Forms, Validation & Focus

For Forms, Validation & Focus, define the source of truth, validation, focus, controller disposal, and minimal rebuild boundary. Keep ephemeral state local and place shared app state in a testable controller or model.

Build a Complete Mental Model

Apply It in Production Flutter

Run and test an original Forms, Validation & Focus example with small and large screens, text scaling, empty/loading/error states, rapid interaction, navigation restoration, and disposal cases. Use flutter analyze, widget tests, and profile mode where relevant.

After This Lesson

Explain constraints, state ownership, and lifecycle
Write an example that passes flutter analyze
Test a widget interaction or failure state

dart

final formKey = GlobalKey<FormState>();

Form(
  key: formKey,
  child: Column(children: [
    TextFormField(
      decoration: const InputDecoration(labelText: 'Email'),
      validator: (value) => (value?.contains('@') ?? false) ? null : 'Enter a valid email',
    ),
    FilledButton(
      onPressed: () => formKey.currentState!.validate(),
      child: const Text('Submit'),
    ),
  ]),
)

You should see

Invalid input shows an inline validation message

Try It Yourself

Lifecycle and Performance Warning

Assuming one screen's sample output proves every rebuild, overflow, accessibility, async, and lifecycle path.

Flutter documentation — Flutter

What you'll walk away with

Build a Complete Mental Model

Apply It in Production Flutter

After This Lesson