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Combine: Asynchronous Programming with Swift

First Edition · iOS 13 · Swift 5.1 · Xcode 11

Before You Begin

Section 0: 3 chapters
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Section I: Introduction to Combine

Section 1: 2 chapters
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Section II: Operators

Section 2: 6 chapters
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Section III: Combine in Action

Section 3: 6 chapters
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Section IV: Advanced Combine

Section 4: 5 chapters
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Section V: Building a Complete App

Section 5: 2 chapters
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16. Error Handling
Written by Shai Mishali

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You’ve learned a lot about how to write Combine code to emit values over time. One thing you might have noticed, though: Throughout most of the code you’ve written so far, you didn’t deal with errors at all, and mostly dealt the “happy path.”

Unless you write error-free apps, this chapter is for you! :]

As you learned in Chapter 1, “Hello, Combine!,” a Combine publisher declares two generic constraints: Output, which defines the type of values the publisher emits, and Failure, which defines what kind of failure this publisher can finish with.

Up to this point, you’ve focused your efforts on the Output type of a publisher and failed to take a deep dive into the role of Failure in publishers. Well, don’t worry, this chapter will change that!

Getting started

Open the starter playground for this chapter in projects/Starter.playground. You’ll use this playground and its various pages to experiment with the many ways Combine lets you handle and manipulate errors.

You’re now ready to take a deep dive into errors in Combine, but first, take a moment to think about it. Errors are such a broad topic, where would you even start?

Well, how about starting with the absence of errors?

Never

A publisher whose Failure is of type Never indicates that the publisher can never fail.

example(of: "Never sink") {
  Just("Hello")
}


public typealias Failure = Never

example(of: "Never sink") {
  Just("Hello")
    .sink(receiveValue: { print($0) })
    .store(in: &subscriptions)
}

——— Example of: Never sink ———
Hello

setFailureType

The first way to turn an infallible publisher into a fallible one is to use setFailureType. This is another operator only available for publishers with a failure type of Never.

enum MyError: Error {
  case ohNo
}

example(of: "setFailureType") {
  Just("Hello")
}

.setFailureType(to: MyError.self)


// 1
.sink(
  receiveCompletion: { completion in
    switch completion {
    // 2
    case .failure(.ohNo):
      print("Finished with Oh No!")
    case .finished:
      print("Finished successfully!")
    }
  },
  receiveValue: { value in
    print("Got value: \(value)")
  }
)
.store(in: &subscriptions)

——— Example of: setFailureType ———
Got value: Hello
Finished successfully!

assign(to:on:)

The assign operator you learned about in Chapter 2, “Publishers & Subscribers,” only works on publishers that cannot fail, same as setFailureType. If you think about it, it makes total sense. Sending an error to a provided key path results in either an unhandled error or an undefined behavior.

example(of: "assign") {
  // 1
  class Person {
    let id = UUID()
    var name = "Unknown"
  }

  // 2
  let person = Person()
  print("1", person.name)

  Just("Shai")
    .handleEvents( // 3
      receiveCompletion: { _ in print("2", person.name) }
    )
    .assign(to: \.name, on: person) // 4
    .store(in: &subscriptions)
}

——— Example of: assign ———
1 Unknown
2 Shai

.setFailureType(to: Error.self)

referencing instance method 'assign(to:on:)' on 'Publisher' requires the types 'Error' and 'Never' be equivalent

assertNoFailure

The assertNoFailure operator is useful when you want to protect yourself during development and confirm a publisher can’t finish with a failure event. It doesn’t prevent a failure event from being emitted by the upstream. However, it will crash with a fatalError if it detects an error, which gives you a good incentive to fix it in development.

example(of: "assertNoFailure") {
  // 1
  Just("Hello")
    .setFailureType(to: MyError.self)
    .assertNoFailure() // 2
    .sink(receiveValue: { print("Got value: \($0) ")}) // 3
    .store(in: &subscriptions)
}

——— Example of: assertNoFailure ———
Got value: Hello 

.tryMap { _ in throw MyError.ohNo }

Playground execution failed:

error: Execution was interrupted, reason: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0).

...

frame #0: 0x00007fff232fbbf2 Combine`Combine.Publishers.AssertNoFailure...

Dealing with failure

Wow, so far you’ve learned a lot about how to deal with publishers that can’t fail at all… in an error-handling chapter! :] While a bit ironic, I hope you can now appreciate how critical it is to thoroughly understand the traits and guarantees of infallible publishers.

try* operators

In Section II, “Operators,” you learned about most of Combine’s operators and how you can use them to manipulate the values and events your publishers emit. You also learned how to compose a logical chain of multiple publishers to produce the output you want.

example(of: "tryMap") {
  // 1
  enum NameError: Error {
    case tooShort(String)
    case unknown
  }

  // 2
  let names = ["Scott", "Marin", "Shai", "Florent"].publisher
  
  names
    // 3
    .map { value in
      return value.count
    }
    .sink(receiveCompletion: { print("Completed with \($0)") },
          receiveValue: { print("Got value: \($0)") })
}

——— Example of: tryMap ———
Got value: 5
Got value: 5
Got value: 4
Got value: 7
Completed with finished

.map { value -> Int in
  // 1
  let length = value.count
  
  // 2
  guard length >= 5 else {
    throw NameError.tooShort(value)
  }
  
  // 3
  return value.count
}

Invalid conversion from throwing function of type '(_) throws -> _' to non-throwing function type '(String) -> _'

——— Example of: tryMap ———
Got value: 5
Got value: 5
Completed with failure(...NameError.tooShort("Shai"))

Mapping errors

The differences between map and tryMap go beyond the fact that the latter allows throwing errors. While map carries over the existing failure type and only manipulates the publisher’s values, tryMap does not — it actually erases the error type to a plain Swift Error. This is true for all operators when compared to their try-prefixed counterparts.

example(of: "map vs tryMap") {
  // 1
  enum NameError: Error {
    case tooShort(String)
    case unknown
  }

  // 2
  Just("Hello")
    .setFailureType(to: NameError.self) // 3
    .map { $0 + " World!" } // 4
    .sink(
      receiveCompletion: { completion in
        // 5
        switch completion {
        case .finished:
          print("Done!")
        case .failure(.tooShort(let name)):
          print("\(name) is too short!")
        case .failure(.unknown):
          print("An unknown name error occurred")
        }
      },
      receiveValue: { print("Got value \($0)") }
    )
    .store(in: &subscriptions)
}

——— Example of: map vs tryMap ———
Got value Hello World!
Done!


.mapError { $0 as? NameError ?? .unknown }

——— Example of: map vs tryMap ———
Got value Hello World!
Done!

.tryMap { throw NameError.tooShort($0) }

——— Example of: map vs tryMap ———
Hello is too short!

Designing your fallible APIs

When constructing your own Combine-based code and APIs, you’ll often use APIs from other sources that return publishers that fail with various types. When creating your own APIs, you would usually want to provide your own errors around that API as well. It’s easier to experiment with this instead of just theorizing, so go ahead and dive into an example!

example(of: "Joke API") {
  class DadJokes {
    // 1
    struct Joke: Codable {
      let id: String
      let joke: String
    }

    // 2
    func getJoke(id: String) -> AnyPublisher<Joke, Error> {
      let url = URL(string: "https://icanhazdadjoke.com/j/\(id)")!
      var request = URLRequest(url: url)
      request.allHTTPHeaderFields = ["Accept": "application/json"]
      
      // 3
      return URLSession.shared
        .dataTaskPublisher(for: request)
        .map(\.data)
        .decode(type: Joke.self, decoder: JSONDecoder())
        .eraseToAnyPublisher()
    }
  }
}

// 4
let api = DadJokes()
let jokeID = "9prWnjyImyd"
let badJokeID = "123456"

// 5
api
  .getJoke(id: jokeID)
  .sink(receiveCompletion: { print($0) },
        receiveValue: { print("Got joke: \($0)") })
  .store(in: &subscriptions)

——— Example of: Joke API ———
Got joke: Joke(id: "9prWnjyImyd", joke: "Why do bears have hairy coats? Fur protection.")
finished

enum Error: Swift.Error, CustomStringConvertible {
  // 1
  case network
  case jokeDoesntExist(id: String)
  case parsing
  case unknown
  
  // 2
  var description: String {
    switch self {
    case .network:
      return "Request to API Server failed"
    case .parsing:
      return "Failed parsing response from server"
    case .jokeDoesntExist(let id):
      return "Joke with ID \(id) doesn't exist"
    case .unknown:
      return "An unknown error occurred"
    }
  }
}

.mapError { error -> DadJokes.Error in
  switch error {
  case is URLError:
    return .network
  case is DecodingError:
    return .parsing
  default:
    return .unknown
  }
}

.getJoke(id: jokeID)

.getJoke(id: badJokeID)

failure(Failed parsing response from server)

{
    message = "Joke with id \"123456\" not found";
    status = 404;
}

.tryMap { data, _ -> Data in
  // 6
  guard let obj = try? JSONSerialization.jsonObject(with: data),
        let dict = obj as? [String: Any],
        dict["status"] as? Int == 404 else {
    return data
  }
  
  // 7
  throw DadJokes.Error.jokeDoesntExist(id: id)
}

——— Example of: Joke API ———
failure(An unknown error occurred)

return .unknown

return error as? DadJokes.Error ?? .unknown

——— Example of: Joke API ———
failure(Joke with ID 123456 doesn't exist)

guard id.rangeOfCharacter(from: .letters) != nil else {
  return Fail<Joke, Error>(error: .jokeDoesntExist(id: id))
          .eraseToAnyPublisher()
}

Catching and retrying

You learned a ton about error handling for your Combine code, but we’ve saved the best for last with two final topics: catching errors and retrying failed publishers.

let photoService = PhotoService()

example(of: "Catching and retrying") {
  photoService
    .fetchPhoto(quality: .low)
    .sink(
      receiveCompletion: { print("\($0)") },
      receiveValue: { image in
        image
        print("Got image: \(image)")
      }
    )
    .store(in: &subscriptions)
}

——— Example of: Catching and retrying ———
Got image: <UIImage:0x600000790750 named(lq.jpg) {300, 300}>
finished


——— Example of: Catching and retrying ———
failure(Failed fetching image with high quality)

.retry(3)

.handleEvents(
  receiveSubscription: { _ in print("Trying ...") },
  receiveCompletion: {
    guard case .failure(let error) = $0 else { return }
    print("Got error: \(error)")
  }
)

——— Example of: Catching and retrying ———
Trying ...
Got error: Failed fetching image with high quality
Trying ...
Got error: Failed fetching image with high quality
Trying ...
Got error: Failed fetching image with high quality
Trying ...
Got error: Failed fetching image with high quality
failure(Failed fetching image with high quality)

.fetchPhoto(quality: .high)

.fetchPhoto(quality: .high, failingTimes: 2)

——— Example of: Catching and retrying ———
Trying ...
Got error: Failed fetching image with high quality
Trying ...
Got error: Failed fetching image with high quality
Trying ...
Got image: <UIImage:0x600001268360 named(hq.jpg) {1835, 2446}>
finished


.replaceError(with: UIImage(named: "na.jpg")!)


——— Example of: Catching and retrying ———
Trying ...
Got error: Failed fetching image with high quality
Trying ...
Got error: Failed fetching image with high quality
Trying ...
Got error: Failed fetching image with high quality
Trying ...
Got error: Failed fetching image with high quality
Got image: <UIImage:0x6000020e9200 named(na.jpg) {200, 200}>
finished

.catch { error -> PhotoService.Publisher in
  print("Failed fetching high quality, falling back to low quality")
  return photoService.fetchPhoto(quality: .low)
}

——— Example of: Catching and retrying ———
Trying ...
Got error: Failed fetching image with high quality
Trying ...
Got error: Failed fetching image with high quality
Trying ...
Got error: Failed fetching image with high quality
Trying ...
Got error: Failed fetching image with high quality
Failed fetching high quality, falling back to low quality
Got image: <UIImage:0x60000205c480 named(lq.jpg) {300, 300}>
finished

Key points

  • Publishers with a Failure type of Never are guaranteed to not emit a failure completion event.
  • Many operators only work with infallible publishers. For example: sink(receiveValue:), setFailureType, assertNoFailure and assign(to:on:).
  • The try-prefixed operators let you throw errors from within them, while non-try operators do not.
  • Since Swift doesn’t support typed throws, calling try-prefixed operators erases the publisher’s Failure to a plain Swift Error.
  • Use mapError to map a publisher’s Failure type, and unify all failure types in your publisher to a single type.
  • When creating your own API based on other publishers with their own Failure types, wrap all possible errors into your own Error type to unify them and hide your API’s implementation details.
  • You can use the retry operator to resubscribe to a failed publisher for an additional number of times.
  • replaceError(with:) is useful when you want to provide a default fallback value for your publisher, in case of failure.
  • Finally, you may use catch to replace a failed publisher with a different fallback publisher.

Where to go from here?

Congratulations on getting to the end of this chapter. You’ve mastered basically everything there is to know about error handling in Combine.

15. In Practice: Combine & SwiftUI
17. Schedulers
Have a technical question? Want to report a bug? You can ask questions and report bugs to the book authors in our official book forum here.
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