released Fri, 19 Jun 2015
Swift Version 5.0

Using try / catch in Swift with asynchronous closures

Swift's try / catch error handling is great. However, you can't use it in an async context. This article briefly explains which options you have if you intend to use Error Handling asynchronously

With Swift 2.0, Apple introduced a new error handling model with the try, throw, and catch keywords. Given the sub par state of error handling in Swift 1.0, this was a welcome addition. It works basically as follows (simplified example):

// We define a struct conforming to the new ErrorType
enum OurError : ErrorType {
    case divisionError
}
func divide(a: Int, b: Int) throws -> Int {
    if b == 0 {
        throw OurError.divisionError
    }
    return a / b
}
do {
    let result = try divide(a: 50, b: 10)
} catch let error {
    print("Could not divide")
}

This is a nice and flexible way of structuring possible errors types and handling them. In synchronous code.

But what about asynchronous operations?

An asynchronous operation

Imagine the following operation as an example of any kind of asynchronous operation where there is a delay between the initiation of a computation and the result of the computation. In Objective-C land, this is solved with delegates, blocks or (though rarely) NSInvocation, in Swift the typical solution is a closure.

func asynchronousWork(completion: (r: [String: Any]?) -> Void) -> Void {
  // do work
  ...
  completion(someValue)
}

How do you handle errors in these situations? We can expect, that within the asynchronousWork function most error handling will be via try/catch. So one possible solution that comes to mind is handling the errors in there, and only forwarding the result, if there is one:

func asynchronousWork(completion: (result: [String: Any]?) -> Void) -> Void {
    URLConnection.sendAsynchronousRequest(request, queue: queue) { 
      (response, data, error) -> Void in
        guard let data = data else { return }
        do {
            let result = try JSONSerialization.JSONObjectWithData(data, options: []) as? [String: Any]
            completion(r: result)
        } catch _ {
            completion(r: nil)
        }
    }
}

// Call
asynchronousWork { (result) -> Void in
    guard let result = result else { return}
    print(result)
}

Result Types

However, this is rather fragile, as it will not gives us any information about the error in question. In Swift 1.0, when such a structure presented itself, people usually implemented a Result type which can be either the result of the computation, or the error:

enum Result {
    case success(r: [String: Any])
    case failure(e: ErrorType)
}

Swift 5 Result

Swift 5 now contains a Result type in the standard library. It is similar to the one we used above. This means the names of the two cases are also success and failure. The name of the type is Result.

Given this enum, we can then change the code as follows:

func asynchronousWork(completion: (result: Result) -> Void) -> Void {
    URLConnection.sendAsynchronousRequest(request, queue: queue) { 
       (response, data, error) -> Void in
        guard let data = data else { return }
        do {
            let result = try JSONSerialization.JSONObjectWithData(data, options: []) 
                as? [String: Any]
            completion(r: Result.success(r: result))
        } catch let error {
            completion(r: Result.failure(e: error))
        }
    }
}

// Call
asynchronousWork { (result: Result) -> Void in
    switch r {
    case .success(let successValue): print(successValue)
    case .failure(let error): print("Error", error)
    }
}

Much nicer. When we look closely, though, there is a bit of overhead. We have to define a ResultType enum and encapsulate our data in it. What's more, if Result enums were the way Apple intends things to be, surely we would have one in the standard library.

Instead, we do have try / catch, a construct which is incompatible with the task at hand because you can't catch something that happens at some point in the future. I.e:


func asynchronousWork(completion: (result: Result) -> Void) throws -> Void {
}

This can't throw, because the function returns before the computation has even begun.

A solution with an inner closure

However, a nice way to solve this is by encapsulating the error into a throwable closure. See for yourself:


func asynchronousWork(completion: (inner: () throws -> [String: Any]) -> Void) -> Void {
    URLConnection.sendAsynchronousRequest(request, queue: queue) { 
        (response, data, error) -> Void in
        guard let data = data else { return }
        do {
            guard let result = try JSONSerialization.JSONObjectWithData(data, options: []) 
                as? [String: Any] else {
                completion(inner: { throw OurError.InvalidJSONType })
            }
            completion(inner: { return result })
        } catch let error {
            completion(inner: { throw error })
        }
    }
}

// Call
asynchronousWork { (inner: () throws -> [String: Any]) -> Void in
    do {
        let result = try inner()
    } catch let error {
        print(error)
    }
}

We successfully performed the asynchronous work while maintaining the possibility to catch the error in our calling function's completion block and being able to match different error types - all without having to resort to enums or switches. How did we do this?

As you can see, the trick is that the asynchronousWork function takes an additional closure called 'inner' of the type () throws -> [String: Any]. This closure will either provide the result of the computation, or it will throw. The closure itself is being constructed during the computation by one of two means:

  • In case of an error: inner: {throw error}
  • In case of success: inner: {return result}

This is a simple and flexible way of throwing errors up a closure chain without any additional means. Much like rethrow but for asynchronous operations.

We also had to add an additional error for the case where our JSON was not of the [String: Any] type.

Addendum

The only downside of this setup is that it can't be used with any asynchronous operation which takes a completion block. You will need to have access to the operation in order to rewrite it to not call the completion block with the result but instead call a completion block with an inner closure. So you can't use it with specific functionality from external libraries, but it is a good approach for your own code, frameworks, and libraries, if you like the idea of using the try/catch approach natively without having to resort to abstraction containers.