Flutter short video #02: How to validate form inputs in flutter (form validation in flutter)?

This video explains flutter forms validation in detail and a signup form is also validated.

The following validation rules were checked:
- Validate email address
- Validate empty fields
- Validate password length
- Validate if passwords match to each other

Some of the concepts that this video covers are as below:
How to implement validation in flutter forms?
How flutter form's input fields validator works?
How to write validation rules?
What to return when there is an error and what to return when rule passes for form input fields in flutter?
How to save values in validation rules, so they can be used in other flutter form input fields?
How to stop values from getting submitted if the validation rules fail in flutter form input fields?
How to use GlobalKey FormState to run validation against all input form fields for flutter?