Question

我搜索了此主题，但找不到解决问题的方法。这是我的loadData函数可以从JSON对象获取数据

 @IBOutlet weak var collectionVitrin: UICollectionView!

 var vitrinDecodePost = [VitrinDecode]() // Decodable

 func loadDatas(limit : Int) {

    guard let url = URL(string: "my Url") else { return }
    let session = URLSession.shared
    let request = NSMutableURLRequest(url: url as URL)
    request.httpMethod = "POST"
    let paramString = "limit=\(limit)"
    request.httpBody = paramString.data(using: String.Encoding.utf8)

    let task = session.dataTask(with: request as URLRequest) {
        (data, response, error) in
            guard let _:NSData = data as NSData? , let _:URLResponse = response, error == nil else {

                print("Mistake")
                return
            }

            guard let data = data else { return }
            do {

                let abc = try JSONDecoder().decode([VitrinDecode].self, from: data)
                self.vitrinDecodePost = abc

                DispatchQueue.main.async {

                    self.collectionVitrin.reloadData()
                }

            } catch { print(error)}

    }
    task.resume()
}

我在ViewDidLoad中运行此功能：

var deger : Int = 10

override func viewDidLoad() {
    super.viewDidLoad()
    loadData(limit: deger)
}

滚动结束后，我想添加更多数据，所以我添加了willDisplay函数

func collectionView(_ collectionView: UICollectionView, 
numberOfItemsInSection section: Int) -> Int {

    return self.vitrinDecodePost.count
}

func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {

    let lastindex = self.vitrinDecodePost.count - 1
    if indexPath.row == lastindex{

        loadDatas(limit: 20)
        self.collectionVitrin.reloadData() 


    }

}

页面加载后，我的单元格中显示10个项目，但是到达页面末尾（最后一个索引）时，collectionview不显示任何内容。它必须在我的collectionview单元格中显示20个项目。我忘记了什么？

Answer 1

您应该删除self.collectionVitrin.reloadData()函数中LoadDatas(limit: 20)之后调用的willDisplay。

但是，如果我可以建议您在collectionview上实现无限滚动，那么我会做一些改变。

首先，我建议您更改api调用。您应该限制每个api调用发回的元素数量的限制，但也应实现偏移量变量。这样效率更高。

在您的情况下，您的第二个api调用只是再次请求相同的对象，但请求中的限制更大。您基本上每次都要求相同的数据。添加偏移量可使您每次仅请求新数据。该偏移量应该是您已经拥有的数据量。

第二，当您到达数据末尾或已经在请求数据时，应该尝试添加故障保护机制。否则，如果您到达集合视图的底部，最终将导致呼叫循环。这是我根据您的代码实现无限加载的方法

 @IBOutlet weak var collectionVitrin: UICollectionView!
 var limitPerCall: Int = 10
 var isLoadindNewData = false
 var shouldLoadMoreData = true

 var vitrinDecodePost = [VitrinDecode]() // Decodable

 func loadDatas(limit : Int, offset: Int) {

     guard let url = URL(string: "my Url") else { return }
     let session = URLSession.shared
     let request = NSMutableURLRequest(url: url as URL)
     request.httpMethod = "POST"
     let parameters = ["limit": "\(limit)", "offset": "\(offset)"] as Dictionary<String, String>
     guard let httpBody = try? JSONSerialization.data(withJSONObject: parameters, options: []) else { return }
     request.httpBody = httpBody

     let task = session.dataTask(with: request as URLRequest) { [weak self]
         (data, response, error) in
         guard let strongSelf = self, let _:NSData = data as NSData? , let _:URLResponse = response, error == nil else {

            print("Mistake")
            return
        }

       /// No More data to gather stop making api calls
        guard let data = data else { 
           strongSelf.shouldLoadMoreData = false
           return 
        }
        do {

            let abc = try JSONDecoder().decode([VitrinDecode].self, from: data)
            strongSelf.vitrinDecodePost.append(contentsOf: abc)

            //// Reload the new data and and indicates that api call can be 
            made again
            DispatchQueue.main.async {
                strongSelf.isLoadingNewData = false
                strongSelf.collectionVitrin.reloadData()
            }

        } catch { print(error)}

}
task.resume()
}

func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection 
    section: Int) -> Int {

    return self.vitrinDecodePost.count
}

func collectionView(_ collectionView: UICollectionView, willDisplay cell: 
    UICollectionViewCell, forItemAt indexPath: IndexPath) {

    let lastindex = self.vitrinDecodePost.count - 1
    if indexPath.row == lastindex && !isLoadindNewData && shouldLoadMoreData {
        /// Forbids multiple api calls to happen at the same time
        isLoadindNewData = true
        loadDatas(limit: limitPerCall, offset: vitrinDecodePost.count)
    }

}

希望有帮助。

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