Pass variables from one ViewController to another in Swift

Question

I have a calculator class, a first ViewController to insert the values and a second ViewController to show the result of the calculation. Unfortunately I get a error called

Answer 1

The problem here is that the FirstViewController has no reference to the instance of SecondViewController. Because of this, this line:

secondViewController.setResultLabel(myResult)

does nothing (except probably causing the Can't unwrap Optional.None error). There are a few ways to solve this problem. If you are using storyboard segues you can use the -prepareForSegue method of UIViewController. Here is an example:

In FirstViewController:

override func prepareForSegue(segue: UIStoryboardSegue!,sender: AnyObject!){         
  //make sure that the segue is going to secondViewController
  if segue.destinationViewController is secondViewController{
    // now set a var that points to that new viewcontroller so you can call the method correctly
    let nextController = (segue.destinationViewController as! secondViewController)
    nextController.setResultLabel((String(myResult)))
  }
}

Note: this code will not run as is because the function has no access to the result variable. you'll have to figure that out yourself :)