Discussion on how Swift handles errors for the Learn Swift Boston Meetup Group on the 16th November 2016

1. Swift Errors
@seanoshea | 16th November 2016

2. XMLDocument
- (instancetype)initWithData:(NSData *)data options:(NSUInteger)mask error:(NSError *
_Nullable *)error;
NSFileManager
- (BOOL)removeItemAtURL:(NSURL *)URL error:(NSError **)error;
NSURLConnectionDelegate
- (void)connection:(NSURLConnection *)connection didFailWithError:(NSError *)error;
Objective-C Error Handling

3. Errors: Objective-C vs Swift
NSFileManager (Objective-C)
// just want to delete an image ...
NSError *error = nil;
// passing a NSError by reference and returns a boolean ???
if (![[NSFileManager defaultManager] removeItemAtPath:filePath error:&error]) {
}
FileManager (Swift)
func removeItem(atPath:filePath) throws Error

4. enum ScoreError: Error {
case negativeScore(message:String)
case unreachableScore(message:String)
}
func updateTeamScore(score:Int) throws {
guard score >= 0 else {
throw ScoreError.negativeScore(message: “Score cannot be negative”)
}
guard isReachableScore(score:score) else {
throw ScoreError.unreachableScore(message: “Score is unreachable”)
}
}
Throwing Errors

5. Catching Errors
// let’s try to update the score for a team
do {
try updateTeamScore(score: -1)
} catch ScoreError.negativeScore {
print("How can a score go negative?")
} catch ScoreError.unreachableScore {
print("How could a team achieve this score?")
} catch _ {
}

6. // similar to optionally unwrapping
guard let jsonString = try? decode(rawString) else { return nil }
let data = JSON(jsonString)
// similar to forced unwrapping
let photo = try! loadImage(atPath:"./Resources/score.png”)
try? and try!

7. func updateTeamScore(score:Integer) throws
defer {
animateScore()
}
guard score >= 0 else {
throw ScoreError.negativeScore(message: “Score cannot be a negative number”)
}
guard isReachableScore(score) else {
throw ScoreError.unreachableScore(message: “Score is unreachable”)
}
this.team.currentScore = score
// animateScore is always called at
// the end of the function's scope
}
defer

8. func testErrorHandling() {
do {
try updateTeamScore(score: -1)
XCTFail("Should never get here")
} catch let e as ScoreError {
XCTAssertEqual(e, ScoreError.negativeScore(message: "Score cannot be a negative
number"))
} catch {
XCTFail("Wrong error")
}
}
Unit Testing

9. Questions/Links
https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programmi
ng_Language/ErrorHandling.html
https://github.com/apple/swift/blob/master/docs/ErrorHandlingRationale.rst
https://gist.github.com/nicklockwood/21495c2015fd2dda56cf