Why is selector being sent to previous view controller? - ios

Question

I have an app with a data model class that declares a protocol, and two view controllers embedded in a navigation controller. The data model class is a shared instance. Both view controllers are delegates of the data model. The second view controller has a UITableView.
On start, calls to data model functions from the first view controller work as expected. When I segue from the first view controller to the second, calls to data model functions work as expected as well.
However, when I navigate back to the first view controller and a data model function is called, the app crashes with this error:
2017-04-03 09:48:12.623027 CoreDataTest[3207:1368182]
-[CoreDataTest.PracticesViewController noDupeWithResult:]: unrecognized selector sent to instance 0x15fe136e0
That PracticesViewController is the second view controller. I don't understand why a selector is being sent to what I am thinking of as the previous view controller. My expectation is that the selector should be sent to the first view controller that has just been navigated back to,
I am self-taught, so I presume there is something basic that I am missing, but I don't know what I don't know. Can someone explain why the crash is happening?
Code for the data model
import Foundation
import CoreData
import UIKit
#objc protocol PracticesDataDelegate {
#objc optional func practicesLoadError(headline:String,message:String)
#objc optional func practicesLoaded(practices:[NSManagedObject])
#objc optional func practiceStored()
#objc optional func practiceDeleted()
#objc optional func noDupe(result:String)
}
class PracticesDataModel {
static let sharedInstance = PracticesDataModel()
private init () {}
var delegate: PracticesDataDelegate?
var practices: [NSManagedObject] = []
// some code omitted . . .
/// Check for a duplicate exercise
func checkForDupe(title:String,ngroup:String,bowing:String,key:String){
guard let appDelegate =
UIApplication.shared.delegate as? AppDelegate else {
return
}
let managedContext = appDelegate.persistentContainer.viewContext
let fetchRequest = NSFetchRequest<NSManagedObject>(entityName: "Practice")
let predicate = NSPredicate(format: "exTitle == %# AND notegroup == %# AND bowing == %# AND key == %#", title, ngroup, bowing, key)
fetchRequest.predicate = predicate
do {
practices = try managedContext.fetch(fetchRequest)
if practices.count == 0 {
self.delegate?.noDupe!(result:"none") // exception happens here
} else {
self.delegate?.noDupe!(result:"dupe")
}
} catch let error as NSError {
// to come
}
}
The top of the first view controller
import UIKit
class galamianSelection: UIViewController, ExcerciseDataModelDelegate, PracticesDataDelegate {
let exerciseModel = ExerciseDataModel.sharedInstance
let pModel = PracticesDataModel.sharedInstance
// some code omitted . . .
//// THE VIEW ////
override func viewDidLoad() {
super.viewDidLoad()
exerciseModel.delegate = self
pModel.delegate = self
exerciseModel.loadExercises()
}
//// RESPOND TO PRACTICE DATA MODEL ////
func practiceStored() {
print("exercise stored")
}
func noDupe(result: String) {
if result == "none" {
let d = Date()
pModel.storePractice(date: d, exTitle: theExerciseTitle.text!, notegroup: theNoteGroup.text!, bowing: theBowings.text!, rythem: theRhythms.text!, key: theKey.text!, notes: "")
} else {
print("dupe")
}
}
The top of the second view controller
import UIKit
class PracticesViewController: UIViewController, UITableViewDelegate, UITableViewDataSource, PracticesDataDelegate {
#IBOutlet weak var tableView: UITableView!
let pModel = PracticesDataModel.sharedInstance
// some code omitted . . .
//// THE VIEW ////
override func viewDidLoad() {
super.viewDidLoad()
pModel.delegate = self
pModel.getPractices()
}
delegate is properly set to self in both view controllers.
I am happy to provide more code is needed, but I suspect someone who knows can diagnose from what I've provided.

Answer 1

There are a few issues here.
You are trying to reuse a delegate but only setting it in viewDidLoad. viewDidLoad is only called when, as the name implies, the view initially loads. That means that if you have VC1 in a navigation controller, then you push to VC2, then pop back to VC1, viewDidLoad will not be called a second time. The view has already loaded. If you want a piece of code to be called every time a view controller comes back into focus, you should put it into viewWillAppear: or viewDidAppear:
You are force unwrapping an optional protocol method which you haven't actually implemented (a bug which you're seeing as a result of the first issue, but it's still an issue on its own). Change the force unwrap to an optional self.delegate?.noDupe?(result:"none")
You also declared your delegate like this var delegate: PracticesDataDelegate?. This makes your class retain its delegate and is generally not the right behavior. In your case it actually causes a retain cycle (until you change the delegate). You should change this declaration to weak var delegate: PracticesDataDelegate?

Answer 2

What appears to be happening is:
In PracticesViewController you set the model's delegate to self, as in pModel.delegate = self.
You navigate back to your first view controller. This means that PracticesViewController gets deallocated. But the model's delegate has not been changed, so it's still pointing to the memory location where the view controller used to be.
Later (but soon), your model tries to call a method on its delegate, but it can't because it was deallocated. This crashes your app.
You could reassign the value of the delegate, for example in viewDidAppear. Or you could have your model check whether its delegate implements a method before attempting to call it. That's a standard practice for optional protocol methods-- since they don't have to be implemented, you check first before calling them.
In general, don't use ! in Swift unless you want your code to crash there if something goes wrong.