import UIKit

class CalendarViewModel: NSObject {

    let dataProvider: EventCalendarProvider

    weak var pager: UIPageViewController?

    var calendar: EventCalendar?

    init(dataProvider: EventCalendarProvider) {
        self.dataProvider = dataProvider
    }

    func setAsDatSource(of pageViewController: UIPageViewController) {
        pager = pageViewController
        pager?.dataSource = self
    }

    func loadCalendar() {
        dataProvider.retrieve { [weak self] result in
            guard let strongSelf = self else { return }
            switch result {
            case .success(let calendar):
                strongSelf.calendar = calendar
                let firstController = strongSelf.dayScheduleViewController(for: 0) ?? UIViewController()
                strongSelf.pager?.setViewControllers([firstController], direction: .forward, animated: false)
            case .failure(let error):
                print(error)
               //show error
            }
        }
    }

    private func dayScheduleViewController(for index: Int) -> UIViewController? {
        guard let calendar = calendar else { return nil }
        guard (0..<calendar.days.count).contains(index) else { return nil }
        let bundle = Bundle(for: Self.self)
        let storyboard = UIStoryboard(name: "Main", bundle: bundle)
        let day = calendar.days[index]
        return storyboard.instantiateViewController(identifier: "DayScheduleViewController") { (coder) -> UIViewController? in
            let viewModel = DayScheduleViewModel(daySchedule: day)
            return DayScheduleViewController(coder: coder, viewModel: viewModel)
        }
    }

}

extension CalendarViewModel: UIPageViewControllerDataSource {

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
        guard let nextController = viewController as? DayScheduleViewController else { return nil }
        guard let calendar = calendar else { return nil }
        let nextDay = nextController.viewModel.daySchedule
        guard let index = calendar.days.firstIndex(of: nextDay) else { return nil }
        return dayScheduleViewController(for: index - 1)
    }

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        guard let previousController = viewController as? DayScheduleViewController else { return nil }
        guard let calendar = calendar else { return nil }
        let previousDay = previousController.viewModel.daySchedule
        guard let index = calendar.days.firstIndex(of: previousDay) else { return nil }
        return dayScheduleViewController(for: index + 1)
    }

}