How to validate a DateTimeField in Flutter?

Question

This Flutter cookbook entry shows validating a text field:

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

However, it doesn't work with a DateTimefield: The getter 'isEmpty' isn't defined for the type 'DateTime'.

Comparing to null works nicely:

DateTimeField(
  format: "MM-dd-yyyy",
  controller: _dateController,
  onShowPicker: (context, currentValue) {
    return showDatePicker(
        context: context,
        firstDate: DateTime(1900),
        initialDate: currentValue ?? DateTime.now(),
        lastDate: DateTime(2100));
  },
  validator: (value) {
    if (value == null) {
      return 'Please enter a date';
    }
    return null;
  },
  onChanged: (value) {
  },
)

HOWEVER, putting this code in an edit widget somehow fails because value is null AT FIRST no matter what the widget is showing:

If I pick a date, which calls onChanged, the value is not null, and everything validates.

But .. how to validate an existing field? Why is value on the screen, but not set for the validator?

Answer 1

等待大神答复