Cannot invoke 'async' with an argument list of type '(execute: ())'

Question

I am trying to make a network call, then update the main thread, but it seems that any variable created in the background thread can not be sent as a parameter to the main thread:

 let task = session.dataTask(with: request as URLRequest,
                                completionHandler: {
                                    (data, response, error) in
                                    guard let _:Data=data, let _:URLResponse = response, error == nil else {


                                        return
                                    }

                                    let json = try? JSONSerialization.jsonObject(with: data!, options: [])


    var car = "hello"
    DispatchQueue.main.async (execute: self.someFunc(car)) <-- wont accept car parameter

    })

    task.resume()

here is the signature:

func someFunc( str:String) {

}

getting the following error: Cannot invoke 'async' with an argument list of type '(execute: ())'

Answer 1

The syntax

DispatchQueue.main.async(execute: () -> Void)

expects a closure without parameter.

---

Use the syntax

DispatchQueue.main.async(execute: { someFunc(str: car) })

or with trailing closure syntax

DispatchQueue.main.async {
    self.someFunc(str: car)
}

and don't forget the label parameter.