Introducing Combine

Description: Combine is a unified declarative framework for processing values over time. Learn how it can simplify asynchronous code like networking, key value observing, notifications and callbacks.

What’s Combine?

  • A unified, declarative API for processing values over time.
  • Combine was designed for composition.

Key Concepts


  • it's a protocol
  • Declarative part of Combine.
  • Describes how values/errors are produced.
  • Sends (sequences of) values over time.
  • Two generic types: Output and Error
  • Completion via .finished or .failure
  • Publishers are not necessary who produced the value/error.


  • it's a protocol
  • Publisher counterpart
  • Receives values from publishers
  • Two generic types: Input and Error
  • Can cancel a subscription
  • Subscription: A subscription is how a Subscriber controls the flow of data from a Publisher to the Subscriber.


  • In-between Publishers and Subscribers, they transform/filter the published value into something desired by the subscriber
  • Operators forward errors, only change values.
  • They also manage scheduling/time, thread/queue movement, and more

Combine provides a lot of operators:
catch, dropFirst, allSatisfy, breakpoint, setFailureType, prepend, replaceError, append, map, filter, count, abortOnError, breakpointOnError, ignoreOutput, first, min, last, output, prefix, replaceEmpty, compactMap, zip, flatMap, removeDuplicates, reduce, combineLatest, collect, retry, contains, switchToLatest, replaceNil, scan, and more!

The idea is that operators are composable:
instead of having a few operators that do a lot, Combine provides many operators that do a little, then it’s up to us to choose which operators to use based on our needs.

If a operator returns nil, it’s like stopping the event/value to proceed further. We can use most of the functional operators we’re used with arrays: prefix, compactMap, filter...

zip converts multiple inputs (from different publishers) into a single tuple. It requires all the inputs (from all upstreams) to be delivered before sending the data downstream. Therefore it acts as a when/and operation.

combineLatest is like zip, but it keeps the latest value of each upstream. Whenever a new value comes in, a new event is sent downstream with all old values except the new one.

Note that both zip and CombineLatest can transform the values into something else (no need to forward necessarily the tuple)

Future vs Publisher

  • If we need to represent a single value asynchronously, we have a future.
  • If we need to represent many values asynchronously, that's a Publisher.
  • Future is a real type in combine

Missing anything? Corrections? Contributions are welcome 😃


Written by

Federico Zanetello

Federico Zanetello

iOS Engineer with strong passion for Swift, minimalism, and design. When he’s not busy automating things, he can be found writing at FIVE STARS and/or playing with the latest shiny toys.