Errors
Errors are represented by values that conform to the Error protocol. They can be thrown, caught, propagated, and recovered.
Complete example
Error declaration
enum DownloadError: Error {
case unauthorized
case notEnoughSpace(mbNeeded: Int)
}
Throwing errors
func downloadFile(mbFree: Int) throws {
let userIsAuthorized = true
let mbFileSize = 1000
if !userIsAuthorized {
throw DownloadError.unauthorized
} else if mbFree < mbFileSize {
throw DownloadError.notEnoughSpace(mbNeeded: mbFileSize)
} else {
// File download logic goes here
}
}
Catching errors
func didPressDownloadFileButton() {
do {
try downloadFile(mbFree: 500)
} catch DownloadError.unauthorized {
print("The user is not authorized to download this file.")
} catch DownloadError.notEnoughSpace(let mbNeeded) {
print("The user needs \(mbNeeded) MB to download this file.")
} catch {
print("Unknown error: \(error).")
}
}
Usage
didPressDownloadFileButton() // The user needs 1000 MB to download this file.
Typed throws
Since Swift 6.0, a function can declare exactly which error type it throws with throws(...). Inside the function, throw can use shorthand member syntax, and at the call site the compiler knows the concrete error type, so a switch over it is exhaustive without a fallback clause:
func downloadFile(mbFree: Int) throws(DownloadError) {
guard mbFree >= 1000 else {
throw .notEnoughSpace(mbNeeded: 1000)
}
// File download logic goes here
}
do {
try downloadFile(mbFree: 500)
} catch {
// error is a DownloadError here, not `any Error`
switch error {
case .unauthorized:
print("The user is not authorized to download this file.")
case .notEnoughSpace(let mbNeeded):
print("The user needs \(mbNeeded) MB to download this file.")
}
}
// Output: The user needs 1000 MB to download this file.
Untyped throws remains the right default for most code; typed throws is most useful in generic code and constrained environments like Embedded Swift.
