validator property

An optional method that validates an input. Returns an error string to display if the input is invalid, or null otherwise.

The returned value is exposed by the FormFieldState.errorText property. It is transformed using errorBuilder.

Alternating between error and normal states can cause the height of the FTextField to change if no other subtext decoration is set on the field. To create a field whose height is fixed regardless of whether or not an error is displayed, wrap the FTextField in a fixed height parent like SizedBox.