A Cheat Sheet of various equivalent of ReactiveSwift for Combine.

This list is by no mean exhaustive, and while each Combine equivalent try to have the same semantic as its ReactiveSwift, there might be cases where the behavior differs in subtle manners.
Please do report these cases so we can update the list accordingly.

Basics

	ReactiveSwift	Combine
Platforms supported	• iOS 8.0+ • macOS 10.9+ • Mac Catalyst 13.0+ • watchOS 2.0+ • tvOS 9.0+ • Linux	• iOS 13.0+ • macOS 10.15+ • Mac Catalyst 13.0+ • watchOS 6.0+ • tvOS 13.0+
Framework Consumption	Third-party	First-party (built-in)
Maintained by	Open-Source / Community	Apple
UI Bindings	ReactiveCocoa (interfacing with UIKit)	SwiftUI[1]

[1] Is actually a full UI framework replacing UIKit, that natively supports Combine.

Components

Note that in Combine, anything can be transformed in an AnyPublisher through eraseToAnyPublisher().
This means that all the equivalent given for Combine can be returned as an AnyPublisher.

Core

ReactiveSwift	Combine	Notes
SignalProducer	Deferred<PassthroughSubject>
Signal	Publisher
Signal.pipe()	PassthroughSubject
Property	private(set) variable annotated with @Published (use $variable to get the underlying publisher)
MutableProperty	CurrentValueSubject, or a variable annotated with @Published (use the $variable to get the underlying publisher)
ValidatingProperty	❌
Action	❌	An equivalent exists at https://github.com/Fueled/ios-utilities/blob/v3-support/FueledUtils/Combine/Action.swift
Disposable	Cancellable
SerialDisposable	❌	No direct equivalent, see ScopedDisposable<SerialDisposable>
CompositeDisposable	❌	No direct equivalent, see ScopedDisposable<CompositeDisposable>
Event	❌	No direct equivalent
Lifetime	❌
Observer	Subscriber	AnySubscriber can be used to create a Subscriber without creating a type that conforms to the former
Scheduler	Scheduler	DispatchQueue, RunLoop, ImmediateScheduler, OperationQueue conforms to Scheduler
DateScheduler	Scheduler
Atomic	❌	An equivalent called AtomicValue exists at https://github.com/Fueled/ios-utilities/blob/v3-support/FueledUtils/Core/Atomic.swift#L19-L46
Bag	❌

Other

ReactiveSwift	Combine	Notes
SignalProducer<Value, Failure>(value: value)	Just(value).setFailureType(Failure.self)
SignalProducer<Value, Failure>(error: error)	Fail(outputType: Value.self, failure: error)
SignalProducer<Value, Failure>.empty	Empty().setFailureType(Failure.self)
SignalProducer<Value, Failure>.never	Empty(completeImmediately: false).setFailureType(Failure.self)
Signal<Value, Failure>.empty	Empty().setFailureType(Failure.self)
Signal<Value, Failure>.never	Empty(completeImmediately: false).setFailureType(Failure.self)
Property<Value>(value: value)	Just(value).setFailureType(Failure.self)
ScopedDisposable<SerialDisposable>	AnyCancellable
ScopedDisposable<CompositeDisposable>	Collection of AnyCancellables	Call anyCancellable.store(in: collection), where collection can be an Array, a Set, or any other RangeReplaceableCollection

Operators

ReactiveSwift	Combine	Notes
combineLatest()	Publishers.CombineLatest/Publishers.CombineLatest3/Publishers.CombineLatest4	Limited to up to 4 parameters, as opposed to 10 for ReactiveSwift.
flatMap(.merge) { <...> }	flatMap { <...> }
flatMap(.latest) { <...> }	map { <...> }.switchToLatest()
mapError	mapError
flatMapError	catch
zip	Publishers.zip/Publishers.zip2/Publishers.zip3
merge(one, two, ..., eight)	Publishers.Merge/Publishers.Merge2/.../Publishers.Merge8
merge([])	Publishers.MergeMany
start()	self.sink(receiveCompletion: { _ in }, receiveValue: { _ in })
startWithValues { value in <...> }	self.sink(receiveValue: { value in <...> })
startWithResult { result in <...> }	self.sink(receiveCompletion { completion in if case .failure(let error) = completion { <...> } }, receiveValue: { value in <...> })
ignoreError()	self.catch { _ in Empty() }
skipNil()	❌	An equivalent can be found at https://github.com/Fueled/ios-utilities/blob/v3-support/FueledUtils/Combine/PublisherExtensions.swift#L36-L38
skip(first:)	dropFirst()
skip(until:)	drop(untilOutputFrom:)
skip(while:)	drop(while:)
scan()	scan()
throttle()	throttle()
SignalProducer<Date, Never>.timer()	Timer.publish().autoconnect()
prefix()	prepend()
on()	handleEvents()
<~	If binding to a property on an class, can use .assign(to:, on:). Keep in mind that this will retain the object.
compositeDisposable += <disposable>	store(in: &cancellables)

Format inspired by https://github.com/CombineCommunity/rxswift-to-combine-cheatsheet