(untilOutputFrom publisher: P) -> Publishers.DropUntilOutput where P : Publisher, Self.Failure == P.Failure ``` ## Parameters `publisher` A publisher to monitor for its first emitted element. ## Return Value A publisher that drops elements from the upstream publisher until the `other` publisher produces a value. ## Discussion Use [`drop(untilOutputFrom:)`](/documentation/Combine/Publisher/drop(untilOutputFrom:)) to ignore elements from the upstream publisher until another, second, publisher delivers its first element. This publisher requests a single value from the second publisher, and it ignores (drops) all elements from the upstream publisher until the second publisher produces a value. After the second publisher produces an element, [`drop(untilOutputFrom:)`](/documentation/Combine/Publisher/drop(untilOutputFrom:)) cancels its subscription to the second publisher, and allows events from the upstream publisher to pass through. After this publisher receives a subscription from the upstream publisher, it passes through backpressure requests from downstream to the upstream publisher. If the upstream publisher acts on those requests before the other publisher produces an item, this publisher drops the elements it receives from the upstream publisher. In the example below, the `pub1` publisher defers publishing its elements until the `pub2` publisher delivers its first element: ``` let upstream = PassthroughSubject() let second = PassthroughSubject() cancellable = upstream .drop(untilOutputFrom: second) .sink { print("\($0)", terminator: " ") } upstream.send(1) upstream.send(2) second.send("A") upstream.send(3) upstream.send(4) // Prints "3 4" ``` --- 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)