"Expression implicitly coerced"警告是什么意思？

Question

func downloadCurrentWeather(completed: @escaping DownloadComplete){
    Alamofire.request(API_URL).responseJSON { (response) in
        let result = response.result
        let json = JSON(result.value) // <-- (EXPRESSION IMPLICITLY COERCED WARNING)
        self._cityName = json["name"].stringValue
        let tempDate = json["dt"].double
        let convertedDate = Date(timeIntervalSince1970: tempDate!)
        let dateFormatter = DateFormatter()
        dateFormatter.dateStyle = .medium
        dateFormatter.timeStyle = .none
        let currentDate = dateFormatter.string(from: convertedDate)
        self._date = "(currentDate)"
        self._weatherType = json["weather"][0]["main"].stringValue
        let downloadedTemp = json["main"]["temp"].double
        self._currentTemp = (downloadedTemp! - 273.15).rounded(toPlaces: 0)
        completed()
    }
}

Answer 1

它

强制使用它，因为value是一个Any?可选值。我建议打开value包装以确保它不是nil：

func downloadCurrentWeather(completed: @escaping DownloadComplete){
    Alamofire.request(API_URL).responseJSON { (response) in
        guard let value = response.result.value else {
            print(response.result.error ?? "Unknown error")
            return
        }
        let json = JSON(value)
        ...
    }
}

作为进一步的改进，我会更改DownloadComplete以包含有关它是否失败的信息。例如，我可能会添加一个Error?参数，然后您可以执行以下操作：

func downloadCurrentWeather(completed: @escaping DownloadComplete){
    Alamofire.request(API_URL).responseJSON { (response) in
        guard let value = response.result.value else {
            completed(response.result.error)
            return
        }
        let json = JSON(value)
        ...
        completed(nil)
    }
}

然后，呼叫者可以看到error是否nil。

---

另一种方法是switch response.result ，因为在.success的情况下，您可以只使用关联的值：

func downloadCurrentWeather(completed: @escaping DownloadComplete){
    Alamofire.request(API_URL).responseJSON { response in
        switch response.result {
        case .failure(let error):
            completed(error)
        case .success(let value):
            let json = JSON(value)
            ...
            completed(nil)
        }
    }
}