Select first Item in CollectionView Swift4 - uicollectionview

Question

I'm trying to select the first Item in CollectionView when the Collectionview loads. I found many solutions for Swift 3 but nothing of that worked for me in Swift4. What I tried(viewDidAppear):
import UIKit
import SDWebImage
class PostsTab_Details: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource, UICollectionViewDelegateFlowLayout {
#IBOutlet weak var PostsSelectRarityCollectionView: UICollectionView!
#IBOutlet weak var RarityTypeLbl: UILabel!
//Center my Collectionview Horizontal
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
let totalCellWidth = 100 * (selectedPosts?.rarity.count)!
let totalSpacingWidth = 10 * (3 - 1)
let leftInset = (PostsSelectRarityCollectionView.frame.width - CGFloat(totalCellWidth + totalSpacingWidth)) / 2
let rightInset = leftInset
return UIEdgeInsetsMake(0, leftInset, 0, rightInset)
}
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return (selectedPosts?.rarity.count)!
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
let cell = PostsSelectRarityCollectionView.dequeueReusableCell(withReuseIdentifier: "PostssTab_DetailCollectionCell", for: indexPath) as! PostsTab_DetailsCollectionViewCell
cell.PostssTab_DetailCollectionImage.sd_setImage(with: URL(string: (selectedPosts?.PostsImageURL)!))
return cell
}
func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
RarityTypeLbl.text = selectedPosts?.rarity[indexPath.row].rarity
}
//Tried this also with IndexPath(item: 0, section: 0)
override func viewDidAppear(_ animated: Bool) {
let selectedIndexPath = IndexPath(row: 0, section: 0)
PostsSelectRarityCollectionView.selectItem(at: selectedIndexPath, animated: true, scrollPosition: .right)
}
var selectedPosts: Posts?
override func viewDidLoad() {
PostsSelectRarityCollectionView.delegate = self
PostsSelectRarityCollectionView.dataSource = self
}
}
But this is not working... I hope someone knows what I'm doing wrong..
Thanks in advance
Update:
This worked for me now, thanks to Razib's answer:
override func viewWillAppear(_ animated: Bool) {
self.collectionView(PostsSelectRarityCollectionView, didSelectItemAt: IndexPath(row: 0, section: 0))
}

Answer 1

Your code seems okay to me. If you like to trigger the didSelectItemAt method, use the following code.
override func viewDidAppear(_ animated: Bool) {
self.PostsSelectRarityCollectionView(self.PostsSelectRarityCollectionView, didSelectItemAt: IndexPath(row: 0, section: 0))
}

Answer 2

To programmatically select the first Item in CollectionViewCell when the Collectionview loads you should code in viewDidAppear:
override func viewDidAppear(_ animated: Bool) {
let indexPath:IndexPath = IndexPath(row: 0, section: 0)
collectionView?.selectItem(at: indexPath, animated: false, scrollPosition: .top)
}

Answer 3

Try this (Swift 5):
let indexPath:IndexPath = IndexPath(row: 0, section: 0)
collectionView?.selectItem(at: indexPath, animated: false, scrollPosition: .top)