCommunityName(String name) {
+  if (isValidMbinCommunityName(name)) return null;
+
+  // Replace every run of unsupported characters (whitespace, punctuation,
+  // accented letters, ...) with a single underscore, then tidy up the
+  // underscores so the result reads naturally.
+  var suggestion = name
+      .replaceAll(_mbinCommunityNameInvalidChars, '_')
+      .replaceAll(RegExp(r'_+'), '_')
+      .replaceAll(RegExp(r'^_+|_+$'), '');
+
+  if (suggestion.length > mbinCommunityNameMaxLength) {
+    suggestion = suggestion
+        .substring(0, mbinCommunityNameMaxLength)
+        .replaceAll(RegExp(r'_+$'), '');
+  }
+
+  if (suggestion.length < mbinCommunityNameMinLength) return null;
+  if (suggestion == name) return null;
+
+  return suggestion;
+}

lib/src/widgets/text_editor.dart

@@ -6,6 +6,8 @@ class TextEditor extends StatelessWidget {
     this.keyboardType,
     this.label,
     this.hint,
+    this.helperText,
+    this.errorText,
     this.onChanged,
     this.enabled,
     this.maxLength,
@@ -17,6 +19,8 @@ class TextEditor extends StatelessWidget {
   final TextInputType? keyboardType;
   final String? label;
   final String? hint;
+  final String? helperText;
+  final String? errorText;
   final void Function(String)? onChanged;
   final bool? enabled;
   final int? maxLength;
@@ -31,6 +35,10 @@ class TextEditor extends StatelessWidget {
         border: const OutlineInputBorder(),
         labelText: label,
         hintText: hint,
+        helperText: helperText,
+        helperMaxLines: 3,
+        errorText: errorText,
+        errorMaxLines: 3,
       ),
       onChanged: onChanged,
       enabled: enabled,

pubspec.yaml

@@ -79,6 +79,10 @@ dependencies:
   unifiedpush_platform_interface: ^4.0.0
   unifiedpush_storage_interface: ^1.0.0
 
+dev_dependencies:
+  flutter_test:
+    sdk: flutter
+
 # Needed for 16kb page, remove once this pr (https://github.com/google/webcrypto.dart/pull/238) is merged and the next version is released.
 dependency_overrides:
   webcrypto:

test/utils/mbin_community_name_test.dart

@@ -0,0 +1,96 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:interstellar/src/utils/mbin_community_name.dart';
+
+void main() {
+  group('isValidMbinCommunityName', () {
+    test('accepts letters, digits and underscores within 2-25 chars', () {
+      expect(isValidMbinCommunityName('ab'), isTrue);
+      expect(isValidMbinCommunityName('under_score'), isTrue);
+      expect(isValidMbinCommunityName('Mixed_Case_123'), isTrue);
+      expect(isValidMbinCommunityName('a' * 25), isTrue);
+    });
+
+    test('rejects empty, too short and too long names', () {
+      expect(isValidMbinCommunityName(''), isFalse);
+      expect(isValidMbinCommunityName('a'), isFalse);
+      expect(isValidMbinCommunityName('a' * 26), isFalse);
+    });
+
+    test('rejects unsupported characters', () {
+      expect(isValidMbinCommunityName('with space'), isFalse);
+      expect(isValidMbinCommunityName('with-hyphen'), isFalse);
+      expect(isValidMbinCommunityName('dot.separated'), isFalse);
+      expect(isValidMbinCommunityName('accenté'), isFalse);
+    });
+  });
+
+  group('mbinCommunityNameIssue', () {
+    const invalid = MbinCommunityNameIssue.invalidCharacters;
+    const tooShort = MbinCommunityNameIssue.tooShort;
+    const tooLong = MbinCommunityNameIssue.tooLong;
+
+    test('returns null for an empty (not yet entered) name', () {
+      expect(mbinCommunityNameIssue(''), isNull);
+    });
+
+    test('returns null for a valid name', () {
+      expect(mbinCommunityNameIssue('valid_Name_123'), isNull);
+    });
+
+    test('reports invalid characters ahead of length problems', () {
+      expect(mbinCommunityNameIssue('hello world'), invalid);
+      expect(mbinCommunityNameIssue('!'), invalid);
+    });
+
+    test('reports names that are too short', () {
+      expect(mbinCommunityNameIssue('a'), tooShort);
+    });
+
+    test('reports names longer than 25 characters', () {
+      expect(mbinCommunityNameIssue('a' * 26), tooLong);
+    });
+  });
+
+  group('suggestMbinCommunityName', () {
+    test('returns null when the name is already valid', () {
+      expect(suggestMbinCommunityName('already_valid'), isNull);
+    });
+
+    test('replaces unsupported runs with a single underscore', () {
+      final result = suggestMbinCommunityName('My Cool Community!');
+      expect(result, 'My_Cool_Community');
+      expect(suggestMbinCommunityName('a...b---c'), 'a_b_c');
+    });
+
+    test('trims leading and trailing underscores', () {
+      expect(suggestMbinCommunityName('  hello!!  '), 'hello');
+    });
+
+    test('truncates to 25 characters without a trailing underscore', () {
+      final s = suggestMbinCommunityName('abcdefghijklmnopqrstuvwx yz');
+      expect(s, 'abcdefghijklmnopqrstuvwx');
+      expect(isValidMbinCommunityName(s!), isTrue);
+    });
+
+    test('returns null when nothing usable can be salvaged', () {
+      expect(suggestMbinCommunityName('a'), isNull);
+      expect(suggestMbinCommunityName('   '), isNull);
+      expect(suggestMbinCommunityName('日本語'), isNull);
+    });
+
+    test('always produces a valid name when it returns one', () {
+      const inputs = [
+        'hello world',
+        'Trailing punctuation???',
+        '***leading',
+        'lots     of     spaces',
+        'cafe-society #2',
+      ];
+      for (final input in inputs) {
+        final suggestion = suggestMbinCommunityName(input);
+        if (suggestion == null) continue;
+        expect(isValidMbinCommunityName(suggestion), isTrue, reason: input);
+      }
+    });
+  });
+}