iOS Unit Testing by Example

Subclass and Override: A Legacy Code Technique

Let’s move on to singletons we don’t own. To add a layer of indirection around the singleton, we’ll extend the calling code. An important dependency-breaking technique from Working Effectively with Legacy Code [Fea04] is Subclass and Override Method.

The idea is to create a subclass of production code that lives only in test code, or a test-specific subclass.^[19] It gives us a way to override methods that are problematic for testing.

Let’s apply this to OverrideViewController, which starts with this initial method:

HardDependencies/HardDependencies/OverrideViewController.swift

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
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