ios - 从DispatchQueue Swift内部返回

Question

我正在构建一个函数，该函数接受CLLocation参数，并返回我自己定义的DarkSkyResponse类型的天气预报。

public func getForecast(at location: CLLocation) -> DarkSkyResponse {
    let request = DarkSkyRequest(key: "KEYGOESHERE")
    let point = DarkSkyRequest.Point(location.coordinate.latitude, location.coordinate.longitude)

    guard let url = request.buildURL(point: point) else {
        preconditionFailure("Failed to construct URL")
    }

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

        DispatchQueue.main.async {
            guard let data = data else {
                fatalError("No Data Recieved")
            }
            guard let forecast = DarkSkyResponse(data: data) else {
                fatalError("Decoding Failed")
            }
            return forecast
        }
    }
    task.resume()
}

一切工作正常，我只想返回在DispatchQueue中找到的 forecast变量。当我尝试执行此操作时，遇到以下错误: Unexpected non-void return value in void function我相信这是因为我要返回DispatchQueue内部。

为了解决这个问题，我将数据任务和解码过程移入了具有完成处理程序的函数中。当我调用该函数时，只需将闭包设置为按以下方式返回输出:

public func getForecast(at location: CLLocation) -> DarkSkyResponse {
    let request = DarkSkyRequest(key: "KEYGOESHERE")
    let point = DarkSkyRequest.Point(location.coordinate.latitude, location.coordinate.longitude)

    guard let url = request.buildURL(point: point) else {
        preconditionFailure("Failed to construct URL")
    }

    func getDataAndDecode(from url: URL, completionBlock: @escaping (DarkSkyResponse) -> Void) -> Void {
        let task = URLSession.shared.dataTask(with: url) {
            data, response, error in

            DispatchQueue.main.async {
                guard let data = data else {
                    fatalError("No Data Recieved")
                }
                guard let forecast = DarkSkyResponse(data: data) else {
                    fatalError("Decoding Failed")
                }
                completionBlock(forecast)
            }
        }
        task.resume()
    }

    getDataAndDecode(from: url) { (output) in
        return output
    }
}

现在我得到一个警告: Expression of type 'DarkSkyResponse' is unused。我正在以正确的方式执行此操作，还是有一种更优雅的方式来返回我想要的值？

Answer 1

到目前为止，swift尚不支持异步等待样式(类似于javascript)，这意味着您无法直接从异步操作中返回。
最好将callback(completionBlock)参数放在getForecast方法上。如果回调函数也处理错误的情况(例如onSuccess和onError)，那就太好了。并且不要忘记将getForecast方法设置为void方法。

因此，就像这样:

func getForecast(at location: CLLocation, onSuccess: (DarkSkyResponse) -> (), onError: (Error) -> ()) {

...
}