Supplementary behaviour code for https://jakubturek.com/bottom-up-unit-testing/

YearsBetweenDatesBehavior.swift

class YearsBetweenDatesBehavior: 
    Behavior<YearsBetweenDatesBehavior.Context> {
  struct Context {
    let sut: (Calendar) -> (Date, Date) -> Int?
    let from: String
    let to: String
    let calendar: Calendar.Identifier
    let expected: Int
  }

  override class func spec(_ aContext: @escaping 
      () -> YearsBetweenDatesBehavior.Context) {
    let calendar = aContext().calendar
    let from = aContext().from
    let to = aContext().to
    let expected = aContext().expected

    describe("years between") {
      context("with calendar \(calendar)") {
        var sut: ((Date, Date) -> Int?)!

        beforeEach {
          sut = aContext().sut(Calendar(identifier: calendar))
        }

        afterEach {
          sut = nil
        }

        context("from \(from) to \(to)") {
          var yearsBetween: Int?

          beforeEach {
            yearsBetween = sut(from.date, to.date)
          }

          afterEach {
            yearsBetween = nil
          }

          it("should return \(expected) years") {
            expect(yearsBetween).toNot(beNil())
            expect(yearsBetween) == expected
          }
        }
      }
    }
  }
}