swift - Swift 中日期之间的天数

Question

我在 Swift 的 Calendar dateComponents(_:from:to:) 函数中发现了一个非常奇怪的行为。

let a = Calendar.current.date(byAdding: DateComponents(day: 7), to: Date())!
let n1 = Calendar.current.dateComponents([.day], from: Date(), to: a).day!
let n2 = Calendar.current.dateComponents([.day], from: Date(), to: Calendar.current.date(byAdding: DateComponents(day: 7), to: Date())!).day!
print(a)
print(Calendar.current.date(byAdding: DateComponents(day: 7), to: Date())!)
print(n1)
print(n2)

打印:

2018-04-21 19:58:16 +0000
2018-04-21 19:58:16 +0000
6
7

我错过了什么？

Answer 1

正如 Martin 在评论中所说，问题在于您在每一步都会获得 Date() 的新值，而最后一个值比第一个值晚了几分之一秒。

请注意，此代码给出了预期的输出:

let now = Date()
let a = Calendar.current.date(byAdding: DateComponents(day: 7), to: now)!
let n1 = Calendar.current.dateComponents([.day], from: now, to: a).day!
let n2 = Calendar.current.dateComponents([.day], from: now, to: Calendar.current.date(byAdding: DateComponents(day: 7), to: now)!).day!
print(a)
print(Calendar.current.date(byAdding: DateComponents(day: 7), to: now)!)
print(n1)
print(n2)

它之所以有效，是因为所有日期都是完全相同的值，而不是相差几分之一秒的值。