Sometimes you’ll want to create your own errors to be thrown once something wrong happens within your application. For this, Swift provides the Error protocol that you can use:

struct InvalidInputError: Error {
    message: String
}

The Error protocol doesn’t have any requirement on its own so you can define whatever information you need inside it.

I’ve used a struct above to create a custom error but Apple’s documentation states that for simple errors enum constructs are well suited as well:

enum ConversionError: Error {
  case unknownCharacter
    case emptySource
}

Here’s an example of how you can use this custom error:

func convertValue(value: String) {
    if value.isEmpty {
        throw ConversionError.emptySource
    }
    // ...
}