Updating a collectionView after dismissing VC

Question

Created Feb ’21

Replies 11

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I have a collection view in VC1 embedded in a navigationController. The collection view contains buttons that go to VC2. When make changes in VC2 and close that VC, I want the collectionView in VC1 to update. However, the functions that make up the collectionView aren't inside viewWillAppear; how can I make VC1 refresh after closing VC2? I have tried a few different things written at the bottom of VC2

VC1

Code Block import UIKit
class HerdViewCell: UICollectionViewCell {
    @IBOutlet weak var horseNameOutlet: UILabel!
    @IBOutlet weak var baseLayer: UIImageView!
}
var myHorses = [
    Horse(name: "Donnerhall", idNumber: 1, gender: "Stallion", age: 1),
    Horse(name: "Celeste", idNumber: 2, gender: "Mare", age: 1),
    Horse(name: "Kara", idNumber: 3, gender: "Mare", age: 1)
]
var horseIndex = 0 
import UIKit
class HerdViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource {
    @IBOutlet weak var herdCollectionView: UICollectionView!
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        herdCollectionView.delegate = self
        herdCollectionView.dataSource = self
    }
    override func viewDidLoad() {
        super.viewDidLoad()
    }
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return myHorses.count
    }
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        var cell = UICollectionViewCell()
        if let horseCell = collectionView.dequeueReusableCell(withReuseIdentifier: "cell", for: indexPath) as? HerdViewCell {
            let horseTag = "\(myHorses[indexPath.row].name ?? "none") \n\(myHorses[indexPath.row].gender), \(myHorses[indexPath.row].age) years"
            horseCell.horseNameOutlet.text = horseTag
            horseCell.baseLayer.image = myHorses[indexPath.row].basePhenotype
            cell = horseCell
            horseIndex = indexPath.row
        }
        return cell
    }
    func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        horseIndex = indexPath.row
        performSegue(withIdentifier: "horseViewerSegue", sender: self)
    }
    }
}

VC2

Code Block import UIKit
class HorseViewController: UIViewController {
    @IBOutlet weak var horseNameOutlet: UILabel!
    @IBOutlet weak var horseGenderOutlet: UILabel!
    @IBOutlet weak var horseAgeOutlet: UILabel!
    @IBOutlet weak var baseLayer: UIImageView!
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)
        updateHorse()
    }
    override func viewDidLoad() {
        super.viewDidLoad()
    }
}
    func updateHorse(){
        horseNameOutlet.text = myHorses[horseIndex].name
        horseGenderOutlet.text = myHorses[horseIndex].gender
        horseAgeOutlet.text = "\(String(myHorses[horseIndex].age)) years"
        baseLayer.image = myHorses[horseIndex].basePhenotype
     }
    @IBAction func sellHorse(_ sender: UIButton) {
        myHorses.remove(at: horseIndex)
       //dismissing won't work, because the app closes the navigationController instead of going back to the root of the navigation controller
        //dismiss(animated: true, completion: nil)
//If I navigate to the root of the navigationController, the collectionView is as I left it instead of removing a horse
        //        self.navigationController?.popViewController(animated: true)
    }
}

Answered by Claude31 in 663651022

Just call

Code Block myCollection.reloadData()

That will trigger

Code Block func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell { }

using the new myHorses dataSource content.

Boost

Answer 1

Claude31 OP

Feb ’21

If you navigate from VC1 to VC2 with NavController, the VCs are stacked. Which means VC1 is still present.

I see at least 2 options:

use delegation to call a function in VC1 from VC2
or
in VC2 viewWillDisappear, call

Code Block     let vcs = vC2.navigationController?.viewControllers

VC1 should be [0] ; then call

Code Block     let vc1 = vcs[0]
    vc1.functionToUpdate

or if VC1 is the first in navigation

Code Block vC2.navigationController?.topViewController.functionToUpdate

0

Answer 2

imey OP

Feb ’21

If I go with the second method and write:

Code Block     func viewWillDisappear() {
        let vcs = vC2.navigationController?.viewControllers
        let vc1 = vcs[0]
            vc1.functionToUpdate
    }

Is "vc1" and "vc2" meant to match the class of the viewControllers in the project?
Is this written correctly or are parts supposed to be in VC1?
The navigation controller starts after the Home Screen, btw, I didn't mention that before. I have three navigation controllers branching off from the initial VC, is this bad practice or normal in IOS development?

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Answer 3

Claude31 OP

Feb ’21

Is "vc1" and "vc2" meant to match the class of the viewControllers in the project?

I just used the info you gave in OP:

I have a collection view in VC1 embedded in a navigationController. The collection view contains buttons that go to VC2.

so vc1 is the instance of VC1 and vc2 instance of VC2

This is in viewWillDisappear of VC2. Exact ?

Then, you could write (I adjusted for optionals):

Code Block     func viewWillDisappear() {
        if let vcs = self.navigationController?.viewControllers {
            let vc1 = vcs[0] // may need to write if let vc1 = vcs[0] as? VC1 { vc1.functionToUpdate }
            vc1.functionToUpdate
       }
    }

The navigation controller starts after the Home Screen, btw, I didn't mention that before. I have three navigation controllers branching off from the initial VC, is this bad practice or normal in IOS development?

initial VC is Home screen ?
How do you go from there to each navigation stack ?
I see nothing wrong there, as long as user can understand where he/she is navigating to.

For the code above to work, what is key is to have the following sequence (in IB):

Home Screen > NavigationController > VC1 > VC2
That would also work with a pattern as

Home Screen > NavigationController > VC1 > VC1bis > VC1ter > VC2

0

Answer 4

Claude31 OP

Feb ’21

Another note: if you want to be sure to return to Home, just use popToRootViewController (line 31)

Code Block         self.navigationController?.popToRootViewController(animated: true)

And in viewWillDisappear:

Code Block     override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
        if let vc1 = self.navigationController?.topViewController as? VC1 {
                vc1.functionToUpdate
       }
    }

0

Answer 5

imey OP

Feb ’21

VC1 is HerdViewController and VC2 is HorseViewController.
The Home Screen has a button for each navigation controller, HerdViewController starts the first navigation controller

I get an error "Value of type 'HerdViewController' has no member 'functionToUpdate'"

Code Block   override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
            if let vc1 = self.navigationController?.topViewController as? HerdViewController {
                    vc1.functionToUpdate
                }
    }

0

Answer 6

Claude31 OP

Feb ’21

functionToUpdate is a generic name. I don't know the name of update function you want to call.

So, in HerdViewController, you should have:

Code Block func functionToUpdate() {
    // update the collectionView here
}

And in HorseViewController

Code Block override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
            if let vc1 = self.navigationController?.topViewController as? HerdViewController {
                    vc1.functionToUpdate()
                }
    }

May be you can pass the list of cells to update or to remove as a function parameter, to avoid reloading the full collectionView.

Note: effectively, you must not dismiss a view in a Navigation stack.

0

Answer 7

imey OP

Feb ’21

Ahh, I thought functionToUpdate didn't make sense. I have now made a function in herdViewController

Code Block     func refreshHerd() {
        print("refresh")
    }

and then updated viewWillDisappear but get an error: "Expression resolves to an unused function"

Code Block     override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
            if let vc1 = self.navigationController?.topViewController as? HerdViewController {
                    vc1.refreshHerd
                }
    }

I know how to remove cells from a stack view and then replace them programmatically, but a collectionView is different, it seems to just "be there" regardless of what happens in the lifecycle of the VC besides these two, but it doesn't make any difference if I place them in viewDidLoad or viewWillAppear:

Code Block         herdCollectionView.delegate = self
        herdCollectionView.dataSource = self

0

Answer 8

Claude31 OP

Feb ’21

Misses ()

Code Block vc1.refreshHerd()

So, you can update by reloading the CollectionView.

0

Answer 9

imey OP

Feb ’21

Ok, now the function is triggered, but how do you reload a collectionView? I think that will be the easiest way to do it

0

Answer 10

Claude31 OP

Feb ’21

Accepted Answer

Just call

Code Block myCollection.reloadData()

That will trigger

Code Block func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell { }

using the new myHorses dataSource content.

0

Answer 11

imey OP

Feb ’21

Nice! It's working perfectly now :)

0