BENEDIKT TERHECHTE

This is my research notebook. I'm an OSX / iOS indie developer. After 8 years of Objective-C I really enjoy Swift nowadays. Trying to publish all my research on Development, Swift & other technologies here.

Fri, 19 Jun 2015 #

Using try / catch in Swift with asynchronous closures

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 Error : ErrorType {
    case DivisionError
}
func divide(a: Int, b: Int) throws -> Int {
    if b == 0 {
	throw Error.DivisionError
    }
    return a / b
}
do {
    let result = try divide(50, 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?

1 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: NSDictionary?) -> Void) -> Void {
  // do work
  ...
  completion(aDictionary)
}

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: (r: NSDictionary?) -> Void) -> Void {
    NSURLConnection.sendAsynchronousRequest(request, queue: queue) { 
      (response, data, error) -> Void in
	guard let data = data else { return }
	do {
	    let result = try NSJSONSerialization.JSONObjectWithData(data, options: []) 
		as! NSDictionary
	    completion(r: result)
	} catch _ {
	    completion(r: nil)
	}
    }
}

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

2 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 1:

enum ResultType {
    case Success(r: NSDictionary)
    case Error(e: ErrorType)
}

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

func asynchronousWork(completion: (r: ResultType) -> Void) -> Void {
    NSURLConnection.sendAsynchronousRequest(request, queue: queue) { 
       (response, data, error) -> Void in
	guard let data = data else { return }
	do {
	    let result = try NSJSONSerialization.JSONObjectWithData(data, options: []) 
		as! NSDictionary
	    completion(r: ResultType.Success(r: result))
	} catch let error {
	    completion(r: ResultType.Error(e: error))
	}
    }
}

// Call
asynchronousWork { (r: ResultType) -> Void in
    switch r {
    case .Success(let e): print(e)
    case .Error(let e): print("Error", e)
    }
}

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: (r: ResultType) -> Void) throws -> Void {
}

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

3 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 -> NSDictionary) -> Void) -> Void {
    NSURLConnection.sendAsynchronousRequest(request, queue: queue) { 
	(response, data, error) -> Void in
	guard let data = data else { return }
	do {
	    let result = try NSJSONSerialization.JSONObjectWithData(data, options: []) 
		as! NSDictionary
	    completion(inner: {return result})
	} catch let error {
	    completion(inner: {throw error})
	}
    }
}

// Call
asynchronousWork { (inner: () throws -> NSDictionary) -> 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 -> NSDictionary. 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.

4 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.

Footnotes:

1

And with Swift 2.0, the dreaded multi-value enum error disappeared, and we can finally implement such a type without having to box it.

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