Publisher

protocol · iOS 13.0+ · SDK 27.1

protocol Publisher<Output, Failure>

Declares that a type can transmit a sequence of values over time. A publisher delivers elements to one or more ``Subscriber`` instances. The subscriber’s ``Subscriber/Input`` and ``Subscriber/Failure`` associated types must match the ``Publisher/Output`` and ``Publisher/Failure`` types declared by the publisher. The publisher implements the ``Publisher/receive(subscriber:)``method to accept a subscriber. After this, the publisher can call the following methods on the subscriber: - ``Subscriber/receive(subscription:)``: Acknowledges the subscribe request and returns a ``Subscription`` instance. The subscriber uses the subscription to demand elements from the publisher and can use it to cancel publishing. - ``Subscriber/receive(_:)``: Delivers one element from the publisher to the subscriber. - ``Subscriber/receive(completion:)``: Informs the subscriber that publishing has ended, either normally or with an error. Every `Publisher` must adhere to this contract for downstream subscribers to function correctly. Extensions on `Publisher` define a wide variety of _operators_ that you compose to create sophisticated event-processing chains. Each operator returns a type that implements the ``Publisher`` protocol Most of these types exist as extensions on the ``Publishers`` enumeration. For example, the ``Publisher/map(_:)-99evh`` operator returns an instance of ``Publishers/Map``. > Tip: A Combine publisher fills a role similar to, but distinct from, the <doc://com.apple.documentation/documentation/Swift/AsyncSequence> in the Swift standard library. A `Publisher` and an `AsyncSequence` both produce elements over time. However, the pull model in Combine uses a ``Combine/Subscriber`` to request elements from a publisher, while Swift concurrency uses the `for`-`await`-`in` syntax to iterate over elements published by an `AsyncSequence`. Both APIs offer methods to modify the sequence by mapping or filtering elements, while only Combine provides time-based operations like ``Publisher/debounce(for:scheduler:options:)`` and ``Publisher/throttle(for:scheduler:latest:)``, and combining operations like ``Publisher/merge(with:)-7fk3a`` and ``Publisher/combineLatest(_:_:)-1n30g``. To bridge the two approaches, the property ``Publisher/values-1dm9r`` exposes a publisher's elements as an `AsyncSequence`, allowing you to iterate over them with `for`-`await`-`in` rather than attaching a ``Subscriber``. # Creating Your Own Publishers Rather than implementing the `Publisher` protocol yourself, you can create your own publisher by using one of several types provided by the Combine framework: - Use a concrete subclass of ``Subject``, such as ``PassthroughSubject``, to publish values on-demand by calling its ``Subject/send(_:)`` method. - Use a ``CurrentValueSubject`` to publish whenever you update the subject’s underlying value. - Add the `@Published` annotation to a property of one of your own types. In doing so, the property gains a publisher that emits an event whenever the property’s value changes. See the ``Published`` type for an example of this approach.

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