Handling errors in Swift

Question

In my application I need to download a JSON file from the web. I have made a ResourceService class that have a download method as seen below. I use this service in "higher level" services of my app. You can see there are multiple of things that may go wrong during the download. The server could be on fire and not be able to successfully respond at the moment, there could be go something wrong during the moving of the temporary file etc.

Now, there is probably not much a user can do with this other than trying later. However, he/she probably want to know that there was something wrong and that the download or the behaviour of the "higher level" methods could not succeed.

Me as a developer is confused as this point because I don't understand how to deal with errors in Swift. I have a completionHandler that takes an error if there was one, but I don't know what kind of error I should pass back to the caller.

Thoughts:

1) If I pass the error objects I get from the NSFileManager API or the NSURLSession API, I would think that I am "leaking" some of the implementation of download method to the callers. And how would the caller know what kind of errors to expect based on the error? It could be both.

2) If I am supposed to catch and wrap those errors that could happen inside the download method, how would that look like?

3) How do I deal with multiple error sources inside a method, and how would the code that calls the method that may throw/return NSError objects look like?

Should you as a caller start intercepting the errors you get back and then write a lot of code that differentiates the messages/action taken based on the error code? I don't get this error handling stuff at all and how it would look like when there are many things that could go wrong in a single method.

func download(destinationUrl: NSURL, completionHandler: ((error: NSError?) -> Void)) {
    let request = NSURLRequest(URL: resourceUrl!)

    let task = downloadSession.downloadTaskWithRequest(request) {
        (url: NSURL?, response: NSURLResponse?, error: NSError?) in

        if error == nil {
            do {
                try self.fileManager.moveItemAtURL(url!, toURL: destinationUrl)
            } catch let e {
                print(e)
            }
        } else {
        }
    }.resume()
}

Your completion handler accepts an error so I would just pass back the error that you got from downloadTaskWithRequest. If you pass back some other error then your caller is left wondering what really happened. If you pass back the error you got then they know — Paulw11
– Paulw11, Commented Dec 18, 2015 at 22:41
But wouldn't that leak some of the internal stuff to the caller? Or is that ok? — LuckyLuke
– LuckyLuke, Commented Dec 18, 2015 at 22:42
And how would the callers know what to "look for" in the error? Do I write what they may expect to find in the error in the documentation then? I am thinking of error codes. — LuckyLuke
– LuckyLuke, Commented Dec 18, 2015 at 22:44
The error is probably most useful to a developer while they are debugging because there is not much a user can do at runtime if the move fails. As a developer the last thing I want is some custom error code that means "something went wrong" that I then have to go and look up. If you pass back the root error then I can just log it and try and work out what is going on. — Paulw11
– Paulw11, Commented Dec 18, 2015 at 22:50
That is true. But what if I as a developer want to show the user "The file couldn't be downloaded because the server was on fire" when the download fails, while showing "The filesystem is cranky at the moment" when the moving went wrong? Do I log the errors and then write the necessary code to do that? Shouldn't I make a difference on those errors with regard to users? — LuckyLuke
– LuckyLuke, Commented Dec 18, 2015 at 22:56

Peter Hornsby · Accepted Answer · 2015-12-23 14:45:28Z

14

+500

First of all this is a great question. Error handling is a specific task that applies to a incredible array of situations with who know's what repercussions with your App's state. The key issue is what is meaningful to your user, app and you the developer.

I like to see this conceptually as how the Responder chain is used to handle events. Like an event traversing the responder chain an error has the possibility of bubbling up your App's levels of abstraction. Depending on the error you might want to do a number of things related to the type of the error. Different components of your app may need to know about error, it maybe an error that depending on the state of the app requires no action.

You as the developer ultimately know where errors effect your app and how. So given that how do we choose to implement a technical solution.

I would suggest using Enumerations and Closures as to build my error handling solution.

Here's a contrived example of an ENUM. As you can see it is represents the core of the error handling solution.

    public enum MyAppErrorCode {

    case NotStartedCode(Int, String)
    case ResponseOkCode
    case ServiceInProgressCode(Int, String)
    case ServiceCancelledCode(Int, String,  NSError)

    func handleCode(errorCode: MyAppErrorCode) {

        switch(errorCode) {
        case NotStartedCode(let code, let message):
            print("code: \(code)")
            print("message: \(message)")
        case ResponseOkCode:
            break

        case ServiceInProgressCode(let code, let message):
            print("code: \(code)")
            print("message: \(message)")
        case ServiceCancelledCode(let code, let message, let error):
            print("code: \(code)")
            print("message: \(message)")
            print("error: \(error.localizedDescription)")
        }
    }
}

Next we want to define our completionHandler which will replace ((error: NSError?) -> Void) the closure you have in your download method.

((errorCode: MyAppErrorCode) -> Void)

New Download Function

func download(destinationUrl: NSURL, completionHandler: ((errorCode: MyAppErrorCode) -> Void)) {
    let request = NSURLRequest(URL: resourceUrl!)

    let task = downloadSession.downloadTaskWithRequest(request) {
        (url: NSURL?, response: NSURLResponse?, error: NSError?) in

        if error == nil {
            do {
                try self.fileManager.moveItemAtURL(url!, toURL: destinationUrl)
                completionHandler(errorCode: MyAppErrorCode.ResponseOkCode)

            } catch let e {
                print(e)
                completionHandler(errorCode: MyAppErrorCode.MoveItemFailedCode(170, "Text you would like to display to the user..", e))

            }


        } else {
            completionHandler(errorCode: MyAppErrorCode.DownloadFailedCode(404, "Text you would like to display to the user.."))

        }
    }.resume()
}

In the closure you pass in you could call handleCode(errorCode: MyAppErrorCode) or any other function you have defined on the ENUM.

You have now the components to define your own error handling solution that is easy to tailor to your app and which you can use to map http codes and any other third party error/response codes to something meaningful in your app. You can also choose if it is useful to let the NSError bubble up.

EDIT

Back to our contrivances.

How do we deal with interacting with our view controllers? We can choose to have a centralized mechanism as we have now or we could handle it in the view controller and keep the scope local. For that we would move the logic from the ENUM to the view controller and target the very specific requirements of our view controller's task (downloading in this case), you could also move the ENUM to the view controller's scope. We achieve encapsulation, but will most lightly end up repeating our code elsewhere in the project. Either way your view controller is going to have to do something with the error/result code

An approach I prefer would be to give the view controller a chance to handle specific behavior in the completion handler, or/then pass it to our ENUM for more general behavior such as sending out a notification that the download had finished, updating app state or just throwing up a AlertViewController with a single action for 'OK'.

We do this by adding methods to our view controller that can be passed the MyAppErrorCode ENUM and any related variables (URL, Request...) and add any instance variables to keep track of our task, i.e. a different URL, or the number of attempts before we give up on trying to do the download.

Here is a possible method for handling the download at the view controller:

func didCompleteDownloadWithResult(resultCode: MyAppErrorCode, request: NSURLRequest, url: NSURL) {

    switch(resultCode) {
    case .ResponseOkCode:
        // Made up method as an example
        resultCode.postSuccessfulDownloadNotification(url, dictionary: ["request" : request])

    case .FailedDownloadCode(let code, let message, let error):

        if numberOfAttempts = maximumAttempts {
            // Made up method as an example
            finishedAttemptingDownload()

        } else {
             // Made up method as an example
            AttemptDownload(numberOfAttempts)
        }

    default:
        break

    }
}

edited Dec 23, 2015 at 14:45

answered Dec 21, 2015 at 15:01

Peter Hornsby

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19 Comments

LuckyLuke Over a year ago

Thank you for answering. So do you create one huge Enum with all the error codes in the application, no matter where they come from? And how should I know which error codes to catch if there are nested closures? I mean if I call a method that takes a closure and that closure calls two to three other closures inside it (async stuff etc) How am I supposed to know which error I should catch? Or should I not care, just give the user a message saying something went bad?

Peter Hornsby Over a year ago

No you can break it down to multiple ENUM's I give you the simplest pattern for the sake of documentation. So you could have one for web services, account management, etc. You can go as granular as you want. As to the nested closures, you need to decide if it's worth bubbling to the very top, or you just exit where you are and clean up.

Peter Hornsby Over a year ago

For the user you have to decide. There are many tasks involved with your download, the user only care if it worked or did not (as you pointed out) in that case you many choose to group errors within the switch statement which are all handled the same.

LuckyLuke Over a year ago

If a user starts my tasks which is to open a catalog file, he/she would expect to be notified if something is wrong and couldn't be opened. In code it would involve downloading, moving and parsing...all creating errors. I would like to see how you or somebody else would write the error handling code around that. It doesn't necessarily have to compile, I am just looking for the error handling.

Peter Hornsby Over a year ago

@LuckyLuke You have the general pattern above (AFIAK). Say for example you have successfully download the file and moved it and then you pass it off to a parser for example, then you define enum values to use in the parser class, where you could change the 'Message' passed to the error. I might not be explaining this very well, please excuse me.

|

theguy · Accepted Answer · 2015-12-23 08:35:00Z

Long story short: yes

... and then write a lot of code that differentiates the messages/action taken based on the error code?

Most code examples leave the programmer alone about how to do any error handling at all, but in order to do it right, your error handling code might be more than the code for successful responses. Especially when it comes to networking and json parsing.

In one of my last projects (a lot of stateful json server communication) I have implemented the following approach: I have asked myself: How should the app possibly react to the user in case of an error (and translate it to be more user friendly)?

ignore it
show a message/ an alert (possibly only one)
retry by itself (how often?)
force the user to start over
assume (i.e. a previously cached response)

To achieve this, I have create a central ErrorHandler class, which does have several enums for the different types of errors (i.e. enum NetworkResponseCode, ServerReturnCode, LocationStatusCode) and one enum for the different ErrorDomains:

enum MyErrorDomain : String {
    // if request data has errors (i.e. json not valid)
    case NetworkRequestDomain   = "NetworkRequest"

    // if network response has error (i.e. offline or http status code != 200)
    case NetworkResponseDomain  = "NetworkResponse"

    // server return code in json: value of JSONxxx_JSON_PARAM_xxx_RETURN_CODE
    case ServerReturnDomain = "ServerReturnCode"

    // server return code in json: value of JSONxxxStatus_xxx_JSON_PARAM_xxx_STATUS_CODE
    case ServerStatusDomain = "ServerStatus"

    // if CLAuthorizationStatus
    case LocationStatusDomain = "LocationStatus"

    ....

}

Furthermore there exists some helper functions named createError. These methods do some checking of the error condition (i.e. network errors are different if you are offline or if the server response !=200). They are shorter than you would expect.

And to put it all together there is a function which handles the error.

func handleError(error: NSError, msgType: String, shouldSuppressAlert: Bool = false){
...
}

This method started with on switch statement (and needs some refactoring now, so I won't show it as it still is one). In this statement all possible reactions are implemented. You might need a different return type to keep your state correctly in the app.

Lessons learned:

Although I thought that I have started big (different enums, central user alerting), the architecture could have been better (i.e. multiple classes, inheritance, ...).
I needed to keep track of previous errors (as some are follow ups) in order to only show one error message to the user -> state.
There are good reasons to hide errors.
Within the errorObj.userInfo map, it exits a user friendly error message and a technicalErrorMessage (which is send to a tracking provider).
We have introduced numeric error codes (the error domain is prefixed with a letter) which are consistent between client and server. They are also shown to the user. This has really helped to track bugs.
I have implemented a handleSoftwareBug function (which is almost the same as the handleError but much less cases). It is used in a lot of else-blocks which you normally do not bother to write (as you think that this state can never be reached). Surprisingly it can.
```
    ErrorHandler.sharedInstance.handleSoftwareBug("SW bug? Unknown received error code string was code: \(code)")
```

How does it look like in code: There are a lot of similar backend network requests where a lot of code looks something like the following:

func postAllXXX(completionHandler:(JSON!, NSError!) -> Void) -> RegisteringSessionTask {
    log.function()
    return postRegistered(jsonDict: self.jsonFactory.allXXX(),
        outgoingMsgType: JSONClientMessageToServerAllXXX,
        expectedIncomingUserDataType: JSONServerResponseAllXXX,
        completionHandler: {(json, error) in

            if error != nil {
                log.error("error: \(error.localizedDescription)")
                ErrorHandler.sharedInstance.handleError(error,
                    msgType: JSONServerResponseAllXXX, shouldSuppressAlert: true)
                dispatch_async(dispatch_get_main_queue(), {
                    completionHandler(json, error)
                })
                return
            }

            // handle request payload
            var returnList:[XXX] = []
            let xxxList = json[JSONServerResponse_PARAM_XXX][JSONServerResponse_PARAM_YYY].arrayValue
            .....
            dispatch_async(dispatch_get_main_queue(), {
                completionHandler(json, error)
            })
    })
}

Within the above code you see that I call a completionHandler and give this caller the chance to customize error handling, too. Most of the time, this caller only handles success.

Whenever I have had the need for retries and other and not so common handling, I have also done it on the caller side, i.e.

private func postXXXMessageInternal(completionHandler:(JSON!, NSError!) -> Void) -> NSURLSessionDataTask {
    log.function()
    return self.networkquery.postServerJsonEphemeral(url, jsonDict: self.jsonFactory.xxxMessage(),
        outgoingMsgType: JSONClientMessageToServerXXXMessage,
        expectedIncomingUserDataType: JSONServerResponseXXXMessage,
        completionHandler: {(json, error) in

            if error != nil {
                self.xxxMessageErrorWaitingCounter++
                log.error("error(\(self.xxxMessageErrorWaitingCounter)): \(error.localizedDescription)")
                if (something || somethingelse) &&
                    self.xxxMessageErrorWaitingCounter >= MAX_ERROR_XXX_MESSAGE_WAITING {
                        // reset app because of too many errors
                        xxx.currentState = AppState.yyy
                        ErrorHandler.sharedInstance.genericError(MAX_ERROR_XXX_MESSAGE_WAITING, shouldSuppressAlert: false)
                        dispatch_async(dispatch_get_main_queue(), {
                            completionHandler(json, nil)
                        })
                        self.xxxMessageErrorWaitingCounter = 0
                        return
                }

           // handle request payload
            if let msg = json[JSONServerResponse_PARAM_XXX][JSONServerResponse_PARAM_ZZZ].stringValue {
                .....
            }
            .....
            dispatch_async(dispatch_get_main_queue(), {
                completionHandler(json, error)
            })
    })
}

Here is another example where the user is forced to retry

        // user did not see a price. should have been fetched earlier (something is wrong), cancel any ongoing requests
        ErrorHandler.sharedInstance.handleSoftwareBug("potentially sw bug (or network to slow?): no payment there? user must retry")
        if let st = self.sessionTask {
            st.cancel()
            self.sessionTask = nil
        }
        // tell user
        ErrorHandler.sharedInstance.genericInfo(MESSAGE_XXX_PRICE_REQUIRED)
        // send him back
        xxx.currentState = AppState.zzz
        return

Thank you, many good points that I will bring with me when I am experimenting. However, I have some questions. It seems like it is the handleError method that do "all" the error handling, am I correct? Then how does that method handle errors that should trigger a download try again for instance? There are many times you get an error, and you would try/do something else in your code without showing an alert or message to the user.
Within handleError a lot is done, but not all. Within my project I have about 40 different backend network calls. So in many cases, the code is very common and sometimes not: This is not in handleError, but on the caller side. I edit my answer with more (artificial) code fragments.

gnasher729 · Accepted Answer · 2015-12-19 13:58:01Z

For any request, you get either an error or an http status code. Error means: Your application never managed to talk properly to the server. http status code means: Your application talked to a server. Be aware that if you take your iPhone into the nearest Starbucks, "your application talked to a server" doesn't mean "your application talked to the server it wanted to talk to". It might mean "your application managed to talk to the Starbucks server which asks you to log in and you have no idea how to do that".

I divide the possible errors into categories: "It's a bug in my code". That's where you need to fix your code. "Something went wrong, and the user can do something about it". For example when WiFi is turned off. "Something went wrong, maybe it works later". You can tell the user to try later. "Something went wrong, and the user can't do anything about it". Tough. "I got a reply from the server that I expected. Maybe an error, maybe not, but something that I know how to handle". You handle it.

I also divide calls into categories: Those that should run invisibly in the background, and those that run as a result of a direct user action. Things running invisibly in the background shouldn't give error messages. (Bloody iTunes telling me it cannot connect to the iTunes Store when I had no interest in connecting to the iTunes Store in the first place is an awful example of getting that wrong).

When you show things to the user, remember that the user doesn't care. To the user: Either it worked, or it didn't work. If it didn't work, the user can fix the problem if it is a problem they can fix, they can try again later, or it's just tough luck. In an enterprise app, you might have a message "call your help desk at xxxxxx and tell them yyyyyy".

And when things don't work, don't annoy the user by showing error after error after error. If you send then requests, don't tell the user ten times that the server is on fire.

There are things that you just don't expect to go wrong. If you download a file, and you can't put it where it belongs, well, that's tough. It shouldn't happen. The user can't do anything about it. (Well, maybe they can. If the storage of the device is full then you can tell the user). Apart from that, it's the same category as "Something went wrong, and the user can't do anything about it". You may find out as a developer what the cause is and fix it, but if it happens with an application out in the user's hands, there's nothing reasonable you can do.

Since all such requests should be asynchronous, you will always pass either one or two callback blocks to the call, one for success and one for failure. I have most of the error handling in the download code, so things like asking the user to turn WiFi on happen only once, and calls may even be repeated automatically if such an error condition is fixed by the user. The error callback is mostly used to inform the application that it won't get the data that it wanted; sometimes the fact that there is an error is useful information in itself.

So what about the situation where the moveItemAtURL fails? How do you handle that, what should I do with it. How would the code example I gave look for the caller? What should the code look like. I want to know how it all looks in code because I don't find anything on it.

gnasher729 · Accepted Answer · 2015-12-20 01:05:41Z

0

For consistent error handling, I create my own errors representing either errors returned by the session, or html status codes interpreted as errors. Plus two additional errors "user cancelled" and "no user interaction allowed" if either there was a UI involved and the user cancelled the operation, or I wanted to use some user interaction but wasn't allowed to. The last two errors are different - these errors will never be reported to the user.

answered Dec 20, 2015 at 1:05

gnasher729

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Comments

Michael · Accepted Answer · 2015-12-21 23:33:53Z

I would wrap the errors in your own, but pass the underlying error as a property on your error class (ala C#'s InnerException). That way you are giving consumers a consistent interface, but also providing lower level error detail if required. However, the main reason I would do this is for unit testing. It makes it much easier to mock your ResourceService class and test the code paths for the various errors that could occur.

I don't like the thought of passing back an array of errors, as it adds complexity for the consumer. Instead I would provide an array of InnerException instances. If they are instances of your own error class, they would potentially have their own InnerException's with underlying errors. However, this would probably only make sense if you were doing your own validations where multiple errors might make sense. Your download method will probably have to bail out after the first error encountered.

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