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iOS Build a Weather App with Swift (Retired) Concurrency Using Our JSON Data

Sebastian Röder
Sebastian Röder
13,878 Points
Posted by Sebastian Röder
Sebastian Röder
13,878 Points

Which types of errors will be catched by `error == nil`?

In the video it is shown that our app crashes when we use an invalid URL. This is the reason why we added the if error == nil statement in the following code snippet:

let downloadTask = sharedSession.downloadTaskWithURL(forecastURL, completionHandler:
            { (location: NSURL!, response: NSURLResponse!, error: NSError!) -> Void in
                // TODO: Check that status code in NSHTTPURLResponse is 200 / OK
                // error will be nil even with status code 404 etc.
                if error == nil {
                    let forecastData = NSData(contentsOfURL: location)
                    let forecastJSON = NSJSONSerialization.JSONObjectWithData(forecastData, options: nil, error: nil) as NSDictionary
                }

However, as it turns out, this code still crashes when our URL is invalid. When the status code in the response is 404, the error is still nil, so the code inside the ifblock gets executed. Since forecastData is also nil, we get fatal error: unexpectedly found nil while unwrapping an Optional value in the next line.

So our error handling does not actually address the problem we wanted to solve. How can this be improved?

I wanted to check for response.statusCode == 200 in the if-statement but had a problem with NSHTTPResponse vs. NSHTTPURLResponse.

Sebastian Röder
Sebastian Röder
13,878 Points
Sebastian Röder
Sebastian Röder
13,878 Points

OK, I found a solution by down casting NSURLResponse to NSHTTPURLResponse. My code looks like this now:

let downloadTask = sharedSession.downloadTaskWithURL(forecastURL, completionHandler:
    { (location: NSURL!, response: NSURLResponse!, error: NSError!) -> Void in

        let statusCode = (response as NSHTTPURLResponse).statusCode

        if error == nil && statusCode == 200 {
            let forecastData = NSData(contentsOfURL: location)
            let forecastJSON = NSJSONSerialization.JSONObjectWithData(forecastData,
                options: nil, error: nil) as NSDictionary
        }
})

1 Answer

Johnny Deep
PLUS
Johnny Deep
Courses Plus Student 286 Points
Johnny Deep
Johnny Deep
Courses Plus Student 286 Points

Sebastian Röder is absolutely right, we can even remove the (error == nil) condition by using statusCode he provide instead, without problem, thanks for the answer!

Regards,