CalendarDraw.swift

import UIKit

struct Day {
    /// Date 인스턴스.
    let date: Date
    
    /// 화면에 표시될 숫자.
    /// 예) Date 인스턴스가 2022년 1월 25일이라면 -> 25
    let number: String
    
    /// 이 날짜가 선택되었는지 여부.
    let isSelected: Bool
    
    /// 이 날짜가 현재 달 내에 있는지 추적.
    /// 예) 1월 달력을 그리고자 할 떄 Date 인스턴스가 1월 25일이라면 true, 2월 1일이라면 false
    let isWithinDisplayedMonth: Bool
}

struct MonthMetadata {
    /// 해당 달의 총 일수, 예를 들어 1월은 31일까지 있으므로 31
    let numberOfDays: Int
    
    /// 해당 달의 첫 Date
    let firstDay: Date
    
    /// 해당 달의 첫 Date가 무슨 요일인지 반환, 일 ~ 토 => 1 ~ 7
    /// 예) 수요일이라면 4
    let firstDayWeekday: Int
}

enum CalendarDataError: Error {
    case metadataGeneration
}

// MARK: - Member Variables

private let baseDate = Date()

private let calendar = Calendar(identifier: .gregorian)

private let selectedDate: Date = baseDate

// lazy
private var dateFormatter_OnlyD: DateFormatter = {
    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = "d"
    return dateFormatter
}()

// lazy
private var dateFormatter_CalendarTitle: DateFormatter = {
    let dateFormatter = DateFormatter()
    dateFormatter.calendar = Calendar(identifier: .gregorian)
    dateFormatter.locale = Locale.autoupdatingCurrent
    dateFormatter.setLocalizedDateFormatFromTemplate("MMMM y")
    return dateFormatter
}()


// MARK: - Generating a Month’s Metadata
 
///  Date를 기준으로 월별 메타데이터인 MonthMetaData 인스턴스를 생성.
func monthMetadata(for baseDate: Date) throws -> MonthMetadata {
    // You ask the calendar for the number of days in baseDate‘s month, then you get the first day of that month.
    guard
        let numberOfDaysInMonth = calendar.range(of: .day, in: .month, for: baseDate)?.count,
        let firstDayOfMonth = calendar.date(from: calendar.dateComponents([.year, .month], from: baseDate))
    else {
        // Both of the previous calls return optional values. If either returns nil, the code throws an error and returns.
        throw CalendarDataError.metadataGeneration
    }
    
    // You get the weekday value, a number between one and seven that represents which day of the week the first day of the month falls on.
    // weekday: 주일, 평일: 일요일 이외의 6일간을 가리키는 경우와 토·일요일 이외의 5일간을 가리키는 경우가 있음.
    let firstDayWeekday: Int = calendar.component(.weekday, from: firstDayOfMonth)
    
    // Finally, you use these values to create an instance of MonthMetadata and return it.
    return MonthMetadata(
        numberOfDays: numberOfDaysInMonth,
        firstDay: firstDayOfMonth,
        firstDayWeekday: firstDayWeekday)
}

/// Adds or subtracts an offset from a Date to produce a new one, and return its result.
func generateDay(offsetBy dayOffset: Int, for baseDate: Date, isWithinDisplayedMonth: Bool) -> Day {
    let date = calendar.date(byAdding: .day, value: dayOffset, to: baseDate) ?? baseDate
    
    return Day(
        date: date,
        number: dateFormatter_OnlyD.string(from: date),
        isSelected: calendar.isDate(date, inSameDayAs: selectedDate),
        isWithinDisplayedMonth: isWithinDisplayedMonth)
}

/// Takes the first day of the displayed month and returns an array of Day objects.
func generateStartOfNextMonth(using firstDayOfDisplayedMonth: Date) -> [Day] {
    // Retrieve the last day of the displayed month. If this fails, you return an empty array.
    guard let lastDayInMonth = calendar.date(
        byAdding: DateComponents(month: 1, day: -1),
        to: firstDayOfDisplayedMonth) else {
        return []
    }
    
    // Calculate the number of extra days you need to fill the last row of the calendar.
    // For instance, if the last day of the month is a Saturday, the result is zero and you return an empty array.
    let additionalDays = 7 - calendar.component(.weekday, from: lastDayInMonth)
    guard additionalDays > 0 else {
        return []
    }
    
    /*
     Create a Range<Int> from one to the value of additionalDays, as in the previous section.
     Then, it transforms this into an array of Days.
     This time, generateDay(offsetBy:for:isWithinDisplayedMonth:) adds the current day in the loop to lastDayInMonth
     to generate the days at the beginning of the next month.
     */
    let days: [Day] = (1...additionalDays)
        .map {
            generateDay(offsetBy: $0, for: lastDayInMonth, isWithinDisplayedMonth: false)
        }
    
    return days
}

/// Takes in a Date and returns an array of Days.
func generateDaysInMonth(for baseDate: Date) -> [Day] {
    // Retrieve the metadata you need about the month, using monthMetadata(for:).
    // If something goes wrong here, the app can’t function. As a result, it terminates with a fatalError.
    guard let metadata = try? monthMetadata(for: baseDate) else {
        fatalError("An error occurred when generating the metadata for \(baseDate)")
    }
    
    let numberOfDaysInMonth = metadata.numberOfDays
    let offsetInInitialRow = metadata.firstDayWeekday
    let firstDayOfMonth = metadata.firstDay
    
    /*
     If a month starts on a day other than Sunday, you add the last few days from the previous month at the beginning.
     This avoids gaps in a month’s first row. Here, you create a Range<Int> that handles this scenario.
     For example, if a month starts on Friday, offsetInInitialRow would add five extra days to even up the row.
     You then transform this range into [Day], using map(_:).
     */
    var days: [Day] = (1..<(numberOfDaysInMonth + offsetInInitialRow))
        .map { day in
            // Check if the current day in the loop is within the current month or part of the previous month.
            let isWithinDisplayedMonth = day >= offsetInInitialRow
            
            // Calculate the offset that day is from the first day of the month. If day is in the previous month, this value will be negative.
            let dayOffset = isWithinDisplayedMonth ? day - offsetInInitialRow : -(offsetInInitialRow - day)
            
            // Call generateDay(offsetBy:for:isWithinDisplayedMonth:), which adds or subtracts an offset from a Date to produce a new one, and return its result.
            return generateDay(offsetBy: dayOffset, for: firstDayOfMonth, isWithinDisplayedMonth: isWithinDisplayedMonth)
        }
    
    days += generateStartOfNextMonth(using: firstDayOfMonth)
    
    return days
}

// MARK: - 달력 그리기
let targetMonths = [
    calendar.date(byAdding: .month, value: -1, to: baseDate),
    baseDate,
    calendar.date(byAdding: .month, value: +1, to: baseDate),
]

for month in targetMonths {
    guard let baseDate = month else { fatalError() }
    
    // lazy
    let days = generateDaysInMonth(for: baseDate)
    
    let lineText = "-------------------------------"
    
    
    // 제목
    print("\(dateFormatter_CalendarTitle.string(from: baseDate))")
    print()

    // 요일
    let weekdayText = ["일", "월", "화", "수", "목", "금", "토"]
    print(weekdayText.joined(separator: "\t"))
    print(lineText)

    // 숫자
    for (index, day) in days.enumerated() {
        print(day.isWithinDisplayedMonth ? day.number : "(\(day.number))", terminator: "\t")
        if (index + 1) % 7 == 0 {
            print("\n\(lineText)")
        }
    }
    
    print("\n\n")
}