Delegation

import UIKit

protocol CalendarDelegate: class {
    func calendar(_ calendar: Calendar, willDisplay year: Int)
    func calendar(_ calendar: Calendar, didSelect date: Date)
    func calendarShouldChangeYear(_ calendar: Calendar) -> Bool
}

protocol CalendarDataSource {
    func calendar(_ calendar: Calendar, eventsFor date: Date) -> [String]
    func calendar(_ calendar: Calendar, add event: String, to date: Date)
}

protocol ReminderPresenting {
    func yearChanged(to year: Int)
}

class Calendar {
    var delegate: CalendarDelegate?
    var dataSource: CalendarDataSource?
    var selectedDate: Date = Date()
    var currentYear: Int = 2019

    func changeDate(to date: Date) {
        selectedDate = date
        delegate?.calendar(self, didSelect: date)

        if let items = dataSource?.calendar(self, eventsFor: date) {
            print("Today's events are...")
            items.forEach { print($0) }
        } else {
            print("There are no events for today.")
        }
    }

    func add(event: String) {
        dataSource?.calendar(self, add: event, to: selectedDate)
    }

    func changeYear(to year: Int) {
        guard delegate?.calendarShouldChangeYear(self) ?? true else {
            return
        }

        delegate?.calendar(self, willDisplay: year)
        currentYear = year
    }
}

class Reminders: ReminderPresenting {

    var title = "Year: 2019"
    var calendar = Calendar()

    init() {
        let delegate = RemindersCalendarDelegate()
        delegate.parentController = self
        calendar.delegate = delegate
        calendar.dataSource = RemindersCalendarDataSource()
    }

    // MARK: - ReminderPresenting

    func yearChanged(to year: Int) {
        title = "Year: \(year)"
    }

}


class RemindersCalendarDataSource: CalendarDataSource {

    // MARK: - CalendarDataSource

    func calendar(_ calendar: Calendar, eventsFor date: Date) -> [String] {
        return [ "Throw away trash", "Wash the dishes" ]
    }

    func calendar(_ calendar: Calendar, add event: String, to date: Date) {
        print("You are going to \(event) on \(date)")
    }
}


class RemindersCalendarDelegate: CalendarDelegate {

    var parentController: ReminderPresenting?

    // MARK: - CalendarDelegate

    func calendarShouldChangeYear(_ calendar: Calendar) -> Bool {
        return true
    }

    func calendar(_ calendar: Calendar, willDisplay year: Int) {
        parentController?.yearChanged(to: year)
    }

    func calendar(_ calendar: Calendar, didSelect date: Date) {
        print("You selected \(date)")
    }
}