A new Swift video series exploring functional programming and more.
#39 • Monday Nov 26, 2018 • Subscriber-only

Witness-Oriented Library Design

We previously refactored a library using protocols to make it more flexible and extensible but found that it wasn’t quite as flexible or extensible as we wanted it to be. This week we re-refactor our protocols away to concrete datatypes using our learnings from earlier in the series.

#39 • Monday Nov 26, 2018 • Subscriber-only

Witness-Oriented Library Design

We previously refactored a library using protocols to make it more flexible and extensible but found that it wasn’t quite as flexible or extensible as we wanted it to be. This week we re-refactor our protocols away to concrete datatypes using our learnings from earlier in the series.


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Exercises

  1. Take our witness-oriented library and define some interesting strategies! Think about your own code base and specialized Snapshotting (and Diffing) instances you can define. Here are some suggestions to get you started!

    • Define a dump strategy on Snapshotting<Any, String> that uses the output of Swift’s dump function. You can reuse logic from the recursiveDescription strategy to remove occurrences of memory addresses.

    • Define a Snapshotting<URLRequest, String> strategy that snapshots a raw HTTP request, pretty-printing the method, headers, and body of the request.

    • Define a Snapshotting<NSAttributedString, UIImage> strategy that snapshots images of attributed strings.

    • Define a Snapshotting<NSAttributedString, String> strategy that snapshots HTML representations of attributed strings.


References

  • Protocol-Oriented Programming in Swift

    Apple • Tuesday Jun 16, 2015

    Apple’s eponymous WWDC talk on protocol-oriented programming:

    At the heart of Swift’s design are two incredibly powerful ideas: protocol-oriented programming and first class value semantics. Each of these concepts benefit predictability, performance, and productivity, but together they can change the way we think about programming. Find out how you can apply these ideas to improve the code you write.

  • uber/ios-snapshot-test-case

    Uber, previously Facebook

    Facebook released a snapshot testing framework known as FBSnapshotTestCase back in 2013, and many in the iOS community adopted it. The library gives you an API to assert snapshots of UIView’s that will take a screenshot of your UI and compare it against a reference image in your repo. If a single pixel is off it will fail the test. Since then Facebook has stopped maintaining it and transfered ownership to Uber.

  • Snapshot Testing in Swift

    Stephen Celis • Friday Sep 1, 2017

    Stephen gave an overview of snapshot testing, its benefits, and how one may snapshot Swift data types, walking through a minimal implementation.

Chapters
Introduction
00:05
A recap
02:35
Asserting with witnesses
11:25
Diffing images with witnesses
15:04
Pulling back witnesses
21:26
Multiple witnesses
28:34
What’s the point?
34:15
Credits

Hosted by Brandon Williams and Stephen Celis. Recorded in Brooklyn, NY.