User input and forms

User input has two layers:

• Gestures — GestureDetector (tap, drag, long press) and InkWell (the same plus a Material ripple). If a ready-made button widget exists, use it; GestureDetector is for the cases it does not cover.
• Text input — TextField on its own, TextFormField inside a Form with validation.

The Form + GlobalKey<FormState> combination is the standard pattern: _formKey.currentState!.validate() runs every field's validator and returns the overall result as a bool.

There are two ways to read a text field's value, and the choice matters:

• `TextEditingController` — when you need to read or set the value at arbitrary times. Created in State, closed in dispose()
• `onChanged` / `onSaved` — when collecting the value is enough; lighter than a controller

autovalidateMode controls when validation runs. AutovalidateMode.onUserInteraction is usually the best UX: it does not shout at a user who has typed nothing yet, but gives immediate feedback once they start.