变量在初始化前从未使用过mutated...or常量。

Question

所以我想要返回jsonMeals数据，并在这个函数之外使用它。然而，不管我把json变量放在哪里，我都会得到一个错误。将其更改为let也可以，尽管是不同的。任何洞察力都将不胜感激！

错误：

Constant 'json' used before being initialized // Variable 'json' was never mutated; consider changing to 'let' constant

        func getApiDetailData(completed: @escaping () -> ()) {
            var json: Any?
            let urlString = "https://www.themealdb.com/api/json/v1/1/lookup.php?i=\(id)"
            let url = URL(string: urlString)
            
            URLSession.shared.dataTask(with: url!) { (data, response, error) in
                    do {
                        let json = try JSONSerialization.jsonObject(with: data!)
                        print("\(json)Testing")
                        DispatchQueue.main.async {
                            completed()
                        }
                    }
                    catch {
                        print("Error getting detail JSON data:\(error)")
                    }
                guard let json = json as? [String : Any],
                      let jsonMeals = json["meals"] as? [String: Any] else {
                          print("No meals in json \(error?.localizedDescription)")
                        return
                    }
                print("testing jsonMeals\(jsonMeals)")
            }.resume()
        }

Answer 1

尝试类似于下面的示例代码：

func getApiDetailData(completed: @escaping () -> ()) {
    //   var json: Any?   // <-- remove, never used
    let urlString = "https://www.themealdb.com/api/json/v1/1/lookup.php?i=\(id)"
    let url = URL(string: urlString)
    
    URLSession.shared.dataTask(with: url!) { (data, response, error) in
        do {
            let jsonData = try JSONSerialization.jsonObject(with: data!)
            print("\(jsonData) Testing")
            
            guard let json = jsonData as? [String : Any],
                  let jsonMeals = json["meals"] as? [[String: Any]] else {
                print("No meals in json \(error?.localizedDescription)")
                completed()  // <-- here
                return
            }
            print("testing jsonMeals \(jsonMeals)")
            completed()  // <-- here
        }
        catch {
            print("Error getting detail JSON data:\(error)")
            completed()  // <-- here
        }
    }.resume()
}

或者，如果您想返回jsonMeals结果：

func getApiDetailData(completed: @escaping ([[String: Any]]?) -> ()) {  // <-- here
  //  var json: Any?   // <-- remove, never used
    let urlString = "https://www.themealdb.com/api/json/v1/1/lookup.php?i=\(id)"
    let url = URL(string: urlString)
    
    URLSession.shared.dataTask(with: url!) { (data, response, error) in
        do {
            let jsonData = try JSONSerialization.jsonObject(with: data!)
            print("\(jsonData) Testing")
            
            guard let json = jsonData as? [String : Any],
                  let jsonMeals = json["meals"] as? [[String: Any]] else {
                print("No meals in json \(error?.localizedDescription)")
                completed(nil)  // <-- here
                return
            }
            print("testing jsonMeals \(jsonMeals)")
            completed(jsonMeals)  // <-- here
        }
        catch {
            print("Error getting detail JSON data:\(error)")
            completed(nil)  // <-- here
        }
    }.resume()
}