Texfield de validación con el proveedor de flutter

campo de texto de validación en flutter

TextFormField(
  // The validator receives the text that the user has entered.
  validator: (value) {
    if (value == null || value.isEmpty) {
      return 'Please enter some text';
    }
    return null;
  },
),

/*
	The TextFormField widget renders a material design 
    text field and can display validation errors when 
    they occur.

	Validate the input by providing a validator() function 
    to the TextFormField. If the user’s input isn’t valid, 
    the validator function returns a String containing an 
    error message. If there are no errors, the validator 
    must return null.

	For this example, We create a validator that ensures the TextFormField 
    isn’t empty. If it is empty, then we return a friendly error message.
*/