ios - 尝试使用 URLsession 从 DataTask 创建图像无效

Question

我创建了一个函数，该函数将从 url 获取图像并将其分配给一个变量，然后它将该变量作为图像返回。虽然它没有返回任何东西并且崩溃了。

func downloadImage(imageUrl:String) -> UIImage {
        var imageDownloaded : UIImage?
        let configuration = URLSessionConfiguration.default
        let session = URLSession(configuration: configuration)
        let task = session.dataTask(with: URL(string: imageUrl)!) { (data, response, error) in
            if error != nil {
                return
            }
            do {
                imageDownloaded = try UIImage(data: data!)
            } catch {
                print("Fail")
            }
        }
        task.resume()
        return imageDownloaded! //Thread 1: Fatal error: Unexpectedly found nil while unwrapping an Optional value
    }

Answer 1

您正试图在变量“imageDownloaded”没有任何值(即 nil)时强行展开它。这是因为下载图像数据需要时间。

要解决这个问题，您应该使用完成处理程序

func downloadImage(imageUrl:String, @escaping completionHandler: (UIImage?)->()) {
    var imageDownloaded : UIImage?
    let configuration = URLSessionConfiguration.default
    let session = URLSession(configuration: configuration)
    let task = session.dataTask(with: URL(string: imageUrl)!) { (data, response, error) in
        if error != nil {
            return
        }
        do {
            imageDownloaded = try UIImage(data: data!)
            completionHandler(imageDownloaded!)
        } catch {
            print("Fail")
            completionHandler(nil)
        }
    }
    task.resume()
}