WIP: Replacing the calendar after DayView initializing

Source/DayView.swift

@@ -50,8 +50,8 @@ public final class DayView: UIView, TimelinePagerViewDelegate {
     }
   }
 
-  public let dayHeaderView: DayHeaderView
-  public let timelinePagerView: TimelinePagerView
+  public private(set) var dayHeaderView: DayHeaderView
+  public private(set) var timelinePagerView: TimelinePagerView
 
   public var state: DayViewState? {
     didSet {
@@ -60,7 +60,23 @@ public final class DayView: UIView, TimelinePagerViewDelegate {
     }
   }
 
-  public var calendar: Calendar = Calendar.autoupdatingCurrent
+  public var calendar: Calendar = Calendar.autoupdatingCurrent {
+    didSet { // TODO: willSet?
+      dayHeaderView.calendar = self.calendar
+      timelinePagerView.calendar = self.calendar
+
+      // recalculate the selectedDate taking into account
+      // the difference between new and old timezones
+      let oldCalendar = state!.calendar
+      let selectedDate = (self.state?.selectedDate ?? Date())
+        .dateOnly(calendar: calendar, oldCalendar: oldCalendar)
+
+      // setting a new state
+      let newState = DayViewState(date: selectedDate, calendar: calendar)
+      state = newState
+      newState.move(to: selectedDate) // TODO: Why I added this :I?
+    }
+  }
 
   private var style = CalendarStyle()
 

Source/DayViewState.swift

@@ -6,6 +6,7 @@ public protocol DayViewStateUpdating: AnyObject {
 
 public final class DayViewState {
   public private(set) var calendar: Calendar
+//    willSet {} // TODO: I think this needs to be implemented, cause timezone can change without creating a new state object
   public private(set) var selectedDate: Date
   private var clients = [DayViewStateUpdating]()
 
@@ -17,6 +18,7 @@ public final class DayViewState {
 
   public func move(to date: Date) {
     let date = date.dateOnly(calendar: calendar)
+//    if (date == selectedDate) { return } // TODO: If necessary? Just trying to make sure that willMoveTo not be called if we are already on this page
     notify(clients: clients, moveTo: date)
     selectedDate = date
   }

Source/Extensions/Date+DateOnly.swift

@@ -14,4 +14,12 @@ extension Date {
     let returnValue = calendar.date(from: newComponents)
     return returnValue!
   }
+
+  /// Cuts off the time, leaving the beginning of the day for the new time zone
+  func dateOnly(calendar: Calendar, oldCalendar: Calendar) -> Date {
+    var newDate = self.dateOnly(calendar: oldCalendar)
+    let diff = oldCalendar.timeZone.secondsFromGMT() - calendar.timeZone.secondsFromGMT()
+    newDate.addTimeInterval(TimeInterval(diff))
+    return newDate
+  }
 }

Source/Header/DayHeaderView.swift

@@ -4,7 +4,13 @@ import DateToolsSwift
 
 public final class DayHeaderView: UIView, DaySelectorDelegate, DayViewStateUpdating, UIPageViewControllerDataSource, UIPageViewControllerDelegate {
   public private(set) var daysInWeek = 7
-  public let calendar: Calendar
+  public var calendar: Calendar {
+    didSet {
+      daySymbolsView.calendar = calendar
+      swipeLabelView.calendar = calendar
+      configure() // TODO: Should I do it that way or better just to set a new vc without adding subview (like in state#didSet)?
+    }
+  }
 
   private var style = DayHeaderStyle()
   private var currentSizeClass = UIUserInterfaceSizeClass.compact
@@ -16,6 +22,14 @@ public final class DayHeaderView: UIView, DaySelectorDelegate, DayViewStateUpdat
     didSet {
       state?.subscribe(client: self)
       swipeLabelView.state = state
+
+      // Fixes the "jump" of the pager from today to the selected date. This is noticeable
+      // when we are on a date outside of today's week, and try to change the time zone.
+      let previousSelectedDate = state!.selectedDate // day selected before timezone change
+      let vc = makeSelectorController(startDate: beginningOfWeek(previousSelectedDate))
+      vc.selectedDate = previousSelectedDate
+      currentWeekdayIndex = vc.selectedIndex
+      pagingViewController.setViewControllers([vc], direction: .forward, animated: false, completion: nil)
     }
   }
 

Source/Header/DaySymbolsView.swift

@@ -3,7 +3,15 @@ import UIKit
 
 public final class DaySymbolsView: UIView {
   public private(set) var daysInWeek = 7
-  private var calendar = Calendar.autoupdatingCurrent
+  public var calendar = Calendar.autoupdatingCurrent {
+    didSet {
+      // TODO: I honestly don't know if I should be doing this. Changes to
+      // the timezone should not affect the displayed days of the week,
+      // however, just in case, I am reconfiguring this view
+      configure()
+      setNeedsLayout()
+    }
+  }
   private var labels = [UILabel]()
   private var style: DaySymbolsStyle = DaySymbolsStyle()
 

Source/Header/SwipeLabelView.swift

@@ -7,7 +7,13 @@ public final class SwipeLabelView: UIView, DayViewStateUpdating {
     case Backward
   }
 
-  public private(set) var calendar = Calendar.autoupdatingCurrent
+  public var calendar = Calendar.autoupdatingCurrent {
+    didSet {
+      // I doubt that someone will use this view without a state, but
+      // I consider it necessary to update text label for such a situation
+      updateLabelText()
+    }
+  }
   public weak var state: DayViewState? {
     willSet(newValue) {
       state?.unsubscribe(client: self)

Source/Timeline/TimelinePagerView.swift

@@ -20,7 +20,17 @@ public final class TimelinePagerView: UIView, UIGestureRecognizerDelegate, UIScr
   public weak var dataSource: EventDataSource?
   public weak var delegate: TimelinePagerViewDelegate?
 
-  public private(set) var calendar: Calendar = Calendar.autoupdatingCurrent
+  public var calendar: Calendar = Calendar.autoupdatingCurrent {
+    didSet {
+      // changing timezone in loaded pages
+      pagingViewController.children.forEach {
+        let vc = $0 as! TimelineContainerController
+        let oldCalendar = vc.timeline.calendar
+        vc.timeline.date = vc.timeline.date.dateOnly(calendar: calendar, oldCalendar: oldCalendar)
+        vc.timeline.calendar = calendar
+      }
+    }
+  }
 
   public var timelineScrollOffset: CGPoint {
     // Any view is fine as they are all synchronized
@@ -173,7 +183,11 @@ public final class TimelinePagerView: UIView, UIGestureRecognizerDelegate, UIScr
 
   private func updateTimeline(_ timeline: TimelineView) {
     guard let dataSource = dataSource else {return}
-    let date = timeline.date.dateOnly(calendar: calendar)
+    // I don't know under what circumstances, but I know for sure that
+    // a situation is possible when these calendars have different time
+    // zones. It is in this case that the call below will prevent a bug
+    // that leads to "jumps" after days when you swipe to the left
+    let date = timeline.date.dateOnly(calendar: calendar, oldCalendar: timeline.calendar)
     let events = dataSource.eventsForDate(date)
     let day = TimePeriod(beginning: date,
                          chunk: TimeChunk.dateComponents(days: 1))
@@ -437,7 +451,7 @@ public final class TimelinePagerView: UIView, UIGestureRecognizerDelegate, UIScr
   // MARK: UIPageViewControllerDataSource
 
   public func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {
-    guard let containerController = viewController as? TimelineContainerController  else {return nil}
+    guard let containerController = viewController as? TimelineContainerController else {return nil}
     let previousDate = containerController.timeline.date.add(TimeChunk.dateComponents(days: -1), calendar: calendar)
     let vc = configureTimelineController(date: previousDate)
     let offset = (pageViewController.viewControllers?.first as? TimelineContainerController)?.container.contentOffset
@@ -446,7 +460,7 @@ public final class TimelinePagerView: UIView, UIGestureRecognizerDelegate, UIScr
   }
 
   public func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
-    guard let containerController = viewController as? TimelineContainerController  else {return nil}
+    guard let containerController = viewController as? TimelineContainerController else {return nil}
     let nextDate = containerController.timeline.date.add(TimeChunk.dateComponents(days: 1), calendar: calendar)
     let vc = configureTimelineController(date: nextDate)
     let offset = (pageViewController.viewControllers?.first as? TimelineContainerController)?.container.contentOffset
@@ -459,7 +473,19 @@ public final class TimelinePagerView: UIView, UIGestureRecognizerDelegate, UIScr
   public func pageViewController(_ pageViewController: UIPageViewController, didFinishAnimating finished: Bool, previousViewControllers: [UIViewController], transitionCompleted completed: Bool) {
     guard completed else {return}
     if let timelineContainerController = pageViewController.viewControllers?.first as? TimelineContainerController {
-      let selectedDate = timelineContainerController.timeline.date
+      // Unfortunately, I cannot explain this change in detail. I fiddled
+      // with a bug in which some days were skipped when swiping, completely
+      // desperate and wrote this. To my surprise, this solved the problem.
+      // I'll do some research on the reasons later.
+
+      // Попытка пофиксить сбой хедера с перескоком через число
+      var selectedDate = timelineContainerController.timeline.date
+      selectedDate = selectedDate.dateOnly(calendar: self.state!.calendar, oldCalendar: timelineContainerController.container.timeline.calendar)
+
+      // попытка пофиксить баг с уменьшением и свайпом
+      timelineContainerController.timeline.date = selectedDate // Я хз как и какие последствия оно имеет, но это сработало
+      timelineContainerController.timeline.calendar = self.state!.calendar
+
       delegate?.timelinePager(timelinePager: self, willMoveTo: selectedDate)
       state?.client(client: self, didMoveTo: selectedDate)
       scrollToFirstEventIfNeeded()