# flatMap(maxPublishers:_:) Transforms all elements from an upstream publisher into a new publisher up to a maximum number of publishers you specify. ``` func flatMap(maxPublishers: Subscribers.Demand = .unlimited, _ transform: @escaping (Self.Output) -> P) -> Publishers.FlatMap where T == P.Output, P : Publisher, Self.Failure == P.Failure ``` ## Parameters `maxPublishers` Specifies the maximum number of concurrent publisher subscriptions, or [`unlimited`](/documentation/Combine/Subscribers/Demand/unlimited) if unspecified. `transform` A closure that takes an element as a parameter and returns a publisher that produces elements of that type. ## Return Value A publisher that transforms elements from an upstream publisher into a publisher of that element’s type. ## Discussion Combine‘s `flatMap(maxPublishers:_:)` operator performs a similar function to the operator in the Swift standard library, but turns the elements from one kind of publisher into a new publisher that is sent to subscribers. Use `flatMap(maxPublishers:_:)` when you want to create a new series of events for downstream subscribers based on the received value. The closure creates the new [`Publisher`](/documentation/Combine/Publisher) based on the received value. The new [`Publisher`](/documentation/Combine/Publisher) can emit more than one event, and successful completion of the new [`Publisher`](/documentation/Combine/Publisher) does not complete the overall stream. Failure of the new [`Publisher`](/documentation/Combine/Publisher) causes the overall stream to fail. In the example below, a [`PassthroughSubject`](/documentation/Combine/PassthroughSubject) publishes `WeatherStation` elements. The `flatMap(maxPublishers:_:)` receives each element, creates a from it, and produces a new , which will publish the data loaded from that . ``` public struct WeatherStation { public let stationID: String } var weatherPublisher = PassthroughSubject() cancellable = weatherPublisher.flatMap { station -> URLSession.DataTaskPublisher in let url = URL(string:"https://weatherapi.example.com/stations/\(station.stationID)/observations/latest")! return URLSession.shared.dataTaskPublisher(for: url) } .sink( receiveCompletion: { completion in // Handle publisher completion (normal or error). }, receiveValue: { // Process the received data. } ) weatherPublisher.send(WeatherStation(stationID: "KSFO")) // San Francisco, CA weatherPublisher.send(WeatherStation(stationID: "EGLC")) // London, UK weatherPublisher.send(WeatherStation(stationID: "ZBBB")) // Beijing, CN ``` --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)