双下划线的目的是什么，在一个下划线之后，作为 function / class 方法的参数？

Question

In my exploration of Polymer Dart 1.0.0, I've found for events, and observer methods, I am forced to use this pattern

@reflectable
void someEvent([_, __]) {
    ...
}

or on an observer method

@Observe('someField')
void someFieldChanged([_, __]) {
    ...
}

I understand what the square brackets are for, optional parameters, I also understand that if you don't care about the passed parameters, you can represent this parameter with the underscore. What surprised me was the examples I looked at used double underscore, __, as the second symbol between the square brackets. When I tried to use just a single underscore again, I get a duplicate formal parameter error. Is there some reason why the second parameter you don't care about has to be different from the first? By this logic, if I include a third one, does it mean it'll have to be a triple underscore ___?

Thanks.

Answer 1

Nothing special. _ as __ as a are just variable identifiers. _ is often used to name an unused variable. Here there are 2 variables unused, the first one is named _ and the second one __ . With multiple unused variables it's common to name them _ , __ , ___ ... or _1 , _2 , _3 ...

Answer 2

In Polymer 3.0 a single underscore stands for a protected property or method and a double underscore stands for members that are private to the class.

Please take a look into this reference for more information: https://polymer-library.polymer-project.org/3.0/docs/devguide/properties