Difference

Better way to identify what’s different between 2 instances.

Have you ever written tests?
Usually they use equality asserts, e.g. XCTAssertEqual, what happens if the objects aren’t equal? Xcode throws a wall of text at you:

This forces you to manually scan the text and try to figure out exactly what’s wrong, what if instead you could just learn which property is different?

If you’d like to support my work and improve your engineering workflows, check out my SwiftyStack course

Installation

CocoaPods

Add pod 'Difference' to your Podfile.

Carthage

Add github "krzysztofzablocki/Difference" to your Cartfile.

SwiftPM

Add .package(url: "https://github.com/krzysztofzablocki/Difference.git", branch: "master") dependency in your Package manifest.

Using lldb

Write the following to see the difference between 2 instances:

po dumpDiff(expected, received)

Integrate with XCTest

Add this to your test target:

public func XCTAssertEqual<T: Equatable>(_ expected: @autoclosure () throws -> T, _ received: @autoclosure () throws -> T, file: StaticString = #filePath, line: UInt = #line) {
    do {
        let expected = try expected()
        let received = try received()
        XCTAssertTrue(expected == received, "Found difference for \n" + diff(expected, received).joined(separator: ", "), file: file, line: line)
    }
    catch {
        XCTFail("Caught error while testing: \(error)", file: file, line: line)
    }
}

Replace #filePath with #file if you’re using Xcode 11 or earlier.

Integrate with Quick

Add this to your test target:

public func equalDiff<T: Equatable>(_ expectedValue: T?) -> Predicate<T> {
    return Predicate.define { actualExpression in
        let receivedValue = try actualExpression.evaluate()

        if receivedValue == nil {
            var message = ExpectationMessage.fail("")
            if let expectedValue = expectedValue {
                message = ExpectationMessage.expectedCustomValueTo("equal <\(expectedValue)>", actual: "nil")
            }
            return PredicateResult(status: .fail, message: message)
        }
        if expectedValue == nil {
            return PredicateResult(status: .fail, message: ExpectationMessage.fail("").appendedBeNilHint())
        }

        return PredicateResult(bool: receivedValue == expectedValue, message: ExpectationMessage.fail("Found difference for " + diff(expectedValue, receivedValue).joined(separator: ", ")))
    }
}

Write the following to see the difference between 2 instances:
expect(received).to(equalDiff(expected))

Integrate with The Composable Architecture

If you are using The Composable Architecture nameLabels configuration to get a diff that’s more appropiate for reducer instrumentation

diff(oldState, newState, indentationType: .pipe, nameLabels: .comparing)

You can use this function in your own variant of ReducerInstrumentation code based on this

That way your diffs will look more like this:

Received action:
  AppAction.home(.howTo(.setSelectedSlide))
State:
home:
|	selectedHowTo:
|	|	selectedSlide:
|	|	|	Current: 8mnkni91h4fe
|	|	|	Previous: exei4wpqsmdk