Optional chaining not working for optional protocol requirements

Jose Marchena Published at Dev

Jose Marchena

I'm following this example from Apple and the optional chaining does not work as expected.

There is a protocol with optional property and function:

@objc protocol CounterDataSource {
  optional func incrementForCount(count: Int) -> Int
  optional var fixedIncrement: Int { get }
}

A class that conform to above protocol:

class ThreeSource: CounterDataSource {
  let fixedIncrement = 3
}

And a class with an optional property (dataSource) that conforms to that protocol:

@objc class Counter {
  var count = 0
  var dataSource: CounterDataSource?
  func increment() {
    if let amount = dataSource?.incrementForCount?(count) {
        count += amount
    } else if let amount = dataSource?.fixedIncrement {
        count += amount
    }
  }
}

Finally when it comes the time to use an instance of Counter, with a non-nil dataSource property, it doesn't behave as expected:

var counter = Counter()
counter.dataSource = ThreeSource()
  for _ in 1...4 {
    counter.increment()
    println(counter.count)
  }

If I'm not wrong and according to the tutorial, we should get printed 3, 6, 9, 12. But I only get 0, 0, 0, 0.

Here is the optional chaining in class Counter that is expected to be assigning the value 3 (due to the fixedIncrement property in ThreeSource) to amount:

        } else if let amount = dataSource?.fixedIncrement {
        count += amount
    }

However this is not working and that branch is not executed.

Is there anything wrong in the code? Or is this maybe a bug?

Price Ringo

It turns out that the website version of the iBook content for The Swift Programming Language (https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Protocols.html) shows the ThreeSource declared with @objc. Either doing this or deriving ThreeSource from NSObject works.

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