在ViewControllers Swift 3之间传递Double

Question

Total noob here to Swift 3. All I need to do is pass a double value from one ViewController to another via the 2 user input text fields. I've tried numerous solutions and have read everything I can find on passing data between ViewControllers. I get a 'fatal error: unexpectedly found nil while unwrapping an Optional value'. I have a real hard time understand the wrapping and unwrapping of the variables and I'm sure it's something simple.

Here is my first ViewController:

import UIKit

var longitude: Double?
var latitude: Double?



class ViewController: UIViewController {

@IBOutlet var getLongitude: UITextField!
@IBOutlet var getLatitude: UITextField!

@IBOutlet var mapbutton: UIButton!

override func viewDidLoad() {
    super.viewDidLoad()

        }

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

@IBAction func MapBtn(_ sender: Any) {

    performSegue(withIdentifier: "segue", sender: self)
    longitude = Double(getLongitude.text!)!
    latitude = Double(getLatitude.text!)!

}

}

And this is the SecondViewController:

class SecondViewController: UIViewController {

override func viewDidLoad() {
    super.viewDidLoad()

    let camera = GMSCameraPosition.camera(withLatitude: latitude!, longitude: longitude!, zoom: 6.0)
    let mapView = GMSMapView.map(withFrame: CGRect.zero, camera: camera)
    view = mapView
    // Creates a marker in the center of the map.
    let marker = GMSMarker()
    marker.position = CLLocationCoordinate2D(latitude: latitude!, longitude: longitude!)
    marker.map = mapView

}

override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    // Dispose of any resources that can be recreated.
}

}

Answer 1

Avoid global mutable state. Send data from one scene to another.

Try this:

class ViewController: UIViewController {

  @IBOutlet weak var longitudeTextField: UITextField!
  @IBOutlet weak var latitudeTextField: UITextField!
  @IBOutlet weak var mapButton: UIButton!

  @IBAction func didTapToMapButton(_ sender: UIButton) {

    // I assume your storyboard' name is Main. If not, change it below accordingly
    let mainStoryboard = UIStoryboard(name: "Main", bundle: nil)

    // I assume your destination view controller' identifier and type is SecondViewController. If not, change it below accordingly.
    if let secondViewController = mainStoryboard.instantiateViewController(withIdentifier: "SecondViewController") as? SecondViewController {

      if let longitude = Double(longitudeTextField.text),
        let latitude = Double(latitudeTextField.text) {

        secondViewController.latitude = latitude
        secondViewController.longitude = longitude
      }

      present(secondViewController, animated: true, completion: nil)
    }
  }
}

class SecondViewController: UIViewController {

  var latitude: Double?
  var longitude: Double?

  override func viewDidLoad() {
    super.viewDidLoad()

    if let latitude = latitude,
      let longitude = longitude {

      let camera = GMSCameraPosition.camera(withLatitude: latitude, longitude: longitude, zoom: 6.0)
      let mapView = GMSMapView.map(withFrame: CGRect.zero, camera: camera)
      view = mapView

      // Creates a marker in the center of the map.
      let marker = GMSMarker()
      marker.position = CLLocationCoordinate2D(latitude: latitude, longitude: longitude)
      marker.map = mapView
    }
  }
}

Answer 2

1st, Unwrapping. when you try to unwrap the variable, rather forcely unwrap it, it's better test if it's nil, or you could use if let to safely unwrap it.

2nd, pass variable from one VC to another. As you know, in iOS, those VCs instances are like any other variable. All you need to do is trying to fetch a reference to that VC, then assign the Double to its accessible variable. Take a look at this link

Answer 3

You could create a segue between the two controllers and pass the variable by overriding prepare for segue.

Pass that variable to the new controller and load that information to its outlet on viewDidAppear or viewWillAppear.

self.prepare(for: "segueName", sender: self)

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {

    if segue.identifier == "segueName" {

        if let vc = segue.destination as? DestinationViewController {

            // Pass Data to New View Controller
        }
    }
}