Swift 检查 NSDate 是否为 nil

Question

我从 userDefaults 得到一个值，它可能是 nil 或者可能是一个 NSDate，所以我想检查一下，因为我需要在那个日期采取行动。我该如何正确地进行检查？

        let timeAtQuit: NSDate = (userDefaults.objectForKey("timeAtQuit") as NSDate)

    if(timeAtQuit){ Type 'NSDate' does not conform to protocol 'BooleanType'

    if(timeAtQuit != nil){ // 'NSDate' is not convertible to 'UInt8'

    another attempt:
        var timeAtQuit:NSDate? = (userDefaults.objectForKey("timeAtQuit") as NSDate)

    if(timeAtQuit != nil){
        let timeSinceQuit:Double = timeAtQuit.timeIntervalSinceNow // 'NSDate?' does not have a member named 'timeIntervalSinceNow'
    }

Answer 1

将可选绑定 (if let) 与可选转换 (as?) 结合使用：

if let timeAtQuit = userDefaults.objectForKey("timeAtQuit") as? NSDate {
    println(timeAtQuit)
} else {
    // default value is not set or not an NSDate
}

有

let timeAtQuit: NSDate = (userDefaults.objectForKey("timeAtQuit") as NSDate)

您正在强行将 return 值转换为 NSDate，所以这如果该值未设置或不是 NSDate，将在运行时崩溃。同样的情况发生在

var timeAtQuit:NSDate? = (userDefaults.objectForKey("timeAtQuit") as NSDate)

将表达式分配给可选 NSDate? 并没有帮助，在评估右侧时已经发生崩溃。

处的编译器错误

let timeSinceQuit:Double = timeAtQuit.timeIntervalSinceNow

发生是因为 timeAtQuit 在这里是可选的，所以你必须打开它

let timeSinceQuit:Double = timeAtQuit!.timeIntervalSinceNow

Swift 检查 NSDate 是否为 nil

Swift check NSDate is not nil

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