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

user_7161429 0
Viewed 4515

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';
}
dart flutter
2 Answers

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.

user_66186220

In Dart (the base language of flutter), if a class, class member, variable starts with an underscore, it means that it is private to its library. This is how dart typically deals with encapsulation as opposed to Java where you have your keywords like private, protected and public.

user_120365000
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