如何等待结果异步代码

Question

How to await result from async method?

    let url = URL(string: "https://www.cbr-xml-daily.ru/daily_json.js")
    var dict = NSDictionary()

    let task = URLSession.shared.dataTask(with: url!) { data, response, error in
        guard error == nil else {
            print(error!)
            return
        }
        guard let data = data else {
            print("Data is empty")
            return
        }

        dict = try! JSONSerialization.jsonObject(with: data, options: []) as! NSDictionary
        print("first print\(dict)")
    }

   task.resume()
   print("second \(dict)")

In first print data not empty but in second print is empty

Answer 1

That's why you need completion closure .

    func load(completion: @escaping (NSDictionary) -> ()) {
        let url = URL(string: "https://www.cbr-xml-daily.ru/daily_json.js")
        var dict = NSDictionary()

        let task = URLSession.shared.dataTask(with: url!) { data, response, error in
            guard error == nil else {
                print(error!)
                return
            }
            guard let data = data else {
                print("Data is empty")
                return
            }

            dict = try! JSONSerialization.jsonObject(with: data, options: []) as! NSDictionary
            print("first print\(dict)")

            // Here is when you finish loading data
            completion(dict)
        }

        task.resume()
        print("second \(dict)")
    } 

EDIT

load(completion: {
    print("My data: \($0)" )
})

Answer 2

The code does not execute in the order you suspect.

Your task.resume() statement starts the task, but the task might not actually run or finish till tomorrow. Meanwhile, print("second (dict)") executes right away, today, right after the task.resume() statement, perhaps running any of before, during, or after the task completion.

The way to have code await the end of your task is to put that code that you want to await inside the task response block, where your print("first print(dict)") statement is already located. Or you can put the waiting code in another block, method, or function, called from inside that task response.

Any code after the task.resume() statement may execute way before the task even really starts.

Answer 3

Assume you download a URL, but just before you do that the server has crashed and was just re-booting, and it takes 50 seconds for your answer to apply. Your code should handle that and can do it easily.

You have data that is missing. Your code displaying data just has to handle that. You say you need the data - no, you don't. You just have written your code in a way that assumes it's there, so change that assumption and make it work when the data isn't there. And when the data arrives later, you update the display and show the correct data.