refactor(create): declare template vars via resolver map

lib/src/commands/create/commands/create_subcommand.dart

@@ -22,6 +22,9 @@ const _defaultDescription = 'A Very Good Project created by Very Good CLI.';
 /// A method which returns a [Future<MasonGenerator>] given a [MasonBundle].
 typedef MasonGeneratorFromBundle = Future<MasonGenerator> Function(MasonBundle);
 
+/// Lazy resolvers for template vars, keyed by the brick var name.
+typedef TemplateVarResolvers = Map<String, Object? Function()>;
+
 /// {@template create_subcommand}
 /// Generic class for sub commands of [CreateCommand].
 /// {@endtemplate}
@@ -259,35 +262,45 @@ abstract class CreateSubCommand({
     return ExitCode.success.code;
   }
 
-  /// Responsible for returns the template parameters to be passed to the
-  /// template brick.
+  /// The template vars this command handles, keyed by the brick var name.
+  ///
+  /// Each resolver is only invoked by [getTemplateVars], so the keys can be
+  /// read without parsing any arguments.
   ///
-  /// Override if the create sub command requires additional template
-  /// parameters.
+  /// Override to handle additional template vars, spreading
+  /// `super.templateVarResolvers` into the returned map.
   ///
   /// For subcommands that mix with [OrgName], it includes 'org_name'.
   /// For subcommands that mix with [Publishable], it includes 'publishable'.
   /// For subcommands that mix with [Workspace], it includes 'workspace'.
   @mustCallSuper
-  Map<String, dynamic> getTemplateVars() {
-    final projectName = this.projectName;
-    final projectDescription = this.projectDescription;
-
-    return <String, dynamic>{
-      'project_name': projectName,
-      'description': projectDescription,
-      if (this is OrgName) 'org_name': (this as OrgName).orgName,
-      if (this is Publishable) 'publishable': (this as Publishable).publishable,
-      if (this is Workspace) 'workspace': (this as Workspace).workspace,
-    };
-  }
+  TemplateVarResolvers get templateVarResolvers => {
+    'project_name': () => projectName,
+    'description': () => projectDescription,
+  };
+
+  /// Returns the template parameters to be passed to the template brick.
+  ///
+  /// Computed from [templateVarResolvers]. Vars that resolve to `null` are
+  /// omitted so the template falls back to its default value.
+  @nonVirtual
+  Map<String, dynamic> getTemplateVars() => <String, dynamic>{
+    for (final MapEntry(:key, value: resolve) in templateVarResolvers.entries)
+      key: ?resolve(),
+  };
 }
 
 /// Mixin for [CreateSubCommand] subclasses that receives the org name
 /// parameter.
 ///
 /// Takes care of parsing from [argResults] and validating the org name.
 mixin OrgName on CreateSubCommand {
+  @override
+  TemplateVarResolvers get templateVarResolvers => {
+    ...super.templateVarResolvers,
+    'org_name': () => orgName,
+  };
+
   /// Gets the organization name.
   String get orgName {
     final orgName = argResults.resolve<String>(
@@ -365,6 +378,12 @@ mixin MultiTemplates on CreateSubCommand {
 /// Takes care of parsing it from [argResults] and pass it
 /// to the brick generator.
 mixin Publishable on CreateSubCommand {
+  @override
+  TemplateVarResolvers get templateVarResolvers => {
+    ...super.templateVarResolvers,
+    'publishable': () => publishable,
+  };
+
   /// Gets the publishable flag.
   bool get publishable => argResults.resolve<bool>(
     'publishable',
@@ -378,6 +397,12 @@ mixin Publishable on CreateSubCommand {
 /// Takes care of parsing it from [argResults] and pass it
 /// to the brick generator.
 mixin Workspace on CreateSubCommand {
+  @override
+  TemplateVarResolvers get templateVarResolvers => {
+    ...super.templateVarResolvers,
+    'workspace': () => workspace,
+  };
+
   /// Gets the workspace flag.
   bool get workspace => argResults.resolve<bool>(
     'workspace',

lib/src/commands/create/commands/dart_cli.dart

@@ -25,14 +25,9 @@ class CreateDartCLI({required super.logger, required super.generatorFromBundle})
   Template get template => VeryGoodDartCLITemplate();
 
   @override
-  Map<String, dynamic> getTemplateVars() {
-    final vars = super.getTemplateVars();
-
-    final executableName =
-        argResults['executable-name'] as String? ?? projectName;
-
-    vars['executable_name'] = executableName;
-
-    return vars;
-  }
+  TemplateVarResolvers get templateVarResolvers => {
+    ...super.templateVarResolvers,
+    'executable_name': () =>
+        argResults['executable-name'] as String? ?? projectName,
+  };
 }

lib/src/commands/create/commands/docs_site.dart

@@ -28,16 +28,14 @@ class CreateDocsSite({
   String get description => 'Generate a Very Good documentation site.';
 
   @override
-  Map<String, dynamic> getTemplateVars() {
-    return <String, dynamic>{
-      ...super.getTemplateVars(),
-      'org_name': argResults.resolve<String>(
-        'org-name',
-        createConfig.orgName,
-        fallbackValue: _defaultOrgName,
-      ),
-    };
-  }
+  TemplateVarResolvers get templateVarResolvers => {
+    ...super.templateVarResolvers,
+    'org_name': () => argResults.resolve<String>(
+      'org-name',
+      createConfig.orgName,
+      fallbackValue: _defaultOrgName,
+    ),
+  };
 
   @override
   Template get template => VeryGoodDocsSiteTemplate();

lib/src/commands/create/commands/flame_game.dart

@@ -37,10 +37,8 @@ class CreateFlameGame({
   Template get template => VeryGoodFlameGameTemplate();
 
   @override
-  Map<String, dynamic> getTemplateVars() {
-    final vars = super.getTemplateVars();
-    final platforms = argResults['platforms'] as List<String>;
-    vars['platforms'] = platforms;
-    return vars;
-  }
+  TemplateVarResolvers get templateVarResolvers => {
+    ...super.templateVarResolvers,
+    'platforms': () => argResults['platforms'] as List<String>,
+  };
 }

lib/src/commands/create/commands/flutter_app.dart

@@ -42,19 +42,11 @@ class CreateFlutterApp({
   String get description => 'Generate a Very Good Flutter application.';
 
   @override
-  Map<String, dynamic> getTemplateVars() {
-    final vars = super.getTemplateVars();
-
-    final applicationId = argResults['application-id'] as String?;
-    if (applicationId != null) {
-      vars['application_id'] = applicationId;
-    }
-
-    final platforms = argResults['platforms'] as List<String>;
-    vars['platforms'] = platforms;
-
-    return vars;
-  }
+  TemplateVarResolvers get templateVarResolvers => {
+    ...super.templateVarResolvers,
+    'application_id': () => argResults['application-id'] as String?,
+    'platforms': () => argResults['platforms'] as List<String>,
+  };
 
   @override
   final List<Template> templates = [VeryGoodCoreTemplate()];

lib/src/commands/create/commands/flutter_plugin.dart

@@ -38,13 +38,8 @@ class CreateFlutterPlugin({
   Template get template => FlutterPluginTemplate();
 
   @override
-  Map<String, dynamic> getTemplateVars() {
-    final vars = super.getTemplateVars();
-
-    final platforms = argResults['platforms'] as List<String>;
-
-    vars['platforms'] = platforms;
-
-    return vars;
-  }
+  TemplateVarResolvers get templateVarResolvers => {
+    ...super.templateVarResolvers,
+    'platforms': () => argResults['platforms'] as List<String>,
+  };
 }

test/src/commands/create/create_subcommand_test.dart

@@ -62,6 +62,19 @@ class _TestCreateSubCommandWithWorkspace({
   required super.generatorFromBundle,
 }) extends _TestCreateSubCommand with Workspace;
 
+class _TestCreateSubCommandWithNullableVar({
+  required super.template,
+  required super.logger,
+  required super.generatorFromBundle,
+}) extends _TestCreateSubCommand {
+  @override
+  TemplateVarResolvers get templateVarResolvers => {
+    ...super.templateVarResolvers,
+    'nullable': () => null,
+    'present': () => 'value',
+  };
+}
+
 class _TestCreateSubCommandMultiTemplate({
   @override required final List<Template> templates,
   required super.logger,
@@ -159,6 +172,34 @@ Run "runner help" to see global options.''';
               .having((o) => o.mandatory, 'mandatory', false),
         });
         expect(command.argParser.commands, isEmpty);
+        expect(command.templateVarResolvers.keys, [
+          'project_name',
+          'description',
+        ]);
+      });
+    });
+
+    group('templateVarResolvers', () {
+      test('getTemplateVars omits vars that resolve to null', () {
+        final argResults = _MockArgResults();
+        when(() => argResults.rest).thenReturn(['test_project']);
+        when(() => argResults['description']).thenReturn('description');
+        when(() => argResults.wasParsed(any())).thenReturn(true);
+
+        final command = _TestCreateSubCommandWithNullableVar(
+          template: template,
+          logger: logger,
+          generatorFromBundle: null,
+        )..argResultOverrides = argResults;
+
+        expect(
+          command.getTemplateVars(),
+          equals({
+            'project_name': 'test_project',
+            'description': 'description',
+            'present': 'value',
+          }),
+        );
       });
     });
 
@@ -611,6 +652,11 @@ Run "runner help" to see global options.''';
               .having((o) => o.aliases, 'aliases', ['org']),
         );
         expect(command.argParser.commands, isEmpty);
+        expect(command.templateVarResolvers.keys, [
+          'project_name',
+          'description',
+          'org_name',
+        ]);
       });
     });
 
@@ -1391,6 +1437,11 @@ Run "runner help" to see global options.''';
               .having((o) => o.aliases, 'aliases', <String>[]),
         );
         expect(command.argParser.commands, isEmpty);
+        expect(command.templateVarResolvers.keys, [
+          'project_name',
+          'description',
+          'publishable',
+        ]);
       });
     });
 
@@ -1585,6 +1636,11 @@ Run "runner help" to see global options.''';
               .having((o) => o.aliases, 'aliases', ['ws']),
         );
         expect(command.argParser.commands, isEmpty);
+        expect(command.templateVarResolvers.keys, [
+          'project_name',
+          'description',
+          'workspace',
+        ]);
       });
     });