Why CustomStringConvertible protocol description being called multiple times?

Question

I have written a struct in iOS playground and want to customize the print format of it.

struct Point {
    let x: Int, y: Int
}

extension Point: CustomStringConvertible {
    var description: String {
       switch (x, y) {
       case let (x, 1..<10):
          print("y in the range")
          return "(\(x), 1..<10)"
       default:
          return "(\(x), \(y))"
      }
   }
}

let p = Point(x: 1, y: 1)
print(p)

The result is

I cannot understand that even though I called print only once but y in the range message is being printed 4 times.

@MartinR really? Because I create a new iOS playground to double confirm, the console prints as I posted — shoujs, Jun 26 '17 at 13:33
A Playground displays the values in the right column, therefore calls `description` multiple times. — Martin R, Jun 26 '17 at 13:35
My general (free!) advice: If *anything* happens in the Playground which you don't understand, try it in a compiled project instead. Then use the debugger, if necessary. — Martin R, Jun 26 '17 at 13:38

score 3 · Accepted Answer · answered Jun 26 '17 at 13:36

If you're using a playground, the description of a value may be computed multiple times because it will be shown in multiple places (e.g. on the right).

If you're executing the code in a more controlled environment (like in compiled code or in the REPL in a terminal), you will notice that y in the range will only be printed once.

Also you should avoid side effects (like print statements) in computed properties.

Why CustomStringConvertible protocol description being called multiple times?

1 Answers1