What's the meaning of passing underscore _ "_" (_) when calling a function in dart/flutter?

Question

When reading dart code I often see some functions called with just an underscore _ parameter. It's bugging me out for some time now and since flutter has improved its analysis messages I have some clues... but I feel like I don't really grasp this concept :-(

Yesterday I wrote the following for a test :

when(mockDevice.getLocalPath()).thenAnswer(() async => fileFolder);

and obtain the following analysis

error: The argument type 'Future Function()' can't be assigned to the parameter type 'Future Function(Invocation)'.

When adding underscore it's working perfectly.

when(mockDevice.getLocalPath()).thenAnswer((_) async => fileFolder);

The most frightenning example I meet come from provider package written by @remi rousselet

builder: (_, counter, __) => Translations(counter.value),

It's from the provider example :

Widget build(BuildContext context) {
  return MultiProvider(
    providers: [
      ChangeNotifierProvider(builder: (_) => Counter()),
      ProxyProvider<Counter, Translations>(
        builder: (_, counter, __) => Translations(counter.value),
      ),
    ],
    child: Foo(),
  );
}

class Translations {
  const Translations(this._value);

  final int _value;

  String get title => 'You clicked $_value times';
}

Answer 1

Underscore is normally an indication that you are not going to use that parameter inside the block it is just a good way to write code, for instance:

method(int useful, int useless) {
  // say I am only going to use 'useful' in this block 
}

Above code can also be written as:

method(int useful, int _) {
  // using '_' means I'm not going to use 2nd parameter in the block
}

---

Answer to your question now:

builder: (_, counter, __) => Translations(counter.value),

means you have 3 parameters _, counter and __, and only counter is what you are using, so 1st and 3rd parameters are denoted with _ and __. This is just cleaner way to write code.