Swift 3：设置时间的内部扩展

Question

我有这个在 Swift 3 中似乎不再工作的内部日期扩展：

internal extension DateComponents {
    func to12pm() {
        self.hour = 12
        self.minute = 0
        self.second = 0
    }
}

错误信息是：

无法分配给属性：'self' 是不可变的

如何在 Swift 3 中实现这一点？

附加信息（如果需要）：由此日期扩展调用：

func endOfWeek(_ weekday: Int) -> Date? {
    guard
        let cal = Calendar.current,
        var comp: DateComponents = (cal as Calendar).components([.weekOfYear], from: self)
        else {
            return nil
    }
    comp.weekOfYear = 1
    comp.day -= 1
    comp.to12pm()
    return (cal as NSCalendar).date(byAdding: comp, to: self.startOfWeek(weekday)!, options: [])!
} 

现在在 Swift 3 中看起来像这样：

func endOfWeek(_ weekday: Int) -> Date? {
    let cal = Calendar.current
    var comp = cal.dateComponents([.weekOfYear], from: self)
    comp.weekOfYear = 1
    comp.day? -= 1
    //This does not have the "comp.to12pm()" that the Swift 2 version did
    return cal.date(byAdding: comp, to: self.startOfWeek(weekday)!)!
}

Answer 1

DateComponents 现在是一个结构，这就是它抛出该错误的原因：

无法分配给属性：'self' 是不可变的

你需要从self创建一个新的var，改变它并返回它如下：

extension DateComponents {
    var to12pm: DateComponents {
        var components = self
        components.hour = 12
        components.minute = 0
        components.second = 0
        components.nanosecond = 0
        return components
    }
}