Swift 3, Successfully passed data but var returns nil when used - ios

Question

So here is my code from VC1 and passing the data to VC2.
override func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
selectedArtist = artists[indexPath.item]
performSegue(withIdentifier: "artistToArtSegue", sender: self)
}
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "artistToArtSegue" {
let artCollectionController = ArtCollectionController()
artCollectionController.artist = selectedArtist
artCollectionController.selectedArtist = selectedArtist
}
}
These codes here in VC2 will print the data
class ArtCollectionController: UICollectionViewController {
var artist = Artist() {
didSet{
print(artist.artistId ?? "did not work")
print(artist.name ?? "what name?")
}
}
var selectedArtist = Artist()
but when I use the the variable in these following test codes in VC2. They return a nil.
func fetchArtForArtist() {
guard let artistId = selectedArtist.artistId else {return}
print(artistId)
let fanRef = FIRDatabase.database().reference().child("art_ref").child(artistId)
fanRef.observeSingleEvent(of: .childAdded, with: { (snapshot) in
let artId = snapshot.key
print(artId)
// let dataRef = FIRDatabase.database().reference().child(artId)
// dataRef.observe(.value, with: { (snapshot) in
// let dictionary = snapshot.value as? [String: AnyObject]
// //let art =
// }, withCancel: nil)
}, withCancel: nil)
}
#IBAction func testButton(_ sender: UIBarButtonItem) {
print(selectedArtist.name ?? "no name")
print(12345)
}
override func viewDidAppear(_ animated: Bool) {
selectedArtist = artist
print(artist.name ?? "non")
print(selectedArtist.artistId ?? "no id")
}
override func viewDidLoad() {
super.viewDidLoad()
fetchArtForArtist()
selectedArtist = artist
print(artist.name ?? "non")
print(selectedArtist.artistId ?? "no id")
}
Im doing this in storyBoard. Im even using 2 vars seeing if there is a difference. I dont understand why the data is successfully passed to VC2 to a couple variables but when the variable is used it returns a nil. Please help.

Answer 1

The other responses are good, but I prefer a slightly different approach:
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
switch segue.destination {
case let artCollectionController as ArtCollectionController:
artCollectionController.artist = selectedArtist
artCollectionController.selectedArtist = selectedArtist
case let otherViewController as OtherViewController:
//Code for some other case
}
}
By using a switch statement, you have a prepareForSegue that will handle multiple different segues cleanly.
The case let construct is a cool trick that only executes that case if the variable in the switch can be case to the desired type. If it can be cast, it creates a local variable of the desired type.
I prefer deciding what code to execute based on the class of the destination view controller because it's less fragile than using the segue identifier. If you forget to set the segue identifier, or add a second segue later to the same type of view controller, or have a typo in the name of the identifier, that code won't work. If you have a typo in your class name, though, the compiler throws an error.

Answer 2

Because you set the artist property on a new instance of ArtCollectionController, which is destroyed upon the exit of the prepareForSegue function:
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "artistToArtSegue" {
let artCollectionController = ArtCollectionController() // created
artCollectionController.artist = selectedArtist
artCollectionController.selectedArtist = selectedArtist
// destroyed here
}
}
Try this instead:
override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "artistToArtSegue",
let artCollectionController = segue.destination as? ArtCollectionController
{
artCollectionController.artist = selectedArtist
artCollectionController.selectedArtist = selectedArtist
}
}

Answer 3

override func prepare(for segue: UIStoryboardSegue, sender: Any?) {
if segue.identifier == "artistToArtSegue" {
let artCollectionController = segue.destination as! ArtCollectionController
artCollectionController.artist = selectedArtist
artCollectionController.selectedArtist = selectedArtist
}
}
try this, you are creating one more ArtCollectionController instead of passing data to segue one