A property wrapper that allows you to enforce that a closure is called exactly once. This is especially useful after the introduction of SE-0293 which makes it legal to place property wrappers on function and closure parameters.
It’s very common to write code where you have a function with a completion handler which must be called with a success or failure value based on the underlying result of the work that the function does. But if you’re not careful, it can be easy to make mistakes where you don’t call the completion handler at all or call it more than once, especially if there’s complicated business logic or error handling.
func fetchSpecialUser(@Once completion: @escaping (Result<User, Error>) -> Void) {
fetchUserImpl { user in
guard let user = user else {
// oops, forgot to call completion(...) here!
return
}
guard user.isPendingEmailVerification == false else {
completion(.failure(.userNotVerified))
return
}
if user.hasSubscription {
switch getSubscriptionType(user) {
case .ultimate, .premium:
completion(.success(user))
case .standard:
completion(.failure(.noPaidSubscription))
}
}
// ... more business logic here
// oops, forgot a 'return' in the if-statement above,
// so execution continues and closure is called twice
// (and with an invalid result!).
completion(.failure(.generic))
}Simply annotate your function parameters with @Once and you will get a runtime error if the closure is not called at all or called more than once.
func fetchSpecialUser(@Once completion: @escaping (Result<User, Error>) -> Void) {
fetchUserImpl { user in
guard let user = user else {
// runtime error: expected closure to have already been executed once!
return
}
guard user.isPendingEmailVerification == false else {
completion(.failure(.userNotVerified))
return
}
if user.hasSubscription {
switch getSubscriptionType(user) {
case .ultimate, .premium:
completion(.success(user))
case .standard:
completion(.failure(.noPaidSubscription))
}
}
// ... more business logic here
// runtime error: closure has already been invoked!
completion(.failure(.generic))
}- Swift 5.2 or above
- The property wrapper can only be used with escaping closures. If you want to use it on a non-escaping closure, you will need to annotate it with
@escaping. This is not ideal, but because the closure is stored inside the property wrapper, there is no way to say to the compiler that the closure does not escape when you know for a fact it doesn't (perhaps because your code is synchronous).
Add the following to your project's Package.swift file:
.package(url: "https://github.com/theblixguy/Once", from: "0.0.1")or add this package via the Xcode UI by going to File > Swift Packages > Add Package Dependency.
MIT License
Copyright (c) 2021 Suyash Srijan
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