Testing, testing

I have unit testing in my list of goals, and if I’m going to throw this space trimming macOS utility up on the web, now might be a good time to figure out how to add unit tests to a project, how to write them, and how to run them. XCode is well set up for this, so it’s really no drama to do.

Although I haven’t worried with any unit testing up to this point in my iOS/Swift learning, in my previous programming work I did a lot of work with the large calculations involved in translating GPS coordinates and robotic positioning models where errors would be bad – so I’ve written a lot of tests over the years. I’ve also definitely felt the confidence you can dramatically refactor code with when you know the code has a robust test suite. I’m a big fan.

Adding Tests to Existing Project

In Xcode, with your project open, you need to add the testing target. File | New | Target... then scroll down to find the Unit Testing Bundle. When you add that, you’ll see a new folder in the Project Navigator, and a new source file – both with the name <app name>Tests.

If you click into the <app name>Test.swift file, you’ll see a couple of methods for setting up the testing environment and cleaning up after it. They are called before and after each test – you’d use them if you needed to set something up like some files in a directory or similar and wanted to ensure it was not affected by the tests run before the current test. Don’t worry about them for the time being.

Writing Tests

Before we can write any tests, we need to import any modules with the code we want to test. In my case, I want to test the function stripSpaces() which is in the ContentView of my app called CodeTrimmer. So in the top of the CodeTrimmerTests.swift file, under the other import, I add the import:

import XCTest
@testable import CodeTrimmer

Then we can write our tests. The test functions must start with the magic prefix test so that Xcode treats them as such. The usual approach in the test is to call a function with some specific input, then test the output is what is expected with a special test version of assert():

Here’s a couple. My function stripSpaces() deletes a number of spaces from each line of a block of code such that the block becomes left aligned. if the XCTAsserts() pass, the test passes and I get a green tick. To run the tests I just click in the breakpoint gutter next to the function declaration.

	func testStripSpaces01() throws {
	let testString =
	"""
	single line string no spaces
	"""
	let resultString = stripSpaces(testString)
	XCTAssertTrue(resultString == testString, testString)
	}
	func testStripSpaces02() throws {
	let testString =
	"""
	single line 4 spaces
	"""
	let resultString = stripSpaces(testString)
	XCTAssertFalse(resultString == testString, testString)
	XCTAssertTrue(testString.dropFirst(4) == resultString, testString)
	}

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