UICollectionView changes problem

CollectionView.swift

import UIKit

class ViewController: UIViewController, UICollectionViewDataSource {
    @IBOutlet weak var collectionView: UICollectionView!
    
    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        return collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath)
    }
    
    func numberOfSections(in collectionView: UICollectionView) -> Int {
        return 1
    }
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return array.count
    }
    
    
    var array : Array<Int> = [1,2,3]

    override func viewDidLoad() {
        super.viewDidLoad()
        collectionView.register(UICollectionViewCell.self, forCellWithReuseIdentifier: "Cell")
        collectionView.reloadData()
        
        addItem(sender: nil)
        addItem(sender: nil)
       
    }
    override func viewWillAppear(_ animated: Bool) {
        super.viewWillAppear(animated)

    }
    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)

    }

    @IBAction func addItem(sender: AnyObject?){
        array.append(10)
        collectionView.insertItems(at: [IndexPath(row: 0, section: 0)])
    }

}