Stanford iOS9 2016 lecture 6 VCL.swift file, errors with line "print("\(logPrefix)Face \(instance)" + msg)"

Question

In lecture 6 Stanford iOS9 2016 lecture series, a file called VCL.swift was added to the program to demonstrate view controller lifecycle.

I have added the file, but for line "print("\(logPrefix)Emotions\(instance)" + msg) and line "print("\(logPrefix)Face \(instance)" + msg)", xcode is displaying an error, telling me that "use of unresolved identifier 'instance' ".

Does anyone has any idea how to fix it?

Below is the VCL.swift file from lecture 6

import UIKit

private var faceMVCinstanceCount = 0
func getFaceMVCinstanceCount() -> Int { faceMVCinstanceCount += 1; return faceMVCinstanceCount }
private var emotionsMVCinstanceCount = 0
func getEmotionsMVCinstanceCount() -> Int { emotionsMVCinstanceCount += 1; return emotionsMVCinstanceCount }

var lastLog = NSDate()
var logPrefix = ""

func bumpLogDepth() {
    if lastLog.timeIntervalSinceNow < -1.0 {
        logPrefix += "__"
        lastLog = NSDate()
    }
}

// we haven't covered extensions as yet
// but it's basically a way to add methods to a given class

extension FaceViewController
{
    func logVCL(msg: String) {
        bumpLogDepth()
        print("\(logPrefix)Face \(instance) " + msg) //error: use of unresolved identifier 'instance'
    }

    override func awakeFromNib() {
        logVCL("awakeFromNib()")
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        logVCL("viewDidLoad()")
    }
    override func viewWillAppear(animated: Bool) {
        super.viewWillAppear(animated)
        logVCL("viewWillAppear(animated = \(animated))")
    }
    override func viewDidAppear(animated: Bool) {
        super.viewDidAppear(animated)
        logVCL("viewDidAppear(animated = \(animated))")
    }
    override func viewWillDisappear(animated: Bool) {
        super.viewWillDisappear(animated)
        logVCL("viewWillDisappear(animated = \(animated))")
    }
    override func viewDidDisappear(animated: Bool) {
        super.viewDidDisappear(animated)
        logVCL("viewDidDisappear(animated = \(animated))")
    }

    override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()
        logVCL("viewWillLayoutSubviews() bounds.size = \(view.bounds.size)")
    }
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        logVCL("viewDidLayoutSubviews() bounds.size = \(view.bounds.size)")
    }

    override func viewWillTransitionToSize(size: CGSize, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator) {
        super.viewWillTransitionToSize(size, withTransitionCoordinator: coordinator)
        logVCL("viewWillTransitionToSize")
        coordinator.animateAlongsideTransition({ (context: UIViewControllerTransitionCoordinatorContext!) -> Void in
            self.logVCL("animatingAlongsideTransition")
            }, completion: { context -> Void in
                self.logVCL("doneAnimatingAlongsideTransition")
        })
    }
}

extension EmotionsViewController
{
    func logVCL(msg: String) {
        bumpLogDepth()
        print("\(logPrefix)Emotions \(instance) " + msg) //error: use of unresolved identifier 'instance'
    }

    override func awakeFromNib() {
        logVCL("awakeFromNib()")
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        logVCL("viewDidLoad()")
    }
    override func viewWillAppear(animated: Bool) {
        super.viewWillAppear(animated)
        logVCL("viewWillAppear(animated = \(animated))")
    }
    override func viewDidAppear(animated: Bool) {
        super.viewDidAppear(animated)
        logVCL("viewDidAppear(animated = \(animated))")
    }
    override func viewWillDisappear(animated: Bool) {
        super.viewWillDisappear(animated)
        logVCL("viewWillDisappear(animated = \(animated))")
    }
    override func viewDidDisappear(animated: Bool) {
        super.viewDidDisappear(animated)
        logVCL("viewDidDisappear(animated = \(animated))")
    }

    override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()
        logVCL("viewWillLayoutSubviews() bounds.size = \(view.bounds.size)")
    }
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        logVCL("viewDidLayoutSubviews() bounds.size = \(view.bounds.size)")
    }

    override func viewWillTransitionToSize(size: CGSize, withTransitionCoordinator coordinator: UIViewControllerTransitionCoordinator) {
        super.viewWillTransitionToSize(size, withTransitionCoordinator: coordinator)
        logVCL("viewWillTransitionToSize")
        coordinator.animateAlongsideTransition({ (context: UIViewControllerTransitionCoordinatorContext!) -> Void in
            self.logVCL("animatingAlongsideTransition")
            }, completion: { context -> Void in
                self.logVCL("doneAnimatingAlongsideTransition")
        })
    }
}

Answer 1

The error: "use of unresolved identifier 'instance'" means that you are trying to use a variable, that hasn't been declared.

In your case you are lacking a line let instance = getEmotionsMVCinstanceCount() in the EmotionsViewController (EmotionsViewController.swift) and/or let instance = getFaceMVCinstanceCount() in the FaceViewController (FacialExpression.swift).

Just take a look on a full source available here: Lecture 6: Multiple MVCs