Minimize code duplication

[ajilo297]

I am working on another PR which would allow to remove country flags from the Selector list. This PR is just to suggest some improvements in the code before I do that. I have attempted to do 2 things in this PR

1. Replace Function(Country) onCountrySelected with ValueChanged<Country> onCountrySelected.
2. Use CountrySelectorNavigator class to hold the common values and return a CountrySelector with appropriate values

Let me know how this looks.

[cedvdb]

Nice PR, thanks.

In the same vein as your other changes, maybe the defaults values should be the same for all and set up on super (specifically the three boolean values).

However the way defaults work in dart might make the code less readable. What do you think? I can merge this as is though.

[ajilo297]

Nice PR, thanks.

In the same vein as your other changes, maybe the defaults values should be the same for all and set up on super (specifically the three boolean values).

However the way defaults work in dart might make the code less readable. What do you think? I can merge this as is though.

@cedvdb So you mean something like super(addSeparator: addSeparator ?? true) and make the booleans required in CountrySelectorNavigator?

[cedvdb]

I meant that the defaults are repeated in every constructors but I don't see how we could resolve that without a bunch of null checks:

    bool addSeparator = true,
    bool showCountryCode = true,
    bool sortCountries = false,

but I don't see how we can cleanly remove that without a bunch of null checks, this might be a bit less readable:

  const CountrySelectorNavigator({
   this.countries,
   this.favorites,
   bool? addSeparator,
   bool? showCountryCode,
   bool? sortCountries,
   this.noResultMessage,
 }):
   addSeparator = addSeparator ?? false,
   showCountryCode = showCountryCode ?? true,
   sortCountries = sortCountries ?? true;
   
   
 const DialogNavigator({
   List<Country>? countries,
   List<String>? favorites,
   bool? addSeparator,
   bool? showCountryCode,
   bool? sortCountries,
   String? noResultMessage,
 }) : super(
         countries: countries,
         favorites: favorites,
         addSeparator: addSeparator,
         showCountryCode: showCountryCode,
         sortCountries: sortCountries,
         noResultMessage: noResultMessage,
       );

I do believe the defaults should be the same for all of them though, and that is a way to enforce it but that's minor.

[cedvdb]

My hope is dart-lang/language#1855 could alleviate this anyway.