flutter-building-forms

Create a GlobalKey<FormState> in your State class, pass it to the Form widget, add validator callbacks to each TextFormField, then call _formKey.currentState!.validate() in your submit button. Validators return an error string on failure or null on success.

Provide a validator function to each TextFormField that receives the current input value. Return a String containing the error message when input is invalid, or return null when the input passes validation. The Form automatically displays error text below the field.

Form state is lost when a new GlobalKey is created inside the build method on every rebuild. Instantiate the GlobalKey<FormState> once as a final variable in the State class so the form identity persists across rebuilds.

Yes, use Form.of(context) from a descendant widget to access the nearest FormState when passing a GlobalKey through a complex widget tree is impractical. This works only for widgets below the Form in the tree.

When validate() returns false, one or more validators returned an error string and the FormState automatically rebuilds the UI to display those error messages below the fields. The user must correct the input and resubmit until validation passes.