近期需要寫一個交互有點DT的日歷控件，具體交互細節這裡略過不表。

不過再怎麼復雜的控件，也是由基礎的零配件組裝起來的，這裡最基本的就是日歷控件。

先上圖：

從圖中可以看出日歷控件就是由一個個小方塊組成的，每一行有7個小方塊，分別表示一周的星期天到星期六。

給定一個月份，我們首先需要知道這個月有多少周。那麼如何確定一個月有多少周呢？

我是這麼想的，在NSDate上做擴展：

@interface NSDate (WQCalendarLogic)

0. 首先需要知道這個月有多少天：

- (NSUInteger)numberOfDaysInCurrentMonth
{
    // 頻繁調用 [NSCalendar currentCalendar] 可能存在性能問題
    return [[NSCalendar currentCalendar] rangeOfUnit:NSDayCalendarUnit inUnit:NSMonthCalendarUnit forDate:self].length;
}

1. 確定這個月的第一天是星期幾。這樣就能知道給定月份的第一周有幾天：

- (NSDate *)firstDayOfCurrentMonth
{
    NSDate *startDate = nil;
    BOOL ok = [[NSCalendar currentCalendar] rangeOfUnit:NSMonthCalendarUnit startDate:&startDate interval:NULL forDate:self];
    NSAssert1(ok, @"Failed to calculate the first day of the month based on %@", self);
    return startDate;
}

- (NSUInteger)weeklyOrdinality
{
    return [[NSCalendar currentCalendar] ordinalityOfUnit:NSDayCalendarUnit inUnit:NSWeekCalendarUnit forDate:self];
}

2. 減去第一周的天數，剩余天數除以7，得到倍數和余數：

- (NSUInteger)numberOfWeeksInCurrentMonth
{
    NSUInteger weekday = [[self firstDayOfCurrentMonth] weeklyOrdinality];
    
    NSUInteger days = [self numberOfDaysInCurrentMonth];
    NSUInteger weeks = 0;
    
    if (weekday > 1) {
        weeks += 1, days -= (7 - weekday + 1);
    }
    
    weeks += days / 7;
    weeks += (days % 7 > 0) ? 1 : 0;
    
    return weeks;
}

到這裡，就可以知道一個月有多少行多少列，從而計算出需要多少個小方塊來展示：

@interface WQCalendarTileView : UIView

這些小方塊用一個大方塊來承載：

@interface WQCalendarGridView : UIView

@property (nonatomic, weak) id dataSource;
@property (nonatomic, weak) id delegate;

- (void)reloadData;

和UITableView類似，當WQCalendarGridView調用reloadData接口時，會開始進行布局。而布局所需要的信息由dataSource和delegate提供：

@class WQCalendarGridView;

@protocol WQCalendarGridViewDataSource 

@required

- (NSUInteger)numberOfRowsInGridView:(WQCalendarGridView *)gridView;

- (WQCalendarTileView *)gridView:(WQCalendarGridView *)gridView tileViewForRow:(NSUInteger)row column:(NSUInteger)column;

@optional

- (CGFloat)heightForRowInGridView:(WQCalendarGridView *)gridView;

@end

@protocol WQCalendarGridViewDelegate 

- (void)gridView:(WQCalendarGridView *)gridView didSelectAtRow:(NSUInteger)row column:(NSUInteger)column;

@end

每一行的高度，heightForRow，我比較傾向於由dataSource提供 ：）

第一個dataSource方法上面已經可以計算出來了，第二個dataSource方法需要對每一個tile進行配置，比如非當前月的以灰色展示，那麼我們就需要知道當前月展示中包含的 上個月殘留部分，和 下個月的開頭部分：

#pragma mark - method to calculate days in previous, current and the following month.

- (void)calculateDaysInPreviousMonthWithDate:(NSDate *)date
{
    NSUInteger weeklyOrdinality = [[date firstDayOfCurrentMonth] weeklyOrdinality];
    NSDate *dayInThePreviousMonth = [date dayInThePreviousMonth];
    
    NSUInteger daysCount = [dayInThePreviousMonth numberOfDaysInCurrentMonth];
    NSUInteger partialDaysCount = weeklyOrdinality - 1;
    
    NSDateComponents *components = [dayInThePreviousMonth YMDComponents];
    
    self.daysInPreviousMonth = [NSMutableArray arrayWithCapacity:partialDaysCount];
    for (int i = daysCount - partialDaysCount + 1; i < daysCount + 1; ++i) {
        WQCalendarDay *calendarDay = [WQCalendarDay calendarDayWithYear:components.year month:components.month day:i];
        [self.daysInPreviousMonth addObject:calendarDay];
        [self.calendarDays addObject:calendarDay];
    }
}

- (void)calculateDaysInCurrentMonthWithDate:(NSDate *)date
{
    NSUInteger daysCount = [date numberOfDaysInCurrentMonth];
    NSDateComponents *components = [date YMDComponents];
    
    self.daysInCurrentMonth = [NSMutableArray arrayWithCapacity:daysCount];
    for (int i = 1; i < daysCount + 1; ++i) {
        WQCalendarDay *calendarDay = [WQCalendarDay calendarDayWithYear:components.year month:components.month day:i];
        [self.daysInCurrentMonth addObject:calendarDay];
        [self.calendarDays addObject:calendarDay];
    }
}

- (void)calculateDaysInFollowingMonthWithDate:(NSDate *)date
{
    NSUInteger weeklyOrdinality = [[date lastDayOfCurrentMonth] weeklyOrdinality];
    if (weeklyOrdinality == 7) return ;
    
    NSUInteger partialDaysCount = 7 - weeklyOrdinality;
    NSDateComponents *components = [[date dayInTheFollowingMonth] YMDComponents];
    
    self.daysInFollowingMonth = [NSMutableArray arrayWithCapacity:partialDaysCount];
    for (int i = 1; i < partialDaysCount + 1; ++i) {
        WQCalendarDay *calendarDay = [WQCalendarDay calendarDayWithYear:components.year month:components.month day:i];
        [self.daysInFollowingMonth addObject:calendarDay];
        [self.calendarDays addObject:calendarDay];
    }
}

到此，就可以順利地展現出給定月份的日歷控件了。最近項目比較忙，隨手記一篇 ：）