Add a basic example for month-view

please note, that configure is invoked in two places for a reason, at
least according to the [docs](https://patchthecode.com/jtapplecalendar-home/calendar-from-scratch/):

> These 2 functions should contain the same code, therefore it is wise
> to have a shared function to reduce code duplication. The only
> difference between these two functions should be the first line of code
> (the dequeuing code). Reasons for the 2 functions having the same code
> are found [here under problem#1](patchthecode/JTAppleCalendar#553).

CalendarPOC.xcodeproj/project.pbxproj

@@ -12,6 +12,7 @@
 		B34582F223ED9B98003460AE /* Assets.xcassets in Resources */ = {isa = PBXBuildFile; fileRef = B34582F123ED9B98003460AE /* Assets.xcassets */; };
 		B34582F523ED9B98003460AE /* LaunchScreen.storyboard in Resources */ = {isa = PBXBuildFile; fileRef = B34582F323ED9B98003460AE /* LaunchScreen.storyboard */; };
 		B34582FF23ED9E51003460AE /* JTAppleCalendar in Frameworks */ = {isa = PBXBuildFile; productRef = B34582FE23ED9E51003460AE /* JTAppleCalendar */; };
+		B345830123EDA2CE003460AE /* DayCell.swift in Sources */ = {isa = PBXBuildFile; fileRef = B345830023EDA2CE003460AE /* DayCell.swift */; };
 /* End PBXBuildFile section */
 
 /* Begin PBXFileReference section */
@@ -22,6 +23,7 @@
 		B34582F423ED9B98003460AE /* Base */ = {isa = PBXFileReference; lastKnownFileType = file.storyboard; name = Base; path = Base.lproj/LaunchScreen.storyboard; sourceTree = "<group>"; };
 		B34582F623ED9B98003460AE /* Info.plist */ = {isa = PBXFileReference; lastKnownFileType = text.plist.xml; path = Info.plist; sourceTree = "<group>"; };
 		B34582FC23ED9C83003460AE /* README.md */ = {isa = PBXFileReference; lastKnownFileType = net.daringfireball.markdown; path = README.md; sourceTree = "<group>"; };
+		B345830023EDA2CE003460AE /* DayCell.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = DayCell.swift; sourceTree = "<group>"; };
 /* End PBXFileReference section */
 
 /* Begin PBXFrameworksBuildPhase section */
@@ -58,6 +60,7 @@
 			children = (
 				B34582E823ED9B96003460AE /* AppDelegate.swift */,
 				B34582EC23ED9B96003460AE /* ViewController.swift */,
+				B345830023EDA2CE003460AE /* DayCell.swift */,
 				B34582F123ED9B98003460AE /* Assets.xcassets */,
 				B34582F323ED9B98003460AE /* LaunchScreen.storyboard */,
 				B34582F623ED9B98003460AE /* Info.plist */,
@@ -142,6 +145,7 @@
 			buildActionMask = 2147483647;
 			files = (
 				B34582ED23ED9B96003460AE /* ViewController.swift in Sources */,
+				B345830123EDA2CE003460AE /* DayCell.swift in Sources */,
 				B34582E923ED9B96003460AE /* AppDelegate.swift in Sources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;

CalendarPOC/DayCell.swift

@@ -0,0 +1,37 @@
+//
+//  DayCell.swift
+//  CalendarPOC
+//
+//  Created by Nathan Mattes on 07.02.20.
+//  Copyright © 2020 Nathan Mattes. All rights reserved.
+//
+
+import UIKit
+import JTAppleCalendar
+
+class DayCell: JTACDayCell {
+    static let reuseIdentifier = "CalendarCell"
+
+    let dateLabel: UILabel
+
+    override init(frame: CGRect) {
+
+        dateLabel = UILabel(frame: .zero)
+        dateLabel.translatesAutoresizingMaskIntoConstraints = false
+
+        super.init(frame: frame)
+
+        contentView.addSubview(dateLabel)
+
+        let constraints = [
+            dateLabel.centerXAnchor.constraint(equalTo: contentView.centerXAnchor),
+            dateLabel.centerYAnchor.constraint(equalTo: contentView.centerYAnchor)
+        ]
+
+        NSLayoutConstraint.activate(constraints)
+    }
+
+    required init?(coder aDecoder: NSCoder) {
+        fatalError("init(coder:) has not been implemented")
+    }
+}

CalendarPOC/ViewController.swift

@@ -7,11 +7,26 @@
 //
 
 import UIKit
+import JTAppleCalendar
 
 class ViewController: UIViewController {
+
+    let calendarView: JTACMonthView
 
     init() {
+
+        calendarView = JTACMonthView(frame: .zero)
+        calendarView.scrollDirection = .vertical
+        calendarView.scrollingMode = .stopAtEachCalendarFrame
+        calendarView.translatesAutoresizingMaskIntoConstraints = false
+        calendarView.backgroundColor = .lightGray
+
         super.init(nibName: nil, bundle: nil)
+
+        calendarView.calendarDataSource = self
+        calendarView.calendarDelegate = self
+        calendarView.register(DayCell.self, forCellWithReuseIdentifier: DayCell.reuseIdentifier)
+
     }
 
     required init?(coder: NSCoder) {
@@ -20,10 +35,85 @@ class ViewController: UIViewController {
 
     // MARK: - UIViewController Lifecycle
 
+    override func loadView() {
+        super.loadView()
+
+        view.addSubview(calendarView)
+
+        setupConstraints()
+    }
+
+    private func setupConstraints() {
+        let constraints = [
+            calendarView.topAnchor.constraint(equalToSystemSpacingBelow: view.safeAreaLayoutGuide.topAnchor, multiplier: 1.0),
+            calendarView.leadingAnchor.constraint(equalToSystemSpacingAfter: view.safeAreaLayoutGuide.leadingAnchor, multiplier: 1.0),
+            calendarView.heightAnchor.constraint(equalTo: view.safeAreaLayoutGuide.heightAnchor, multiplier: 0.5),
+            view.safeAreaLayoutGuide.trailingAnchor.constraint(equalToSystemSpacingAfter: calendarView.trailingAnchor, multiplier: 1.0),
+            view.safeAreaLayoutGuide.bottomAnchor.constraint(greaterThanOrEqualToSystemSpacingBelow: calendarView.bottomAnchor, multiplier: 1.0)
+        ]
+        NSLayoutConstraint.activate(constraints)
+    }
+
     override func viewDidLoad() {
         super.viewDidLoad()
 
-        view.backgroundColor = .orange
+        view.backgroundColor = .white
+
+        navigationItem.rightBarButtonItem = UIBarButtonItem(title: "Today", style: .done, target: self, action: #selector(scrollToToday(_:)))
+    }
+
+    //MARK: - Actions
+
+    @objc
+    func scrollToToday(_ sender: Any) {
+        calendarView.scrollToDate(Date())
+    }
+}
+
+extension ViewController: JTACMonthViewDataSource {
+    func configureCalendar(_ calendar: JTACMonthView) -> ConfigurationParameters {
+        let startDate = Date(timeIntervalSinceNow: -60*60*24*365)
+        let endDate = Date(timeIntervalSinceNow: 60*60*24*365)
+
+        return ConfigurationParameters(startDate: startDate, endDate: endDate)
     }
 }
 
+extension ViewController: JTACMonthViewDelegate {
+    func calendar(_ calendar: JTACMonthView, willDisplay cell: JTACDayCell, forItemAt date: Date, cellState: CellState, indexPath: IndexPath) {
+        guard let cell = cell as? DayCell else {
+            NSLog("This should never happen")
+            assert(false)
+        }
+
+        configure(cell, withCellState: cellState)
+    }
+
+    func calendar(_ calendar: JTACMonthView, cellForItemAt date: Date, cellState: CellState, indexPath: IndexPath) -> JTACDayCell {
+        guard let cell = calendarView.dequeueReusableJTAppleCell(withReuseIdentifier: DayCell.reuseIdentifier, for: indexPath) as? DayCell else {
+            NSLog("This should never happen")
+            assert(false)
+        }
+
+        configure(cell, withCellState: cellState)
+        return cell
+    }
+
+    private func configure(_ cell: DayCell, withCellState cellState: CellState) {
+        cell.dateLabel.text = cellState.text
+
+        if cellState.dateBelongsTo == .thisMonth {
+            cell.isHidden = false
+        } else {
+            cell.isHidden = true
+        }
+    }
+
+    func calendar(_ calendar: JTACMonthView, shouldSelectDate date: Date, cell: JTACDayCell?, cellState: CellState, indexPath: IndexPath) -> Bool {
+        return true
+    }
+
+    func calendar(_ calendar: JTACMonthView, didSelectDate date: Date, cell: JTACDayCell?, cellState: CellState, indexPath: IndexPath) {
+        print(date)
+    }
+}