In our previous post, I explained how to hook up our new CLManagerDelegate publishers for CLAuthorizationStatus to the ObservableObject using Combine. In this post, we be transforming the CLLocation publisher data into something useful using flatMap and the built-in publisher, Sequence.

Creating Publishers from Delegates
Using Function Reactive Programming to Transform Values
Understanding FlatMap and Built-In Publishers

Using Combine for Complex Transformations

While we can simply assign the value from the authorizationPublisher to our authorizationStatus property, locationPublisher requires more work.

class CoreLocationObject: ObservableObject {
  @Published var location: CLLocation?
  ...
}

protocol CLLocationManagerCombineDelegate: CLLocationManagerDelegate {
  func locationPublisher() -> AnyPublisher<[CLLocation], Never>
  ...
}

While our locationPublisher returns a [CLLocation] result, our location property is CLLocation?.

That is to say we are going to need to do some functional programming to transform the result of our publisher:

Turn the Array itself into a Sequential Publisher of each value
Use flatMap to return a new Publisher
Cast the CLLocation into the matching CLLocation? (i.e. Optional<CLLocation>)

Here is our result:

      locationPublisher
        // convert the array of CLLocation into a Publisher itself
        .flatMap(Publishers.Sequence.init(sequence:))
        // in order to match the property map to Optional
        .map { $0 as CLLocation? }
        // since this is used in the UI,
        //  it needs to be on the main DispatchQueue
        .receive(on: DispatchQueue.main)
        // store the value in the location property
        .assign(to: &$location)

Let’s break each of these steps down starting with Publishers.Sequence.

Built-In Publishers from Combine

Combine comes with several built-in Publishers we can use.

Just - single value
Future - handling asynchronous callbacks
Sequence - publishing individual values from a sequence in order

We already talked about Just which takes in a single value. Another popular built-in publisher to use with asynchronous calls is Future which takes in a result type for when a call is completed. In this case, we are using Sequence which transforms a sequence type object into a Publisher stream of sequential values. Therefore when CLLocationManagerDelegate receives multiple locations, the publisher acts as if each location from that array received in order.

However, as Just is a Publisher so is Sequence, therefore we can not use .map but .flatMap since we want the values from the Publisher not the publisher itself. flatMap is especially powerful in Combine since it allows us to integrate Publishers from multiple technologies.

Integrations in Combine

I’ve talked previously about what flatMap does in the functional sense whether that be Arrays or Optionals. The same goes for Publishers as well.

Here’s an example of a piece of code to download json from the Bacon Ipsum JSON API:

  let url = URL(string: "https://baconipsum.com/api/?type=all-meat&sentences=1&format=json")!

  let urlPublisher = URLSession.shared.dataTaskPublisher(for: url)
    .map(\.data)
    .decode(type: [String].self, decoder: JSONDecoder())
    .replaceError(with: [String]())
    .compactMap{ $0.first }

This uses the dataTaskPublisher available on URLSession and transforms it to a single String. Here is step-by-step how:

Create DataTaskPublisher from URL
Use the map function to get the data property from the tuple
Decode the JSON into an Array of Strings
In order to assign the value, we change the Failure type to Never. In this case, we are replacing each Error instance with an empty Array of Strings.
Lastly, we use compactMap to get the first string and therefore ignore all empty Arrays.

Depending on your app and the data you are looking for, you’ll want change the transformations and error handle but typically this is the basics for handling a data task in Combine.

Combine System Publishers with `flatMap`

Along with URLSession.dataTaskPublisher there are other System publishers available to use:

URLSession.dataTaskPublisher
NotificationCenter.publisher
Key Value Observation via NSObject.publisher
Timer.publisher

With this in mind, let’s use the Timer publisher and use flatMap to update our String from the Bacon Ipsum JSON API at a regular interval. Firstly, let’s create a Timer publisher:

let timerInterval:TimeInterval = 1.0
let timerPublisher = Timer.publish(every: timerInterval, on: .current, in: .default)
      .autoconnect()

In this instance, we are creating a Timer publisher on the current RunLoop using the default mode for every 1 second. Lastly is autoconnect which ensure the publisher connects when subscribed.

Now we can use flatMap to initiate a new dataTaskPublisher each second:

timerPublisher.flatMap {_ in urlPublisher }
      .receive(on: DispatchQueue.main)
      .assign(to: &$publishedStringProperty)

With this we can automate the assignment of the String value, as each dataTask is completed. In other words, our string property will update every second as the url is called.

Let’s again take a look at our code for updating CLLocation:

      locationPublisher
        // convert the array of CLLocation into a Publisher itself
        .flatMap(Publishers.Sequence.init(sequence:))
        // in order to match the property map to Optional
        .map { $0 as CLLocation? }
        // since this is used in the UI,
        //  it needs to be on the main DispatchQueue
        .receive(on: DispatchQueue.main)
        // store the value in the location property
        .assign(to: &$location)

By using Sequence to create a Publisher and flatMap to flatten and return the values from the Sequence Publisher, we have transformed the value from [CLLocation] to CLLocation without losing any data.

However the location property type and the Publisher result type must match. Therefore since our property takes into not having CLLocation as a possibility, we need to transform the CLLocation to an Optional<CLLocation> (i.e. CLLocation?).

Combine with other Delegates

While I've demonstrated how to use CoreLocation with Combine, this can be useful for other uses of Delegation APIs.

Recently, I've used these Publicists in my app, Heartwitch, which is an Apple Watch app for broadcasting your heart rate to a live stream, as you are playing games or doing presentations. Likewise with HealthKit workouts, I've used the same pattern of creating publishers.

...   
    public let coalesce: CoalescePublicist
    public let failurePublisist: FailurePublicist
    public let fullIdentifier: FullIdentifierPublicist
    public let metadata: MetadataPublicist
    public let optional: OptionalPublicist
    public let transition: TransitionPublicist
    public let workoutCreation: WorkoutCreationPublicist
    public let workoutDataUpdate: WorkoutDataUpdatePublicist
    public let workoutStart: WorkoutStartPublicist
...

This same pattern can be used with other Delegation APIs such as:

CoreBluetooth
UINotificationCenter
AVFoundation etc...

Other Resources

If you are interested in learning more about Combine, I highly recommend checking out:

Donny also appeared on my podcast EmpowerApps.Show:

Practical Combine with Donny Wals

EmpowerApps.Show

For code samples, check out my repositories here:

If you have any questions or comments on this series, reach out to me on Twitter.

Leo G Dion still @Home

@leogdion

The last on my series of posts on Combine and CoreLocation is out. In this one I explain:
🎞Built-in publishers like Just and Sequence
🗺Power of flatMap in Combine
🚜Using Publishers with other Delegation Pattern APIs
buff.ly/32Jlxn9

13:52 PM - 01 Sep 2020

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