How to assign variable from inside an if expression?

Question

I want to assign value to a variable inside the if expression.

func message() -> Int?{
  var y = "" 
  if x.Sender(msg, reply: { message in
      replyFunc {
          if message.command == .success {
               y = "Hello World"
               print(y) //It prints hello world
          }   
       }   
  }) {   
      print("Error") 
  }
  return y //Here I don't get Hello World
}

I want to assign 'y' from inside the expression.

Edit: This issue is of asynchronous callback and please refer to Matt's comment for the tutorial.

And what prevents you from doing so? Is there a compile error? — matt, Jul 23 '19 at 13:26
You won't get the output. Because your print executes before closure executes — Loren Rogers, Jul 23 '19 at 13:33
But I know that the variable is set to Hello World. But the string becomes empty when I print it at the end. — curiously77, Jul 23 '19 at 13:36
Set a breakpoint at the "print" statement and set a breakpoint at the setting statement. The print will hit before the set will — Loren Rogers, Jul 23 '19 at 13:38
Unfortunately, I'm using playground so I can't set breakpoint. — curiously77, Jul 23 '19 at 13:38
This has nothing to do with `if` expressions, and everything with asynchronous processing. — Gereon, Jul 23 '19 at 13:39
So, what can I do about it? Is there anyway to delay it or make it execute at the end? — curiously77, Jul 23 '19 at 13:40
Simply do the printing inside the closure, when the value has been set. — Gereon, Jul 23 '19 at 13:40
What you need to do is break out the handling of the return of message() and have the closure call THAT when it gets executed — Loren Rogers, Jul 23 '19 at 13:43
The code doesn't even compile. You cannot **`return`** something from an asynchronous completion handler because it doesn't have a return value. — vadian, Jul 23 '19 at 13:54

Moris Kramer · Accepted Answer · 2019-07-23T20:54:21.567

This is a good question a lot of new to swift/async programmers fall.

Either Sender or ReplyFunc are async functions: Meaning they will open a new thread(for simplicity sake) and do the assignment there. Two things will happen in parallel and you cant know which going to be first. Because Entering closure and then entering another closure is more work in reality order of execution might be:

First: return y
Second: y = "whatever string you had to assign"

Solution: Return completion closure.

    func message(completion: (String)->()) {
        if x.Sender(msg, reply: { message in
            replyFunc {
                if message.command == .success {
                    completion("Hello World")//It send hello world
                } else {
                    completion("Failure")
                }
            }
        }) {
            completion("Error")
        }
    }

Consuming Y:

    message { y in
        print(y) // here is my "Hello World" or "Error" when error
    }

NOTE: You must always, with all flows, run completion, this is a best practice.

Loren Rogers · Answer 2 · 2019-07-23T13:51:16.977

-3

func someMethod() {
    _ = message()
}

func handleReply(message: SomeType) {
    if message.command = .success {
        y = "Hello World"
    }
}

func message() -> Int?{
      var y = "" 
      if x.Sender(msg, reply: { message in
          replyFunc {
              handleReply(message)
           }   
      }) {   
          print("Error") 
      }
      return y //Here I don't get Hello World
    }

edited Jul 23 '19 at 13:51

answered Jul 23 '19 at 13:26

Loren Rogers

325
3
13

I have edited the code. I am not getting the output for y as Hello World. Instead I get an empty string. – curiously77 Jul 23 '19 at 13:34
See my comment above. The print() executes before the closure does. – Loren Rogers Jul 23 '19 at 13:35
Okay, so you mean the print statement at the end executes before the if expression? – curiously77 Jul 23 '19 at 13:38
Correct. The 'if' statement is inside a closure – Loren Rogers Jul 23 '19 at 13:39

How to assign variable from inside an if expression?

2 Answers2