SLIDE 1
ERROR HANDLING IN COCOA AGENDA Objective-Cs NSError Swifts - - PowerPoint PPT Presentation
ERROR HANDLING IN COCOA AGENDA Objective-Cs NSError Swifts - - PowerPoint PPT Presentation
ERROR HANDLING IN COCOA AGENDA Objective-Cs NSError Swifts try/catch/throws Swifts Result<S,F> type Exceptions & Assertions OBJECTIVE-C ERROR HANDLING NSERROR IN OBJECTIVE-C NSError *error = nil; NSString *string =
SLIDE 2
SLIDE 3
AGENDA
Objective-C’s NSError Swift’s try/catch/throws Swift’s Result<S,F> type Exceptions & Assertions
SLIDE 4
OBJECTIVE-C ERROR HANDLING
SLIDE 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); }
SLIDE 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
SLIDE 7
NSERROR IN OBJECTIVE-C… …IN THE SAD REAL WORLD
NSString *string = [NSString stringWithContentsOfFile:@"test.txt" encoding:NSUTF8StringEncoding error:nil];
SLIDE 8
SWIFT ERROR HANDLING
SLIDE 9
SWIFT ERROR HANDLING
Swift 2 uses throws instead of NSError Objective-C methods are bridged to Swift accordingly:
SLIDE 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
SLIDE 11
DO/CATCH IN SWIFT
do { let content = try String(contentsOfFile: “test.txt", encoding: NSUTF8StringEncoding) } catch let error as NSError { print(error) }
SLIDE 12
DECLARING ERROR TYPES
How do you know which types of errors to catch? Header documentation! Unfortunately throws has no type information
SLIDE 13
DECLARING ERROR TYPES
/**
- Returns: the content of the file
- Throws: `NSError` if file could not be read
*/ func readFile() throws -> String { //... }
SLIDE 14
DEFINING SWIFT ERROR TYPES
enum FileReadError: ErrorType { case InvalidFilePath case InvalidEncoding case IncorrectFileFormat(actualFileFormat: String) }
SLIDE 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 }
SLIDE 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!") }
SLIDE 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 }
SLIDE 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
SLIDE 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!" ) } }
SLIDE 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)) }
SLIDE 21
ASSERTIONS AND EXCEPTIONS
SLIDE 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
SLIDE 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
SLIDE 24
ASSERTIONS
SLIDE 25
SUMMARY
SLIDE 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
SLIDE 27
ADDITIONAL RESOURCES
SLIDE 28