Unit test cases - Swift 3.2

I have to write unit test case to check whether a particular method gets called or not from within the method which is being called from unit test case.

class A{
    func fetchPersonsData() {
       let b = B()
       b.sendDataToserver([])
     }
}

class B {
  func sendEventToServer(_:[]) {
  }
}

In my unit test case I will call fetchPersonsData() method and want to check whether it calls sentEventToServer(_:) or not and what are the params.

How to write test case for this in Swift 3.2?

Should I mock by creating subclass of class B and override required method and pass my test if required method gets called?

First add a protocol:

protocol ServerSender {
    func sendEventToServer(_:[String])
}

Make B conform to that protocol:

class B: ServerSender {
    func sendEventToServer(_:[String]) {
    }
}

In your test target add a mock:

class BMock: ServerSender {
    var sendDataToServerCalled = false
    func sendEventToServer() {
        sendDataToServerCalled = true
    }
}

Change A to allow dependency injection:

class A {
    lazy var b: ServerSender = B()
    func fetchPersonsData() {
        b.sendDataToServer([])
    }
 }

In your test, inject the mock:

func test_fetchPersonsData_sendsDataToServer() {
    let sut = A()
    let mock = BMock()
    sut.b = mock

    a.fetchPersonsData()

    XCTAssertTrue(mock.sendDataToServerCalled)
}