Combine — Publisher Subscriber

Subscriber

Subscribers have three functions:

1. receive(subscription:) when subscribing the publisher will call this function exactly once
2. receive(_:) the publisher will provide 0 or more values via this method
3. receive(completion:) the publisher can eventually provide a completion only once, meaning that it has completed or a failure has risen

Once receive(completion:) is called, no further value will be forwarded.

Special Subscriptions

Sink

Just provide a closure and now for every value received, the closure is going to get called with the value.

This Sink will return a canceller, which is like a token where we can call cancel when we no longer want to get new values.

Subjects

Behaves like Publisher and Subscriber.

Supporting multi casting a single value (broadcast to multiple subscribers)

Two kinds of subjects:

• Passthrough: doesn’t store any value, therefore we’ll get a value only once a new one will be sentWe can inject a passthrough subjects in a stream by calling onePublisher.share()
• CurrentValue: like Passthrough, however it stores the last value, so new subscribers have an opportunity to catch up

SwiftUI

• SwiftUI owns the subscriber
• We only need to provide a publisher

@BindableObject

• @BindableObjects in SwiftUI have a single associated type.
• It’s a Publisher that is constrained to never fail.
• @ObjectBinding allows SwiftUI to automatically discover and subscribe to our Publisher.
• SwiftUI will automatically generate a new body whenever we signal that the model has changed.
• Note how the “combine upstream” value is Void, we do not use as the View owns the model anyway, so we can access it from there instead.

@Published

• Adds a publisher to a property
• Access the published via wrapped value (in the example below, $password is the publisher aka wrapped value)

eraseToAnyPublisher

Advertise the exact contract we want for our API boundary and hide all the implementation details along the way.

AnyCancellable

• What is returned after creating a subscriber
• This class automatically calls cancel() on deinit.