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We’ve spent the past four episodes taking a deep study of protocols and exploring how their features can be represented using concrete datatypes and functions, and we’ve seen how in doing so we can address some of the shortcomings and complexities of protocols while unlocking a world of composition that was previously hidden. We’ve even suggested that the way we design our APIs could be dramatically improved by eschewing protocols in favor of these concrete datatypes and functions. That’s quite the claim, seeing as we’ve spent four episodes on the subject without ever demonstrating this! So much for “what’s the point,” huh!?
Well this week we finally put our theory to the test! We’re going to refactor a small library to be more flexible and extensible by abstracting away part of its design. We’ll start in the protocol-oriented way because that’s how we’re told to design these kinds of things in Swift. It’ll give us an opportunity to analyze the problems that protocols have that we think concrete datatypes and functions can solve.
We want to stress that we do not consider the actual library we are design to be the most interesting aspect of this topic, although it is quite cool. The real topic we are dissecting here is that of how we approach library design in Swift, and what lessons from our series on protocol witnesses can we apply so that we avoid the pitfalls of protocols.
There’s one protocol requirement we missed: the fact that we hard-code the path extension of the snapshot reference to “png”. Move this requirement into our protocols. Which protocol does it belong to?
Add a default implementation of path extension so that those conforming their own types to be
Snapshottable do not need to declare
"png" every time.
Showing the difference between two images is a matter of using a Core Image difference filter. Unfortunately, Apple provides no such API to show the difference between two strings. Implement a line diff algorithm to describe the difference between two strings in Swift.
A popular, human-readable algorithm is called the “patience diff”. Here are some resources:
Enumerating longest increasing subsequences and patience sorting: a paper that describes the algorithm.
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.
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.
Stephen gave an overview of snapshot testing, its benefits, and how one may snapshot Swift data types, walking through a minimal implementation.