Using #function to record messages sent to Swift test doubles

Tests.swift

import XCTest

class Tests: XCTestCase {
    func test_visit_startsTheDelegate() {
        let delegate = TestVisitDelegate()
        let visitor = Visitor(delegate: delegate)
        visitor.visit("/")
        XCTAssertTrue(delegate.didCall("visitDidStart()"))
    }

    func test_visit_completesTheDelegate() {
        let delegate = TestVisitDelegate()
        let visitor = Visitor(delegate: delegate)
        visitor.visit("/")
        XCTAssertTrue(delegate.didCall("visitDidComplete()"))
    }
}

TestVisitDelegate.swift

class TestVisitDelegate: VisitDelegate {
    func visitDidStart() {
        record(#function)
    }

    func visitDidComplete() {
        record(#function)
    }

    // MARK: Testing verification helpers.

    private var methodsCalled: Set<String> = []

    func didCall(_ method: String) -> Bool {
        methodsCalled.contains(method)
    }

    private func record(_ string: String) {
        methodsCalled.insert(string)
    }
}

Visitor.swift

protocol VisitDelegate {
    func visitDidStart()
    func visitDidComplete()
}

class Visitor {
    private let delegate: VisitDelegate

    init(delegate: VisitDelegate) {
        self.delegate = delegate
    }

    func visit(_ path: String) {
        delegate.visitDidStart()
        // Do the visiting.
        delegate.visitDidComplete()
    }
}