Apps Dissected Posts

Combine provides a wide variety of Publishers that can be combined to fulfill a wide variety of use cases. But as with any framework, you may find at some point that you need something custom that Combine does not provide out of the box.

You may have your own custom operation or you may want to wrap another Combine chain in a convenient wrapper. But how? If you look at Apple’s documentation for even how to add a method to the Publisher as an extension, the signature looks something like this:

😖

The steps are a little complicated, because Combine heavily uses a couple of features of Swift to perform its magic, but once you’ve followed along with an example, those difficult steps become much easier to apply to your own unique case.

Over the next several articles, we’ll trace through the development of a caching operator, which will take an upstream input and a Combine chain that performs the operation we’d like to cache, running the operation if the upstream value received hasn’t been seen before, or sending the cached result if it has. In part 1, we will build out the Combine publisher chain that implements the cache operation. In part 2, we’ll learn how we can make it so that somebody can use upstream.cache(...) to add our operator to their own Combine chains. Part 3 will cover taking our cache operator and turning it into a full-blown Publisher type. Part 4 will move away from Combine and talk about how to package that code into a Swift package that can be easily downloaded and included in your own, and everybody else’s app.

Lots to cover, so let’s get going!

So you’re starting to work with Combine, because Apple is finally jumping on the reactive programming bandwagon (yay!). So you build your first Combine workflow, and it looks something like this:

func retrieveData() {
    URLSession.shared.dataTaskPublisher(for: url)
      .map({$0.data})
      .eraseToAnyPublisher()
      .sink(receiveCompletion: { (status) in
        switch status {
        case .failure(let error):
          print(error.localizedDescription)
        case .finished:
          break
        }
      })
      { (data) in
        let str = String(data: data, encoding: .utf8)
        print(str!)
      }
}

And then you try and use it, and instead of getting the data response you were expecting, you get an error:

2019-10-25 14:59:34.452071-0400 TestApp[2127:98883] Task <663D6D3A-48B8-49E6-9103-AA1D89513D84>.<1> finished with error [-999] Error Domain=NSURLErrorDomain Code=-999 "cancelled" …

Cancelled? Why did my request get cancelled?

The key in the above code is that you missed something important. I made this exact same mistake when I started using Combine, and sadly I don’t think it will be the last time, because it is just that easy to miss. Let’s talk about the solution, and why you get the error when you don’t do it.