Swift NSDate 扩展错误 : Mutating isn't valid on methods in classes or class-bound protocols

Question

我正在尝试扩展 NSDate 但出现两个错误:

extension NSDate { //'mutating' isn't valid on methods in classes or class-bound protocols
    mutating func addMonth() {
        let calendar = NSCalendar.currentCalendar()
        let component = NSDateComponents()
        component.month = 1
        self = calendar.dateByAddingComponents(component, toDate: self, options: []) //Cannot assign to value: 'self' is immutable
    }
} 

我的猜测是 NSDate 是一个类而不是 Swift 类型，并且错误状态是 mutating 在类中的方法上是不可能的。如果我返回该值并为其分配一切正常，但我想知道这不起作用的确切原因以及是否有更好的解决方法。

Answer 1

NSDate objects encapsulate a single point in time, independent of any particular calendrical system or time zone. Date objects are immutable, representing an invariant time interval relative to an absolute reference date (00:00:00 UTC on 1 January 2001).

NSDate Documentation

因为 NSDate 对象是不可变的，所以您不能随意将一个月加到一。您可以修改您的扩展以从现有日期返回一个新的 NSDate 对象，并添加一个月:

extension NSDate {
    func addMonth() -> NSDate? {
        let calendar = NSCalendar.currentCalendar()
        let component = NSDateComponents()
        component.month = 1
        let newDate = calendar.dateByAddingComponents(component, toDate: self, options: [])
        return newDate
    }
}