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

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
}

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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.