Add Change Matcher

[paulz]

What did you do?

Write a spec that verifies change in the collection

What did you expect to happen?

To use

expect({code}).toChange(collection.count).by(1)

What actually happened instead?

let count = collection.count
subject()
expect(collection.count - count) == -1

Environment

• Quick (1.1.0)
• Nimble (6.0.1)
• Xcode Version: 8.2.1
• Swift Version: 3

Please also mention which package manager you used and its version. Delete the
other package managers in this list:

• Cocoapods: 1.2.0

Project that demonstrates the issue

Here is an example of change matcher from RSpec:
https://relishapp.com/rspec/rspec-expectations/docs/built-in-matchers/change-matcher

[ikesyo]

I'm sorry for the really late response but this looks interesting!

[paulz]

thank you for the reply @ikesyo. This still would be useful for our projects. With the change matcher, we can be assured that tests did not pass accidentally because the value was already matching expected. Also, it will help us writing tests without storing initial values in the variable that we have to name. Currently, we are writing preconditions for tests in "before each" block, which is less readable.

[ikesyo]

A very simple PoC:

func change(_ subject: @autoclosure @escaping () -> Int, by delta: Int) -> Predicate<Void> {
    return Predicate.define { expr -> PredicateResult in
        let before = subject()
        try expr.evaluate()
        let after = subject()

        let diff = after - before
        return PredicateResult(
            bool: diff == delta,
            message: .expectedTo("change the subject by \(delta), but did change by \(diff)")
        )
    }
}

[paulz]

Love it!
and could that be even generic <T: Equatable> or Comparable?

[ikesyo]

For change(_:by:) matcher AdditiveArithmetic would be appropriate to calculate a diff.

Equatable could be used for change(_:from:to:) matcher.