How I can return value from async block in swift

Question

Please have a look into the code below:

backgroundthread.async {
    return self.mycallback() //return string, int etc
}

I want to return a value from an async block. I don't want any completion handler or any other workaround.

func getAppConfigFromDB(_ key: String) -> String 
{
   let value = String()
   backgroundthread.async {
      let inst = AppConfigDB.init(_APP_CONFIG_DB_PATH)
      value = inst.getConfigurationInfo(key) // I want to return from here.
   }
   return value
}


getAppConfigFromDB("path")

You have no choice but to use a completion handler. You can't return a value from an async call. — rmaddy, Sep 12 '18 at 05:18
ok, is there any work around or custom template, we can use. — vikkuu, Sep 12 '18 at 05:20
I agree with @rmaddy, completion handler is the only way to handling the results when the async task completes. They named it well! — SagarScript, Sep 12 '18 at 05:20
Why do you wish to avoid a completion handler? What problem are you trying to solve where the simplest and proper solution is to be avoided for some reason? — rmaddy, Sep 12 '18 at 05:22
The way to get a return value from your function is to change the async block to a synchronous block, or an inefficient workaround to do the same thing. — hotpaw2, Sep 12 '18 at 05:35
@hotpaw2, yes, I can try with sync block. Its long way to change my code but thats fine. Thanks. !! — vikkuu, Sep 12 '18 at 05:40

Rakesha Shastri · Accepted Answer · 2021-03-10T06:04:48.433

Like @rmaddy said, you have no other way than to use completion handlers.

func getAppConfigFromDB(_ key: String, completion: @escaping ((String) -> Void)) {
    let value = String()
    backgroundthread.async {
        let inst = AppConfigDB.init(_APP_CONFIG_DB_PATH)
        value = inst.getConfigurationInfo(key) // I want to return from here.
        completion(value)
    }
}

You call the method like this.

getAppConfigFromDB("") { (value) in
    // Use value to do something
}

score 1 · Answer 2 · answered Sep 12 '18 at 05:40

Your function would need a closure like so

func getAppConfigFromDB(_ key: String, completion: @escaping (String?) -> Void) {
    backgroundthread.async {
        completion("string here")
    }    
}

When you call your function you would do

getAppConfigFromDB("key") { (returnedString) in
    //returnedString is Optional("string here")
}

score 0 · Answer 3 · answered Sep 12 '18 at 06:15

What you are describing with return self.mycallback() is something like in

TypeScript:

String text = await someTask();

or in C#:

String text = someTask().result;

and from an async function:

String text = await someTask();

However this concept does not exist in swift (and i think Java too).

The only other method I can think of other than using a completion handler, is passing the results to another function (NOTE: If you intend on working on the main thread/UI Thread you will get an exception - you will need to wrap your call in DispatchQueue.main.async {/*Do stuff...*/}) like so

    func startAsync {
        let url = URL(string: "https://jsonplaceholder.typicode.com/todos/1")!
        let task = URLSession.shared.dataTask(with: url) {(data, response, error) in
            guard let data = data else { return }
            guard let jsonString = String(data: data, encoding: .utf8) else { return }
            DispatchQueue.main.async {
                self.setResults(text: jsonString);
            }
        }
        task.resume()
    }

    func setResults(text: String?){
        textView.text = text;
    }

full project: https://github.com/AppLogics/SwiftAsyncTaskWithoutCompletionHandler

How I can return value from async block in swift

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