Combine framework tutorial: transforming Operators part 1 – map, compactMap and tryMap

This is the first part of a series for the Combine framework. I will start by showing you transforming operators like map, compactMap, tryMap and, decode. You will learn how to use Combine to load JSON files.

Combine framework tutorial – Part 2 – nested publisher streams with switchToLatest and flatMap

I will show you an example of nested publisher streams. First, we will fetch images with URLSession dataTaskPublisher inside map and use the switchToLatest operator.

The second example will show how to do the same with flatMap. I will talk a little bit about multithreading in Combine. Publisher streams pass and process values in serial. With flatMap we can create multiple nested publisher streams that run concurrently in the case of URLSession.

Flatmap is a good example to see how backpressure works in Combine.

Combine framework tutorial – Part 3 – Error handling with mapError, setFailureType, & flatMap

Error handling is important otherwise your data streams will complete and data processing they are supposed to handle will stop. This results in an UI that is no longer responsive.

You can create nested publisher streams with flatMap. If you handle the error inside the nested stream, the main data streams will not complete.

To handle errors you can use:

– catch { error in …}

– replaceError(with)

– assertNoFailure 

Combine framework tutorial – Part 4 – How to include unit testing and dependency injection

Let’s see how we can write unit tests for code that uses the Combine framework. We need to first use dependency injection to replace URLSession data task publisher with a Result publisher in the mock service.

I will write a few tests to show you different test cases and use Combine operators like collect, first, output(at). You can find the full list of Combine operators at the official Apple documentation here:
https://developer.apple.com/do…
You need to scroll down a lot. Some of the operators have examples. They don’t show a good use case but are a good example for implementation details.

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