Swift `in` keyword meaning?

问题

I am trying to implement some code from parse.com and I notice a keyword in after the void.

I am stumped what is this ? The second line you see the Void in

PFUser.logInWithUsernameInBackground("myname", password:"mypass") {
  (user: PFUser?, error: NSError?) -> Void in
  if user != nil {
    // Do stuff after successful login.
  } else {
    // The login failed. Check error to see why.
  }
}

The docs don't document this. I know the in keyword is used in for loops.

Anyone confirm?

回答1:

In a named function, we declare the parameters and return type in the func declaration line.

func say(s:String)->() {
    // body
}

In an anonymous function, there is no func declaration line - it's anonymous! So we do it with an in line at the start of the body instead.

{
    (s:String)->() in
    // body
}

(That is the full form of an anonymous function. But then Swift has a series of rules allowing the return type, the parameter types, and even the parameter names and the whole in line to be omitted under certain circumstances.)

回答2:

The start of the closure’s body is introduced by the in keyword. This keyword indicates that the definition of the closure’s parameters and return type has finished, and the body of the closure is about to begin.

Source: https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Closures.html

回答3:

Closure expression syntax has the following general form:

来源：https://stackoverflow.com/questions/30379108/swift-in-keyword-meaning

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