swift - 收到错误无法在属性初始值设定项中使用实例成员 'url'；属性初始值设定项在 'self' 可用之前运行

Question

我想知道为什么会出现此错误，我很好奇是否是因为我在同一文件夹中使用类和结构？

class ViewController: UIViewController {

    let url = "http://roadfiresoftware.com/feed/json"

    struct Blog: Decodable {
        let title: String
        let homepageURL: URL
        let articles: [Article]

        enum CodingKeys : String, CodingKey {
            case title
            case homepageURL = "home_page_url"
            case articles = "items"
        }
    }

    struct Article: Decodable {
        let id: String
        let url: URL
        let title: String
    }

    let task = URLSession.shared.dataTask(with: URL(string: url)!) { (data, response, error) in
        guard let data = data else {

            NSLog("Error: No data to decode")
            return
        }

        guard let blog = try? JSONDecoder().decode(Blog.self, from: data) else {
            NSLog("Error: Couldn't decode data into Blog")
            return
        }

        NSLog("blog title: \(blog.title)")
        NSLog("blog home: \(blog.homepageURL)")

        NSLog("articles:")
        for article in blog.articles {
            NSLog("- \(article.title)")
        }
    }
}

Answer 1

问题出在这一行:

let task = URLSession.shared.dataTask(with: URL(string: url)!) { (data, response, error) in

您试图直接在类中运行代码块，但您不能这样做，此代码需要包含在代码块中，例如方法调用。

我建议将此逻辑包装在 viewDidLoad() 方法中

override func viewDidLoad() {
    super.viewDidLoad()

    fetchBlog() 
}

func fetchBlog() {
    let task = URLSession.shared.dataTask(with: URL(string: url)!) { (data, response, error) in
        guard let data = data else {

            print("Error: No data to decode")
            return
        }

        guard let blog = try? JSONDecoder().decode(Blog.self, from: data) else {
            print("Error: Couldn't decode data into Blog")
            return
        }

        print("blog title: \(blog.title)")
        print("blog home: \(blog.homepageURL)")

        print("articles:")
        for article in blog.articles {
            print("- \(article.title)")
        }
    }
}

您还可以从其他地方调用此函数，例如 init、viewWillAppear、另一个函数等。

注意:除非确实需要，否则不要在 Swift 中使用 NS 类，例如 NSLog，几乎总是有 Swift 版本