结构内部的变异函数答案

【问题标题】：Mutating function inside structure结构内部的变异函数
【发布时间】：2018-10-22 03:47:47
【问题描述】：

我使用的是 Swift 4，我有一个使用默认值初始化的结构。我在里面创建了一个函数，它应该读取 JSON 并用它得到的内容更改这些默认值，但它似乎不起作用。

错误：闭包无法隐式捕获变异的 self 参数

代码：

struct Workspace: Decodable {
    var guid: String
    var name: String

private enum CodingKeys : String, CodingKey {
    case guid = "guid"
    case name = "name"
}

init(){
    self.guid = "blbl"
    self.name = "oops"
}

mutating func getUserWorkspace(base: String, completed: @escaping () -> ()){
    let url = URL(string: "some url")!
    var request = URLRequest(url: url)

    request.addValue("Basic \(base)", forHTTPHeaderField: "Authorization")
    request.addValue("application/json", forHTTPHeaderField: "Accept")

    URLSession.shared.dataTask(with: request){ (data, response, error) in
        if error == nil {
            do {
                let res: Workspace = try JSONDecoder().decode(Workspace.self, from: data!)
                self.guid = res.guid  //Error here
                self.name = res.name  //Error here
                DispatchQueue.main.async {
                    completed()
                }
            }catch {
                print ("JSON error")
            }
        }
    }.resume()
}

我把 let 改成了 var，但我想还是有什么我不明白的地方..

【问题讨论】：

如果 Swift 允许您捕获变异的 self 参数，那么它可以有效地让您使用值类型（例如结构的实例）作为引用类型。

标签： swift structure mutating-function

【解决方案1】：

我实际上发现了问题.. 它非常愚蠢.. 我只需要更改函数的“已完成”部分，如下所示：

 mutating func getUserWorkspace(base: String, completed: @escaping (_ arr:[Workspace]?) -> ()){
    let url = URL(string: "SOME URL")!
    var request = URLRequest(url: url)

    request.addValue("Basic \(base)", forHTTPHeaderField: "Authorization")
    request.addValue("application/json", forHTTPHeaderField: "Accept")

    URLSession.shared.dataTask(with: request){ (data, response, error) in
        if error == nil {
            do {
                let rep = try JSONDecoder().decode([Workspace].self, from: data!)
                DispatchQueue.main.async {
                    completed(rep)
                }
            }catch {
                print(error)
            }
        }
    }.resume()
}

【讨论】：

【解决方案2】：

我建议你使用 class 而不是 struct。无论如何，如果你喜欢使用你的代码，那么在你的变异方法中捕获 self，如下所示：

mutating func getUserWorkspace(base: String, completed: @escaping () -> ()){
        let url = URL(string: "some url")!
        var request = URLRequest(url: url)
        var myself = self

        request.addValue("Basic \(base)", forHTTPHeaderField: "Authorization")
        request.addValue("application/json", forHTTPHeaderField: "Accept")

        URLSession.shared.dataTask(with: request){ (data, response, error) in
            if error == nil {
                do {
                    let res: Workspace = try JSONDecoder().decode(Workspace.self, from: data!)
                    myself.guid = res.guid  //Error here
                    myself.name = res.name  //Error here
                    DispatchQueue.main.async {
                        completed()
                    }
                }catch {
                    print ("JSON error")
                }
            }
            }.resume()
  }

【讨论】：

【解决方案3】：

//Try this one and let me know
struct Workspace: Decodable {
    static var shared = Workspace()
    var guid: String
    var name: String

    private enum CodingKeys : String, CodingKey {
        case guid = "guid"
        case name = "name"
    }

    init(){
        self.guid = "blbl"
        self.name = "oops"
    }

    mutating func getUserWorkspace(base: String, completed: @escaping () -> ()){
        let url = URL(string: "some url")!
        var request = URLRequest(url: url)

        request.addValue("Basic \(base)", forHTTPHeaderField: "Authorization")
        request.addValue("application/json", forHTTPHeaderField: "Accept")

        URLSession.shared.dataTask(with: request){ (data, response, error) in
            if error == nil {
                do {
                    let res: Workspace = try JSONDecoder().decode(Workspace.self, from: data!)
                    Workspace.shared.guid = res.guid  //Error here
                    Workspace.shared.name = res.name  //Error here
                    DispatchQueue.main.async {
                        completed()
                    }
                }catch {
                    print ("JSON error")
                }
            }
            }.resume()
    }
}

【讨论】：