Question

我正在调用我的webservice来返回JSON数据，然后将其用于填充UICollectionView中的单元格。为了确定单元格的数量，我计算JSON数组中返回的项目数。但是，问题是我的collectionView（）函数在我的dataTask（）函数完成之前被调用（我的猜测是dataTask（）是异步的），这反过来给我一个错误，详细说明我访问了一个nil值（TestController.swift中的jsonCount变量）。我尝试过使用各种completionHandler和返回值，但我无法让它工作。你能帮我们指点一下正确的方向吗？

非常感谢您的帮助/意见。

HomeModel.swift

class func getPostings(completionHandler: @escaping (_: Any) -> ()) {
        let url: URL = URL(string: "url")!
        let request: URLRequest = URLRequest(url: url)
        let task = URLSession.shared.dataTask(with: request) {data, 
                   response, error in
                guard error == nil else {
                print(error!)
                completionHandler("Failed")
                return
                }
        let httpResponse = response as! HTTPURLResponse
        let statusCode = httpResponse.statusCode

        if statusCode == 200 {
            let json = try? JSONSerialization.jsonObject(with: data!, options: .allowFragments) as! [[String: Any]]
            completionHandler(json!)
        }
    }
    task.resume()

TestController.swift

var jsonCount: Int?
var jsonData: [[String: Any]]?

override func viewDidLoad() {
    super.viewDidLoad()

    HomeModel.getPostings(completionHandler: {(response: Any) -> Void in
        guard response as? String != "Failed" else {
            //Code to send a pop-up alert to the user
            return
        }
        self.jsonData = response as? [[String: Any]]
        self.jsonCount = (self.jsonData?.count)!
    })
}

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

Answer 1

CollectionView或TableView默认情况下会在ViewDidLoad方法上重新加载。但是当您从服务器下载某些内容时，只需在主线程上完成下载任务更新UI。

HomeModel.getPostings(completionHandler: {(response: Any) -> Void in
        guard response as? String != "Failed" else {
            //Code to send a pop-up alert to the user
            return
        }
        self.jsonData = response as? [[String: Any]]
        self.jsonCount = (self.jsonData?.count)!
        DispatchQueue.main.async {
            // Update UI on Main Thread
            collectionView.reloadData()
        }
    })

在dataTask（）方法完成之前调用func collectionView

1 个答案: