DispatchQueue不会快速更新数据

Question

I'm need to do a search operation in Swift and me using UISearchbar for it.

On textDidChange event, I need to call a web api, parse the response and then update the array and then begin the search on updated array.

But not sure my code does not work.

func searchBar(_ searchBar: UISearchBar, textDidChange searchText: String) {

    let group = DispatchGroup()
    group.enter()
    // Perform some asynchronous operation
    let queue1 = DispatchQueue(label: "abc")
    queue1.async {
        self.callWebAPI() // This function calls the web api and parses it’s response
        group.leave()
    }

    DispatchQueue.global(qos: .utility).async {
        DispatchQueue.main.async {
            self.filteredCountry = self.arrCountry.filter({$0.name.prefix(searchText.count) == searchText})
            self.searching = true
            self.tableView.reloadData()
        }
    }
}

func callWebAPI() {
    let urlString = URL(string: "https://restcountries.eu/rest/v2/all")
    if let url = urlString {
        let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
            if error != nil {
                print(error!)
            } else {
                if let usableData = data {
                    do{
                        //here dataResponse received from a network request
                        let jsonResponse = try JSONSerialization.jsonObject(with:
                            data!, options: [])
                        print(jsonResponse) //Response result

                        guard let jsonArray = jsonResponse as? [[String: Any]] else {
                            return
                        }


                        print(jsonArray)

                        print("done")

                    } catch let parsingError {
                        print("Error", parsingError)
                    }

                }
            }
        }
        task.resume()
    }

}

Please guide on my above code as not sure where I'm wrong

Answer 1

The issue is that callWebAPI is asynchronous (it returns immediately before the request is done), so you are calling leave immediately. You could give this method a completion handler and call leave in that. And you would also call the UI update in a notify block for your group, not just dispatch it.

Easier, just retire the DispatchGroup entirely and just update your UI in the completion handler you supply to callWebAPI .

For example, give callWebAPI a completion handler:

func callWebAPI(completionHandler: @escaping ([[String: Any]]?, Error?) -> Void) {
    let urlString = URL(string: "https://restcountries.eu/rest/v2/all")
    if let url = urlString {
        let task = URLSession.shared.dataTask(with: url) { (data, response, error) in
            guard let data = data, error == nil else {
                completionHandler(nil, error)
                return
            }

            do {
                let jsonResponse = try JSONSerialization.jsonObject(with:
                    data)

                completionHandler(jsonResponse as? [[String: Any]], nil)
            } catch let parsingError {
                completionHandler(nil, parsingError)
            }
        }
        task.resume()
    }
}

And then, you can eliminate the dispatch groups, the global queues (because it's already an asynchronous method, you don't need to invoke this from background queue), etc., and it's simplified to just:

func searchBar(_ searchBar: UISearchBar, textDidChange searchText: String) {
    callWebAPI { jsonResponse, error in
        guard let jsonResponse = jsonResponse, error == nil else {
            print("Error:", error ?? "Response was not correct format")
            return
        }

        print(jsonResponse)

        // Note, you don’t appear to be using `jsonResponse` at all,
        // so I presume you’d update the relevant model objects.

        DispatchQueue.main.async {
            self.filteredCountry = self.arrCountry.filter({$0.name.prefix(searchText.count) == searchText})
            self.searching = true
            self.tableView.reloadData()
        }
    }
}

---

As an aside, nowadays we use JSONDecoder to parse JSON to populate model objects directly, but that's beyond the scope of this question.