如何同步闭包？

Question

How can I synchronize closures?

I have this code:

    private func getWeather(parameters: [String : Any],  failure: ((String) -> Void)? = nil ,completion: (() -> Void)? = nil) {

        for _ in 0...10 {

            RequestManager.sharedInstance.request(url: baseURL, parameters: parameters, completion:  { (result) in
                if JSON.parse(result)["name"].string == nil {
                    failure?("Something went wrong. Please, try again later")
                } else {
                    let weatherModel: WeatherModel = WeatherModel(json: JSON.parse(result))
                }
            })
        }
        completion?()

    }

In my code, completion?() will call, not when all requests will ended And I need call completion?() when all requests will ended. Ho can I do it?

Answer 1

Since the currently accepted answer isn't correct, here is a version that properly uses a DispatchGroup .

private func getWeather(parameters: [String : Any],  failure: ((String) -> Void)? = nil ,completion: (() -> Void)? = nil) {
    let dispatchGroup = DispatchGroup()
    for _ in 0...10 {
        dispatchGroup.enter()
        RequestManager.sharedInstance.request(url: baseURL, parameters: parameters) { result in
            if JSON.parse(result)["name"].string == nil {
                failure?("Something went wrong. Please, try again later")
            } else {
                let weatherModel: WeatherModel = WeatherModel(json: JSON.parse(result))
            }
            dispatchGroup.leave()
        }
    }

    dispatchGroup.notify(queue: DispatchQueue.main) {
        completion?()
    }
}

Answer 2

An easy way to do this is just to count completed calls.

private func getWeather(parameters: [String : Any],  failure: ((String) -> Void)? = nil ,completion: (() -> Void)? = nil) {
    let numCalls = 11;
    var completedCalls = 0;

    for _ in 0..<numCalls {

        RequestManager.sharedInstance.request(url: baseURL, parameters: parameters, completion:  { (result) in
            if JSON.parse(result)["name"].string == nil {
                failure?("Something went wrong. Please, try again later")
            } else {
                let weatherModel: WeatherModel = WeatherModel(json: JSON.parse(result))
            }
            completedCalls += 1
            if completedCalls == numCalls {
                completion?()
            }
        })
    }
}

Your completion callback runs when each request ends. Making each of your completion callbacks update a value in its enclosing scope allows you to keep track of how many requests have ended. When all the requests you expect have ended, you call completion?() .