You can’t do what you’re trying to do.
First off, a block (which is what dispatch_async() takes) is defined to take no
parameters and return nothing. That’s the source of your compilation error,
you’re trying to return something from a block defined not to return anything.
Secondly, what you’re trying to do doesn’t make sense, you are trying to return
a value from a method which enqueues work for later execution. You can’t return
the return value of that block because at the time you return, the block hasn’t
been executed. Your code is basically this
func thing()->Banana
{
dispatch_async( dispatch_get_main_queue() )
{
()->Banana in
return Banana( curveyness: 1.0, smellsGross: true )
}
}
The dispatch_async queues the block and instantly returns, at this point there
is no Banana, the banana will be created later. dispatch_async() returns
nothing, there is nothing to return. When the banana is eventually created and
returned, it’s returned into nothingness, the method which called it long-since
finished.
You need your method to create its banana and then call asynchronously back to
another method which consumes it. You can’t synchronise asynchronous calls like
that.
> On 22 Dec 2015, at 22:25, Eric E. Dolecki <[email protected]> wrote:
>
> I am trying to return a dictionary from a method that uses a queue. I'm not
> sure how to actually return though as I get an error (can't convert value
> of type '() -> Dictionary<String,String>' to expected argument type
> 'dispatch_block_t'. I've tried a number of things but none prevent an error
> somewhere. I could ditch the queue altogether, but it feels better to use
> one.
>
> - Eric
>
>
> func getForecast(cityName:String) -> Dictionary<String,String> {
> let baseUrl: String = "
> http://api.openweathermap.org/data/2.5/forecast";
> let url: String =
> "\(baseUrl)?q=\(theCityName)&units=imperial&appid=\(APIKey)"
> let finalUrl: NSURL = NSURL(string: url)!
> let session = NSURLSession.sharedSession()
> let task = session.dataTaskWithURL(finalUrl, completionHandler:
> {data, response, error -> Void in
> if error != nil{
> print(error!.localizedDescription)
> }
> var err: NSError?
> let qualityOfServiceClass = QOS_CLASS_BACKGROUND
> let backgroundQueue =
> dispatch_get_global_queue(qualityOfServiceClass, 0)
> dispatch_async(backgroundQueue, {
>
> let json = JSON(data: data!, options:
> NSJSONReadingOptions(), error: &err)
> //print("response is \(json)")
> let weatherCurrent = json["list"][0]["main"]["temp"]
> let weatherHigh = json["list"][0]["main"]["temp_max"]
> let weatherLow = json["list"][0]["main"]["temp_min"]
> let conditionForecast =
> json["list"][0]["weather"][0]["description"].stringValue
>
> * dispatch_async(dispatch_get_main_queue(), { () ->
> Dictionary<String,String> in*
> * print("Forecast: Current: \(weatherCurrent)ºF.
> H:\(weatherHigh), L:\(weatherLow). Cond:
> \(conditionForecast.capitalizedString).")*
> * let dict = ["current":"\(weatherCurrent)",
> "high":"\(weatherHigh)",*
> * "low":"\(weatherLow)",
> "conditions":"\(conditionForecast.capitalizedString)"]*
> * return dict*
> })
> })
> })
> task.resume()
> }
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