How to test PublishSubject in RxSwift

class Recorder<T> {
    var items = [T]()
    let bag = DisposeBag()

    func on(arraySubject: PublishSubject<[T]>) {
        arraySubject.subscribe(onNext: { value in
            self.items = value
        }).disposed(by: bag)
    }

    func on(valueSubject: PublishSubject<T>) {
        valueSubject.subscribe(onNext: { value in
            self.items.append(value)
        }).disposed(by: bag)
    }
}

final class BookViewModelTests: XCTestCase {
    func testBooks() throws {
        let expectation = self.expectation(description: #function)
        let recorder = Recorder<Book>()
        let viewModel = BookViewModel(bookClient: MockBookClient())
        recorder.on(arraySubject: viewModel.books)
        viewModel.load()

        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 0.5, execute: {
            expectation.fulfill()
        })

        wait(for: [expectation], timeout: 2)
        XCTAssertEqual(recorder.items.count, 3)
    }
}

extension XCTestCase {
    func waitOrFail(timeout: TimeInterval) {
        let expectation = self.expectation(description: #function)

        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + timeout, execute: {
            expectation.fulfill()
        })

        wait(for: [expectation], timeout: timeout + 2)
    }
}