Singleton with Swift 3.0

Question

I had this implementation with Swift 2.0 and the Xcode suggestion is not only baffling but causes compilation error as well. it\'s a library where users are passing callfunc

Answer 1

The answers to date are great, but I like to be able to test my classes that use singleton with unit tests. The existing solutions require that you get a real singleton, not one that you can Fake, Mock, and Stub. In order to make testing easier, I use something like this:

protocol ScheduleManaging {
    func add(_ schedule: TimerSchedule)
}

class ScheduleManager: ScheduleManaging {
    var schedule: TimerSchedule?

    static var sharedInstance: ScheduleManaging = {
        return ScheduleManager()
    }()

    private init() {
    }
}

typealias ScheduleManagingMethods = ScheduleManager
extension ScheduleManagingMethods {
    func add(_ schedule: TimerSchedule) {
        self.schedule = schedule
    }
}

In classes under test that use the singleton, I can do something like this:

class OtherTests: XCTestCase {

    class FakeSharedManager: ScheduleManaging {
        var addMethodCalled = false
        func add(_ schedule: TimerSchedule) {
            addMethodCalled = true
        }
    }

    func test_SomeMethodThatShouldCallAdd_CallsAdd() {
        ScheduleManager.sharedInstance = FakeSharedManager()
        // Test that some class that uses the ScheduleManager
        // calls the "add" method of the ScheduleManager
        let someClass = ManagerUsingClass()
        someClass.someMethodThatShouldCallAdd()
        XCTAssertTrue(ScheduleManager.sharedInstance. addMethodCalled)
    }
}