Swiftly

Swift 5.1 references for busy coders

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 {
  var userIsAuthorized = true
  var mbFileSize = 1000

  if !userIsAuthorized {
    throw DownloadError.unauthorized
  } else if mbFree < mbFileSize {
    throw DownloadError.notEnoughSpace(mbNeeded: apiManager.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.