docs: add submit button state example #1490

Author: rednikisfun

README.md

@@ -154,6 +154,10 @@ FormBuilder(
 ),
 ```
 
+For a complete example that enables or disables the submit button from form
+state and validates an email field conditionally, see
+[Submit Button State](example/lib/sources/submit_button.dart).
+
 #### Building your own custom field
 
 To build your own field within a `FormBuilder`, we use `FormBuilderField` which will require that you define your own field.

example/README.md

@@ -10,6 +10,7 @@ This example showcases various form building patterns and use cases:
 - **Sign Up Form** - A practical sign-up form example
 - **Dynamic Fields** - Forms with fields that can be added/removed at runtime
 - **Conditional Fields** - Fields that show/hide based on other field values
+- **Submit Button State** - Submit button enabled from form state with conditional validation
 - **Related Fields** - Forms with interdependent field values
 - **Grouped Radio/Checkbox** - Selection inputs with grouping
 - **Decorated Radio/Checkbox** - Custom styling for selection fields
@@ -48,6 +49,7 @@ lib/
 │   ├── signup_form.dart            # Sign up form
 │   ├── dynamic_fields.dart         # Dynamic field management
 │   ├── conditional_fields.dart     # Conditional visibility
+│   ├── submit_button.dart          # Submit button state and validation
 │   ├── related_fields.dart         # Field dependencies
 │   ├── grouped_radio_checkbox.dart # Grouped selections
 │   ├── decorated_radio_checkbox.dart # Styled selections

example/lib/main.dart

@@ -3,6 +3,7 @@ import 'package:example/sources/decorated_radio_checkbox.dart';
 import 'package:example/sources/dynamic_fields.dart';
 import 'package:example/sources/grouped_radio_checkbox.dart';
 import 'package:example/sources/related_fields.dart';
+import 'package:example/sources/submit_button.dart';
 import 'package:flutter/material.dart';
 import 'package:flutter_localizations/flutter_localizations.dart';
 import 'package:form_builder_validators/form_builder_validators.dart';
@@ -132,6 +133,23 @@ class _HomePage extends StatelessWidget {
             },
           ),
           const Divider(),
+          ListTile(
+            title: const Text('Submit Button State'),
+            trailing: const Icon(Icons.arrow_right_sharp),
+            onTap: () {
+              Navigator.of(context).push(
+                MaterialPageRoute(
+                  builder: (context) {
+                    return const CodePage(
+                      title: 'Submit Button State',
+                      child: SubmitButtonForm(),
+                    );
+                  },
+                ),
+              );
+            },
+          ),
+          const Divider(),
           ListTile(
             title: const Text('Related Fields'),
             trailing: const Icon(Icons.arrow_right_sharp),

example/lib/sources/submit_button.dart

@@ -0,0 +1,102 @@
+import 'package:flutter/material.dart';
+import 'package:flutter_form_builder/flutter_form_builder.dart';
+import 'package:form_builder_validators/form_builder_validators.dart';
+
+class SubmitButtonForm extends StatefulWidget {
+  const SubmitButtonForm({super.key});
+
+  @override
+  State<SubmitButtonForm> createState() => _SubmitButtonFormState();
+}
+
+class _SubmitButtonFormState extends State<SubmitButtonForm> {
+  final _formKey = GlobalKey<FormBuilderState>();
+  bool _canSubmit = false;
+  bool _subscribe = false;
+  bool _submitted = false;
+
+  void _updateSubmitButton() {
+    final formState = _formKey.currentState;
+    if (formState == null) {
+      return;
+    }
+
+    final isSubscribed = formState.instantValue['subscribe'] == true;
+    final isValid = formState.validate(focusOnInvalid: false);
+
+    setState(() {
+      _canSubmit = isValid;
+      _subscribe = isSubscribed;
+      _submitted = false;
+    });
+  }
+
+  String? _validateEmail(String? value) {
+    final emailRequired =
+        _formKey.currentState?.instantValue['subscribe'] == true;
+    final hasEmail = value?.isNotEmpty ?? false;
+
+    if (!emailRequired && !hasEmail) {
+      return null;
+    }
+
+    return FormBuilderValidators.compose([
+      FormBuilderValidators.required(),
+      FormBuilderValidators.email(),
+    ])(value);
+  }
+
+  void _submit() {
+    if (_formKey.currentState?.saveAndValidate() ?? false) {
+      debugPrint(_formKey.currentState?.value.toString());
+      setState(() {
+        _submitted = true;
+      });
+    }
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    return SingleChildScrollView(
+      child: FormBuilder(
+        key: _formKey,
+        autovalidateMode: AutovalidateMode.onUserInteraction,
+        initialValue: const {'subscribe': false},
+        onChanged: _updateSubmitButton,
+        child: Column(
+          children: [
+            FormBuilderTextField(
+              name: 'name',
+              decoration: const InputDecoration(labelText: 'Name'),
+              validator: FormBuilderValidators.required(),
+            ),
+            const SizedBox(height: 10),
+            FormBuilderSwitch(
+              name: 'subscribe',
+              title: const Text('Receive product updates'),
+            ),
+            const SizedBox(height: 10),
+            FormBuilderTextField(
+              name: 'email',
+              enabled: _subscribe,
+              decoration: InputDecoration(
+                labelText: _subscribe ? 'Email' : 'Email (optional)',
+              ),
+              keyboardType: TextInputType.emailAddress,
+              validator: _validateEmail,
+            ),
+            const SizedBox(height: 10),
+            ElevatedButton(
+              onPressed: _canSubmit ? _submit : null,
+              child: const Text('Submit'),
+            ),
+            if (_submitted) ...[
+              const SizedBox(height: 10),
+              Text('Submitted values: ${_formKey.currentState?.value}'),
+            ],
+          ],
+        ),
+      ),
+    );
+  }
+}