在隐式展开可选值时意外发现 nil (UICollectionView)

Question

我想制作一个在获取新项目后重新加载其数据的集合视图。但是当我尝试重新加载集合视图时，我得到了这个错误

这是视图控制器的代码

import UIKit

class GalleryViewController: UIViewController {
var presenter : ViewToPresenterPhotoProtocol?

@IBOutlet var collectionView: UICollectionView!

let reuseIdentifier = "cell"

override func viewDidLoad() {
    super.viewDidLoad()
    presenter?.viewDidLoad()
    // Do any additional setup after loading the view.
}
}

extension GalleryViewController: UICollectionViewDelegate, UICollectionViewDataSource {

func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return self.presenter?.photos?.count ?? 0
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as! PhotoCollectionViewCell
    
    cell.label.text = self.presenter?.photos?[indexPath.item].name
    cell.backgroundColor = UIColor.yellow
    
    return cell
}

// MARK: - UICollectionViewDelegate protocol

private func collectionView(collectionView: UICollectionView, didSelectItemAtIndexPath indexPath: NSIndexPath) {
    // handle tap events
    print("You selected cell #\(indexPath.item)!")
}
}

extension GalleryViewController: PresenterToViewPhotoProtocol{
func onFetchPhotoSuccess() {
    self.collectionView.reloadData()
}

func onFetchPhotoFailure(error: String) {
    print("View receives the response from Presenter with error: \(error)")
}


}

我不知道如何在故事板的集合视图中正确重新加载数据，所以如果您能帮我解决这个问题，我将非常感激

Answer 1

也许你的 GalleryViewController 是在没有 xib/storyboard 的情况下初始化的。

如果您使用情节提要，试试这个：

let storyboard = UIStoryboard(name: 'yourStoryboardName', bundle: .main)
let controller = storyboard.instantiateViewController(identifier: "yourControllerIdentifier") as! GalleryViewController

如果你使用的是 xib，试试这个：

let controller = UIViewController(nibName: "yourControllerNibName", bundle: .main) as! GalleryViewController

Answer 2

1. Set identifier in Attributes Inspector

2. 注册您的自定义单元格。

    @IBOutlet private var collectionView: UICollectionView! {
        didSet {
            collectionView.register(PhotoCollectionViewCell.self,   
                                    forCellWithReuseIdentifier: reuseIdentifier)
        }
    }
   

Answer 3

您强制转换了一个 nil 值。我相信你需要使用as 或as?。

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath) as? PhotoCollectionViewCell
    
    ...
}