Thankfully, migrating from using a built-in publisher to a custom one is usually quite easy, since any custom publishers that we’ll end up building can make use of the exact same features and operators as the built-in ones can — which is, in general, a huge advantage of writing our own asynchronous abstractions on top of Combine. But sometimes it is very handy to receive some system-wide notifications in the view layer. Instead, we’ll define two partial models that we’ll use within our TaskGroupsLoader, and since both of those models should be considered private implementation details of our loader, we’ll place them within the same file using a private extension — like this: To learn more about the above way of using extensions, check out last week’s “The power of extensions in Swift”. Apple’s Combine framework provides a general-purpose abstraction for asynchronous programming, by enabling various values and events to be emitted, transformed and observed over time. Getting started with the Combine framework in Swift A blog post to get you up and running with all Combine basics To read … // (at which point we'll send a completion event): // Simply creating a Feed instance doesn't make it start loading It shows a simple login validation using combine publisher. Since we’re now able to observe both query and filter as publishers, we can actually construct all of our view model’s internal logic using a single Combine pipeline. To also enable errors to be correctly propagated to the user, we’ll make that output property contain a Result value — giving us the following class declaration: Finally, let’s implement the loadResults method that we’re calling above whenever our query or filter was changed. Getting started with the Combine framework in Swift. Combine Swift Playground A set of pages explaining Combine in detail 3. Since that’ll always be an empty query string in this case, we’re simply ignoring it. One way to do that would be by constructing a dictionary of indexes before we start our nested loading operations, and to then base our final sorting on those indexes — like this: With either of the above sorting strategies in place, we’re now able to load our data from multiple sources, and to then turn that data into a single, predictable array of output values — which is really nice, but we’re still just scratching the surface of what Combine is actually capable of. In this tutorial, you've learned the basics behind Combine's Publisher and Subscriber. // a new object started observing it. Combine Swift is one of the newest frameworks introduced by Apple. // and will start emitting values. For example, the following Feed publisher continuously emits new values as long as its provider closure doesn’t return nil: To decide when a Subscription instance created by the above publisher should emit its values, we’ll then use the request method that we previously ignored, along with its demand parameter — which contains an Int-based value that indicates how many output values that the current subscriber is interested in receiving — giving us an implementation that looks like this: The benefit of the above approach is that we won’t start loading and emitting values until there’s some form of demand for those values — which Combine will automatically manage based on the subscribers that’ll end up connecting to our publisher: So, in general, whenever we’re building a custom publisher that can decide when to emit values on its own, it’s typically a good idea to use the request method and its demand parameter to decide what amount of values to send to a given subscriber, and to not start emitting values until there’s demand for them. Another possible definition for SwiftUI is that it’s a framework for building UIs in a declarative programming style using Swift in the form of Domain-specific language(DSL). In the next series of examples, we’re going to use the following SearchResultsLoader, which enables us to load an array of SearchResult values using a String-based query, along with an optional SearchFilter: To connect the above SearchResultsLoader to our UI, we’ll then use a view model that’ll let us observe a Published-marked output property from either a SwiftUI view or a view controller. These values can represent many kinds of asynchronous events. Mention Swift by Sundell when requesting a free demo to get a 20% discount on your license for a whole year. The beauty of the above type of abstraction is that it completely hides all of the complexity involved in dealing with multiple inputs, network calls and JSON decoding from our UI layer — which, especially when implemented using SwiftUI, can be kept really simple: Our SearchViewModel is also fully UIKit-compatible, since we can manually assign new values to both query and filter, and we can use Combine’s sink operator to observe our view model’s output property in order to bind our search results to something like a UITableView or a UICollectionView. Introduced during WWDC 2019, Apple’s Combine framework lets us model all sorts of asynchronous events and operations as “values over time”. A reasonable approach in this case might be to sort our groups based on when they were last updated, with the most recently updated group first: But what if we don’t have a specific piece of data that we can use for sorting — how could we still ensure a stable output order based on our initial array of Entry values? Swift on the server is an amazing new opportunity to build fast, safe and scalable backend apps. Instead, we’re using the eraseToAnyPublisher method to convert our subject into a read-only publisher, which outside objects can then observe: Like its name implies, a PassthroughSubject simply passes any values that were sent to it along to its observers, without storing those values. To learn more about that approach, and a couple of more powerful alternatives to it, check out the “Managing URLs and endpoints” Swift clip. Open Combine Open source implementations of Combine 2. You also learned about using operators and timers. We’ll also remove our didSet property observers, and we’re now calling a new configureDataPipeline method from within our initializer: Now, here comes the really cool part. // we'll need to create a subscription instance and Articles, podcasts and news about Swift development, by John Sundell. For example, here’s how we could create convenience APIs for both UIButton and UITextField, simply by combining our new publisher with additional operators: But perhaps the biggest advantage of our new publisher-based control event API is Combine’s ability to, well, combine various data streams in really powerful ways. One of the advantages of this approach is that Combine’s various APIs were designed to be incredibly composable — which means that we can use the above to quite easily create increasingly specialized APIs for specific controls. So, currently, each Task.Group array that our TaskGroupsLoader will emit will have an order determined by when each nested network call was finished, which gives us a quite substantial race condition within that part of the app. For example, the following CanvasView uses a PassthroughSubject to emit a CGPoint value whenever it was tapped by the user — but it keeps that subject private, since we only want the canvas itself to be able to send values to it. Updated for Xcode 12.0 @Published is one of the most useful property wrappers in SwiftUI, allowing us to create observable objects that automatically announce when changes occur. 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