Return to function from closure

Question

I'm learning swift and caught in a situation:

I have a function which returns array. This function calls another closure inside and I want to pass the closure result as return to the function.

This is how my code has been setup

func setData() -> [City] {
  let conn = ApiConnection()
  conn.get_cities{  citiesList in
    return (citiesList)
  }
 }

When I do this I get error saying "Cannot convert return expression of type '()' to return type 'City?'"

Please advice how I can return the citiesList to my function

`conn.get_cities` will be executing asynchronously - that is, the data is retrieved *after* `setData` has already returned. This means you can't return the array. You need to pass a closure to `setData` and then your `get_cities` closure can call *that* closure and pass the cities array. — Paulw11, Apr 07 '19 at 07:26

score 1 · Accepted Answer · edited Apr 07 '19 at 08:59

If the closure get_cities is a non-escaping closure, then simply return the closure like

func setData() -> [City] {
    let conn = ApiConnection()
    return conn.get_cities{  citiesList in
        return (citiesList)
    }
}

If it is an escaping closure, then rather than returning a list from the function directly, use a completion handler like

func setData(completion: @escaping (([CityList]) -> Void)) {
    let conn = ApiConnection()
    conn.get_cities{  citiesList in
        completion(citiesList)
    }
}

score 0 · Answer 2 · answered Apr 07 '19 at 07:28

You cannot directly return the closure results. Closure results are meant to be full-filled later. To achieve this, you have to pass a closure which takes similar parameter.

func setData(completion: (([YOUR_CITY_CLASS]) -> void) ) {
    let conn = ApiConnection()
    conn.get_cities(completion)
}

NB: As this is an api call, most probably this is an closure which escapes, in that case the declaration of the function would be like following

func setData(completion: @escaping (([YOUR_CITY_CLASS]) -> void))

Return to function from closure

2 Answers2