Those values are sent one by one using the subscriber’s receive(_:) method until the maximum demand is reached. The subscription receives the demand and starts to process information and emit values.This is done by sending the number of values it wants, by calling request(_:). Now that the subscriber has the subscription and a contract between it and the publisher has been made, it can request for values to the publisher.
![swift publisher 3 tutorials swift publisher 3 tutorials](https://i.ytimg.com/vi/Kl-p08LqpfM/hqdefault.jpg)
The subscriber receives the subscription via the receive(subscription:) method.
![swift publisher 3 tutorials swift publisher 3 tutorials](https://i1.wp.com/www.macbed.com/wp-content/uploads/2017/06/18896.png)
That isn’t great, as ideally we’d like to enforce that only Counter can send new values - to avoid multiple sources of truth.
SWIFT PUBLISHER 3 TUTORIALS CODE
Since our PassthroughSubject is both a publisher and a subject, any code can send new values to it, even if that code lives outside of our Counter class - simply by calling send(): counter. However, while the above approach works, it does come with a quite major downside. Note how we’re able to just pass a single closure into our above call to sink, since our publisher can’t throw any errors, which means that we don’t need to handle its completion event (if we don’t want to). increment call will result in a value being printed: Since we're filtering out all values below 3, only our final With the above in place, we can now subscribe to our new publisher just like we did earlier when performing network requests using URLSession - for example like this: let counter = Counter() We’re using Never as our publisher’s error type, which means that it’ll never be able to throw any errors - which is perfect in this case, since we’re only sending new Int values to it. Once we’ve created a publisher, we can then attach subscriptions to it, for example by using the sink API - which lets us pass a closure to be called whenever a new value was received, as well as one that’ll be called once the publisher was completed: let cancellable = publisher. For example, here’s how we could use Foundation’s URLSession type to create a publisher for making a network request to a given URL: let url = URL(string: "")! When introducing Combine, Apple also went through some of their core libraries in order to retrofit them with Combine support. Publishers can either be active indefinitely or eventually be completed, and can also optionally fail when an error was encountered. Let’s start with Publishers, which are observable objects that emit values whenever a given event occurred. So in this article, let’s take a look at the basics of Combine, what some of the core principles of reactive programming are, and how they can become really useful in practice. While that’s a phrase that’s commonly used within the reactive programming world, as a concept and way of constructing logic, it can initially be quite difficult to fully grasp. Introduced during WWDC 2019, Apple’s Combine framework lets us model all sorts of asynchronous events and operations as “values over time”.