The Combine framework provides a declarative Swift API for processing values over time using publishers, subscribers, and operators (FRP). It introduces the concept of back pressure to prevent overflow and includes built-in support for assigning values and debugging streams. Combine is integrated with SwiftUI and uses the same supported platforms, providing a standardized way to work with asynchronous data in Apple ecosystems.

1. COMBINE
A TOUR OF

2. VY-SHANE XIE
I build software things at WiseTime
https://vyshane.com
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3. The Combine framework
provides a declarative Swift API
for processing values over time.


4. The Combine framework
provides a declarative Swift API
for processing values over time.


5. STREAMS
Publisher SubscriberOperator Subscriber
Functional Reactive Programming
FRP is a programming paradigm for asynchronous
stream processing using the building blocks of
functional programming (e.g. map, reduce, filter).

6. FRP
RxSwift
ReactiveSwift
Is not new in the Swift ecosystem
Combine

7. HOW IS COMBINE DIFFERENT?

8. COMBINE IS
A foundational  framework
Deeply integrated with SwiftUI
Closed source
Not available on Linux

9. https://forums.swift.org/t/will-swiftnio-adapt-to-the-new-combine-framework/25166/11
ONE MORE THING

10. REACTIVE STREAMS SPEC?
Enables JVM streaming libraries to interoperate
Provides a design for implementing back pressure

11. BACK
PRESSURE?

12. userIDs getProfile save
Slow IO

13. PRODUCER CONSUMERPROCESSOR CONSUMERuserIDs getProfile save
Slow IO
😰
Incoming rate higher than we can process

14. PRODUCER CONSUMERuserIDs save
Slow IO
getProfile
💥We're out of memory

15. PRODUCER CONSUMERuserIDs save
Slow IO
getProfile
On overflow
drop elements
Not Great!
😒

16. PRODUCER CONSUMERPROCESSOR CONSUMERgetProfile save
Slow IO
🚦 😎
Signal demand upstream
THIS IS BACK PRESSURE
userIDs

17. COMBINE IS
A low level  framework
Deeply integrated with SwiftUI
Closed source
Not available on Linux
The first Swift FRP library that has back pressure built in

18. KEY CONCEPTS
COMBINE

19. SUBSCRIBEROperator
3 MAIN CONCEPTS
Publisher Subscriber

20. PUBLISHER
Knows how to deliver a stream of values over time
Publisher

21. PUBLISHER
The stream may complete successfully
Publisher

22. PUBLISHER
Or fail with an error
Publisher

23. PUBLISHER
An infinite stream that never ends is legal
Publisher

24. PUBLISHER
A stream that completes without emitting anything is OK
Publisher

25. THE PUBLISHER PROTOCOL
A publisher declares an Output type as well as a Failure type

26. SOME BUILT-IN WAYS TO PUBLISH VALUES

27. SUBSCRIBER
‣ Requests values from a publisher
‣ Acts on values as it receives them
‣ Declares an Input type and a Failure type

28. SUBSCRIBING TO A PUBLISHER
A subscriber can subscribe to a publisher if:
‣ Its input type matches the output type of the publisher
‣ Its failure type matches that of the publisher
Publisher Subscriber
O
F
I
F

29. SUBSCRIBING TO A PUBLISHER
A subscriber can subscribe to a publisher if:
‣ Its input type matches the output type of the publisher
‣ Its failure type matches that of the publisher

30. TWO BUILT-IN SUBSCRIBERS

31. Assign Subscriber
Assigns elements it receives to an object property at a keypath
The assign(to:on:) operator creates an Assign subscriber
and subscribes it to the upstream publisher

32. Sink Subscriber
Accepts closures that receive values and completion events
Useful when writing tests

33. OPERATORS
Operate on streams and allow you to:
‣ Transform values
‣ Filter values
‣ Combine publishers together
‣ Manipulate timing
‣ Perform sequence and aggregation operations

34. Operators are instance methods on the Publisher protocol

35. Many operators subscribe to the upstream publisher
and return a publisher of their own
This allows you to compose them into pipelines
EXAMPLE: CALLING A WEB SERVICE

36. EXAMPLE: CALLING A WEB SERVICE

37. EXAMPLE: CALLING A WEB SERVICE
assign(to:on:) is a subscription operator,
returns an AnyCancellable on which you can call .cancel()

38. RETURN TYPES OF CHAINED OPERATIONS

39. TYPE-ERASING OPERATOR
Whew!

40. DEBUG OPERATORS
Combine ships with some handy operators that you can
insert into your pipelines to debug them:
‣ print(_:to:)
‣ breakpoint(receiveSubscription:receiveOutput:
receiveCompletion:)
‣ breakpointOnError()
‣ handleEvents(receiveSubscription:receiveOutput:
receiveCompletion:receiveCancel:receiveRequest:)

41. ... all yuor operators

42. SUBSCRIPTION LIFECYCLE
COMBINE

43. PUBLISHER SUBSCRIPTION SUBSCRIBER
Subscribes
Creates Sends
Requests values
Emits value
Returns demand
Updates
demand
Emits value
Demand
fulfilled,
pauses
Returns demand
Requests values
Emits value
Updates
demand
Returns demand
No more
values Emits completion
Receives
demand
Stream ends
Publishers are lazy
(cold)

44. PUBLISHER SUBSCRIPTION SUBSCRIBER
Subscribes
Creates Sends
Requests values
Emits value
Returns demand
Receives
demand
Stream cancelled
Deallocated

45. EXAMPLE - DEALLOCATED SUBSCRIPTION
Times out, test fails

46. EXAMPLE - DEALLOCATED SUBSCRIPTION

47. CANCELLABLE
‣ Can be cancelled
‣ Can be stored/retained

48. SUPPORTED PLATFORMS
Same as SwiftUI

49. SUPPORTED PLATFORMS
Swift 5.1+
iOS 13.0+
macOS 10.15+
tvOS 13.0+
watchOS 6.0+
Mac Catalyst 13.0+

50. RESOURCES
‣ Using Combine web book (free):
https://heckj.github.io/swiftui-notes/
‣ Combine: Asynchronous Programming with Swift (ebook):
https://store.raywenderlich.com/products/combine-
asynchronous-programming-with-swift
‣ WWDC: Introducing Combine (video)
https://developer.apple.com/videos/play/wwdc2019/722/
‣ WWDC: Combine in Practice (video)
https://developer.apple.com/videos/play/wwdc2019/721/

51. ANY QUESTIONS?
THANKS FOR LISTENING