Types of Swift Error Handling

March 8, 2016

I recently did an informal presentation on error handling in Swift to inform others on the variety of options we have with Swift. I used an Xcode playground to present this topic and it ended up surprising all of us with how many interesting features playgrounds have. That playground can be downloaded here or you can just read this post for info (be sure to check out the playground fun at the bottom).

There are 4 main types of error handling Apple mentions with Swift.

1. Propagate the error from a function to the code that calls the function

enum WindStrength: ErrorType {
    case Gust
    case Gale
    case Hurricane
}

class WeatherManager {
    
    // in MPH
    var currentWindSpeed = 34
    
    func isWeatherSafe() throws -> Bool {
        
        if currentWindSpeed >= 25 && currentWindSpeed <= 38 {
            throw WindStrength.Gust
        } else if currentWindSpeed >= 39 && currentWindSpeed <= 72 {
            throw WindStrength.Gale
        } else if currentWindSpeed >= 73 {
            throw WindStrength.Hurricane
        }
        
        // good for cleanup, such as closing a file that was opened
        defer {
            print("do clean up task")
        }
        
        return true
        
    }
    
}

I would use this when I have a method I will call a lot and a method that could have multiple things go wrong for which the method caller will need to know about. No user or dev likes the description-less error, such as “An error occurred”.

2. Handle the error using a do-catch statement

var weatherManager = WeatherManager()
var isSafe: Bool? {
    didSet {
        print("Must be safe")
    }
}

do {
    isSafe = try weatherManager.isWeatherSafe()
} catch WindStrength.Gust {
    print("You should go somewhere safe")
} catch WindStrength.Gale {
    print("Strong winds, to say the least")
} catch WindStrength.Hurricane {
    print("Hurricane winds, best of luck")
} catch {
    print(error)
}

Very similar to try-catch statements in other languages, it is more focused on how you handle an error result rather than translating something else into an error case. For example, method #1 in this list is more for the person writing a framework, however simple, and method #2 here, is for the person consuming that framework.

3. Handle the error as an optional value

// If any error, value is nil
if let _ = try? weatherManager.isWeatherSafe() {
    print("Weather is fine, wind is calm")
} else {
    print("error")
}

// Bonus: ?? operator takes the value that is not nil, if possible
var myValue = try? weatherManager.isWeatherSafe()
let safeWeather = myValue ?? false

This is essentially doing the optional unwrap you all should be familiar with by now. The difference with this one is that the try? keyword is used because we don’t care what type of error we get back, just that we get an error. In addition, there is a try! which should very rarely be used, in fact, you shouldn’t ever have to use it, nevertheless, here is an example:

let photo = try! loadImage("./Resources/John Appleseed.jpg")

4. Assert that an error will not occur

func transformString(string: String?) -> String {
    assert(string != nil, "Invalid parameter")
    return string! + "_transformed"
}

transformString("hi")

This is probably the least common type of “error handling” because it simply crashes your app if the error occurs. You should check out this more thorough blog article on asserts by Marcin Krzyżanowski.

###

Keep in Mind

Exceptions do not exist in Swift, but they do exist in Objective-C. It is really easy to get these two things mixed up in your head, but they have a very key difference. Exceptions are more for informing the developer, because they will crash your app when thrown and not handled. While errors, are more for informing the user of an issue while keeping your app running.

For reference, here is an example of error handling in Objective-C

NSError *anyError;
BOOL success = [receivedData writeToURL:someLocalFileURL
                                options:0
                                  error:&anyError];

if (!success) {
    NSLog(@"Write failed with error: %@", anyError);
    // present error to user
}

Pretty different from Swift, considering error pointers are passed to methods by reference to be updated by the method being called.

One more feature, is that you can cast do-catch error objects to an NSError object when by default, they are of type ErrorType

// Catch clauses

let fileManager = NSFileManager.defaultManager()
let fromURL = NSURL(fileURLWithPath: "/path/to/old")
let toURL = NSURL(fileURLWithPath: "/path/to/new")
do {
    try fileManager.moveItemAtURL(fromURL, toURL: toURL)
} catch let error as NSError {
    print("Error: \(error.localizedDescription)")
}

For additional reading and an exhaustive explanation of why Swift error handling is the way it is, go to Swift Error Handling Rationale

Wrap Up

For many iOS tasks, I can get by with just optional unwrapping, but sometimes you need a little something more that explains errors more effectively, such as a do-catch statement. This is especially true when you are writing a framework with a lot of network calls. I hope this post was a good overview and that Swift error handling is not something you need to shy away from, but rather a great tool for developers and your user experience.

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