Swift: Extra argument 'error' in call

Question

I'm currently developing my first iOS app using Swift 2.0 and Xcode Beta 2. It reads an external JSON and generates a list in a table view with the data. However, I'm getting a strange little error that I can't seem to fix:

Extra argument 'error' in call

Here is a snippet of my code:

let task = session.dataTaskWithURL(url!, completionHandler: {data, response, error -> Void in
            print("Task completed")

            if(error != nil){
                print(error!.localizedDescription)
            }

            var err: NSError?

            if let jsonResult = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.MutableContainers, error: &err) as? NSDictionary{

                if(err != nil){
                    print("JSON Error \(err!.localizedDescription)")
                }

                if let results: NSArray = jsonResult["results"] as? NSArray{
                    dispatch_async(dispatch_get_main_queue(), {
                        self.tableData = results
                        self.appsTableView!.reloadData()
                    })
                }
            }
        })

The error is thrown at this line:

if let jsonResult = NSJSONSerialization.JSONObjectWithData(data, options: NSJSONReadingOptions.MutableContainers, error: &err) as? NSDictionary{

Can someone please tell me what I'm doing wrong here?

Solution 1

With Swift 2, the signature for NSJSONSerialization has changed, to conform to the new error handling system.

Here's an example of how to use it:

do {
    if let jsonResult = try NSJSONSerialization.JSONObjectWithData(data, options: []) as? NSDictionary {
        print(jsonResult)
    }
} catch let error as NSError {
    print(error.localizedDescription)
}

With Swift 3, the name of NSJSONSerialization and its methods have changed, according to the Swift API Design Guidelines.

Here's the same example:

do {
    if let jsonResult = try JSONSerialization.jsonObject(with: data, options: []) as? [String:AnyObject] {
        print(jsonResult)
    }
} catch let error as NSError {
    print(error.localizedDescription)
}

Solution 2

Things have changed in Swift 2, methods that accepted an error parameter were transformed into methods that throw that error instead of returning it via an inout parameter. By looking at the Apple documentation:

HANDLING ERRORS IN SWIFT: In Swift, this method returns a nonoptional result and is marked with the throws keyword to indicate that it throws an error in cases of failure.

You call this method in a try expression and handle any errors in the catch clauses of a do statement, as described in Error Handling in The Swift Programming Language (Swift 2.1) and Error Handling in Using Swift with Cocoa and Objective-C (Swift 2.1).

The shortest solution would be to use try? which returns nil if an error occurs:

let message = try? NSJSONSerialization.JSONObjectWithData(receivedData, options:.AllowFragments)
if let dict = message as? NSDictionary {
    // ... process the data
}

If you're also interested into the error, you can use a do/catch:

do {
    let message = try NSJSONSerialization.JSONObjectWithData(receivedData, options:.AllowFragments)
    if let dict = message as? NSDictionary {
        // ... process the data
    }
} catch let error as NSError {
    print("An error occurred: \(error)")
}

Comments

[Dharmbir Singh]

This line doesn't giving this error i think may b you are getting in different line.

[Race B]

With 2.0 you need to create a do catch block. error is no longer a parameter with NSJSONSerialization. There are a number of other answers to this question here. Look for do, catch with Swift 2.0

[jeffery_the_wind]

I am using swift 2.2, and tried your first version. It is compiling correctly but it is dying silently, does not hit either of the the print lines. When I remove the if part in front of jsonResult and run I can infer that the jsonResult is being set as nil no errors. I am sure the data starts as a valid json string...

[Eric Aya]

@jeffery_the_wind Your JSON is probably different from what you think. Try this: stackoverflow.com/a/33510776/2227743

[jeffery_the_wind]

Thanks, yes that was it! Replacing as? NSDictionary with as? NSArray in your first example works for me.