Swift Language Error Handling Error handling basics
Example
Functions in Swift may return values, throw errors, or both:
func reticulateSplines() // no return value and no error
func reticulateSplines() -> Int // always returns a value
func reticulateSplines() throws // no return value, but may throw an error
func reticulateSplines() throws -> Int // may either return a value or throw an errorAny value which conforms to the ErrorType protocol (including NSError objects) can be thrown as an error. Enumerations provide a convenient way to define custom errors:
2.02.2
enum NetworkError: ErrorType {
case Offline
case ServerError(String)
}
3.0
enum NetworkError: Error {
// Swift 3 dictates that enum cases should be `lowerCamelCase`
case offline
case serverError(String)
}
An error indicates a non-fatal failure during program execution, and is handled with the specialized control-flow constructs do/catch, throw, and try.
func fetchResource(resource: NSURL) throws -> String {
if let (statusCode, responseString) = /* ...from elsewhere...*/ {
if case 500..<600 = statusCode {
throw NetworkError.serverError(responseString)
} else {
return responseString
}
} else {
throw NetworkError.offline
}
}Errors can be caught with do/catch:
do {
let response = try fetchResource(resURL)
// If fetchResource() didn't throw an error, execution continues here:
print("Got response: \(response)")
...
} catch {
// If an error is thrown, we can handle it here.
print("Whoops, couldn't fetch resource: \(error)")
}Any function which can throw an error must be called using try, try?, or try!:
// error: call can throw but is not marked with 'try'
let response = fetchResource(resURL)
// "try" works within do/catch, or within another throwing function:
do {
let response = try fetchResource(resURL)
} catch {
// Handle the error
}
func foo() throws {
// If an error is thrown, continue passing it up to the caller.
let response = try fetchResource(resURL)
}
// "try?" wraps the function's return value in an Optional (nil if an error was thrown).
if let response = try? fetchResource(resURL) {
// no error was thrown
}
// "try!" crashes the program at runtime if an error occurs.
let response = try! fetchResource(resURL)
