what do "_" and "in" mean in Swift programming language?

Question

I found some code written in Swift on github,

And get a little confused about this line

var done: (NSError?, NSData, NSString?) -> () = { (_, _, _) -> () in }

could you please explain the real meaning of this line? Thank you very much!

Answer 1

_ means don't name that thing. It can be used in a number of places. In your case, it is saying ignore the variable being passed into the closure. The code you gave is ignoring all parameters but you can also just ignore some parameters.

in is the start of the implementation of the closure. In your example code, the implementation of the closure is empty.

Overall, that line is defining a closure called "done" that takes an Optional NSError (NSError?), NSData (NSData), and Optional NSString (NSString?) and returns nothing (-> ()). The actual implementation of the closure does nothing.