快速检查 NSDate 不为零 [英] Swift check NSDate is not nil

问题描述

我从 userDefaults 获取一个值，该值可能为零或可能是 NSDate，因此我想检查一下，因为我需要在该日期采取行动.我将如何正确进行检查?

I'm getting a value from userDefaults which might be nil or might be an NSDate, so I want to check since I need to act on that date. How would I correctly do the check?

        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'
    }

推荐答案

使用可选绑定(if let)和可选类型转换(as?):

Use optional binding (if let) with an optional cast (as?):

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

<小时>

与

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

你强行将返回值转换为 NSDate，所以这如果该值未设置或不是 NSDate，将在运行时崩溃.同样发生在

you are forcefully casting the return value to NSDate, so this will crash at runtime if the value is not set or not an NSDate. The same happens in

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

将表达式分配给可选的 NSDate? 并没有帮助，在评估右侧时已经发生崩溃.

It does not help that the expression is assigned to an optional NSDate?, the crash already occurs when evaluating the right-hand side. The compiler error at

let timeSinceQuit:Double = timeAtQuit.timeIntervalSinceNow

发生是因为 timeAtQuit 在这里是可选的，所以你必须打开包装

occurs because timeAtQuit is an optional here, so you would have to unwrap it

let timeSinceQuit:Double = timeAtQuit!.timeIntervalSinceNow

这篇关于快速检查 NSDate 不为零的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！