Swift uses throws and do/catch for error handling. It has limitations like lacking type information and not working for asynchronous code. The Result type is a good alternative that represents values or errors for both synchronous and asynchronous functions. Exceptions are for crashes and unrecoverable errors, while assertions verify assumptions at debug time.

2. ERROR HANDLING IN
COCOA

3. AGENDA
Objective-C’s NSError
Swift’s try/catch/throws
Swift’s Result<S,F> type
Exceptions & Assertions

4. OBJECTIVE-C ERROR
HANDLING

5. NSERROR IN OBJECTIVE-C
NSError *error = nil;
NSString *string = [NSString stringWithContentsOfFile:@"test.txt"
encoding:NSUTF8StringEncoding error:&error];
if (!string) {
NSLog(@"File could not be read!");
}
if (error) {
NSLog(@"%@", error.localizedDescription);
}

6. NSERROR IN OBJECTIVE-C
Return value is most relevant for checking
whether an operation was successful or not [1]
Methods can return errors, but still successfully
return a value
[1] Apple Error Handling Documentation

7. NSERROR IN OBJECTIVE-C…
…IN THE SAD REAL WORLD
NSString *string = [NSString stringWithContentsOfFile:@"test.txt"
encoding:NSUTF8StringEncoding error:nil];

8. SWIFT ERROR HANDLING

9. SWIFT ERROR HANDLING
Swift 2 uses throws instead of NSError
Objective-C methods are bridged to Swift
accordingly:

10. CALLING THROWING
FUNCTIONS
You need to choose one of these 3 approaches:
handle the error with a do/catch block
Use forced-try: try!
propagate the error further up the call stack by
declaring calling function as throws

11. DO/CATCH IN SWIFT
do {
let content = try String(contentsOfFile: “test.txt", encoding:
NSUTF8StringEncoding)
} catch let error as NSError {
print(error)
}

12. DECLARING ERROR TYPES
How do you know which types of errors to catch?
Header documentation!
Unfortunately throws has no type information

13. DECLARING ERROR TYPES
/**
- Returns: the content of the file
- Throws: `NSError` if file could not be read
*/
func readFile() throws -> String {
//...
}

14. DEFINING SWIFT ERROR
TYPES
enum FileReadError: ErrorType {
case InvalidFilePath
case InvalidEncoding
case IncorrectFileFormat(actualFileFormat: String)
}

15. THROWING SWIFT ERROR
TYPES
/**
- Returns: The content of the file
- Throws: `FileReadError.InvalidFilePath` if file could not be read;
`FileReadError.InvalidEncoding` if file encoding does not match expected encoding;
`FileReadError.IncorrectFileFormat` if file format does not match specified one
*/
func readFile(path: String) throws -> NSData {
// ...
throw FileReadError.InvalidFilePath
}

16. HANDLING SWIFT ERROR
TYPES
do {
try readFile("test.txt")
} catch FileReadError.InvalidFilePath {
print("Invalid filepath")
} catch FileReadError.InvalidEncoding {
print("Invalid encoding")
} catch let FileReadError.IncorrectFileFormat(actualFileFormat) {
print("Unexpected file format: (actualFileFormat)")
} catch {
print("Unhandled Error!")
}

17. LIMITATIONS OF SWIFT
ERROR HANDLING
Errors don’t have type information
Error handling doesn’t work for asynchronous code
let request = NSURLRequest(URL: NSURL(string: "https://www.google.com")!)
let session = NSURLSession.sharedSession()
session.dataTaskWithRequest(request) { (data, response, error) -> Void in
// error handling happens in callback
}

18. RESULT TYPE
Result type can represent value or error depending on
result of operation. Popular Open Source
implementation: [2]
Can be used for synchronous and asynchronous code
func search(searchString: String) -> Result<Predictions, SearchError>
[2] https://github.com/antitypical/Result

19. CONSUMING RESULT TYPE
func handleSearchResult(result: Result<Predictions, Reason>) -> Void {
switch result {
case let .Success(predictions):
self.locations = predictions.predictions
case .Failure(_):
self.errorHandler.displayErrorMessage(
"The search returned an error, sorry!"
)
}
}

20. PRODUCING RESULT TYPE
func fetchAllTrips(callback: Result<[JSONTrip], Reason> -> Void) {
// in case of success
var trips: [JSONTrip] = [/*...*/]
callback(.Success(trips))
// in case of error
var reason: Reason = .NoData
callback(.Failure(reason))
}

21. ASSERTIONS AND
EXCEPTIONS

22. EXCEPTIONS
Objective-C provides exceptions, Swift does not
Objective-C exceptions should not be caught, they are
not intended for error handling [1]
Exceptions are used to crash the app to make you
aware of a programming error
[1] Apple Error Handling Documentation

23. ASSERTIONS
Are used to state and verify assumptions
Typically only used at debug time
Objective-C: NSAssert…
Swift: assert, assertionFailure,
fatalError,… [3]
[3] Swift asserts the missing manual

24. ASSERTIONS

25. SUMMARY

26. SUMMARY
Swift 2 uses ErrorType and throws for error
handling
Swift 2 error handling has limitations (no type info,
not suitable for async code) - Result type is a
good alternative
Exceptions and assertions are used for
unrecoverable errors

27. ADDITIONAL RESOURCES

28. ADDITIONAL RESOURCES
WWDC 106: What’s new in Swift
Javi Soto: Swift Sync and Async Error
Handling
Benjamin Encz: Swift Error Handling and
Objective-C Interop in Depth