Swift NSUserDefaults NSArray 使用 objectForKey [英] Swift NSUserDefaults NSArray using objectForKey

问题描述

我对 Swift 还很陌生，但我已经陷入了困境.

I'm pretty new to Swift, and I've managed to get pretty stuck.

我正在尝试从 NSUserDefaults 检索数据并将其存储在一个数组 (tasks) 中:

I'm trying to retrieve data from NSUserDefaults and store it in an array (tasks):

@lazy var tasks: NSArray = {
    let def = NSUserDefaults.standardUserDefaults()
    let obj: AnyObject? = def.objectForKey("tasks")
    return obj as NSArray
}()

我得到的只是一个警告:EXE_BAD_INSTRUCTION 在第 3 行.

All I'm getting is a warning: EXE_BAD_INSTRUCTION on line 3.

还要注意，我实际上还没有设置任何数据，但我的目标是如果没有数据，我希望数组为空.我将使用这些数据来填充表格视图.

Also to note that I haven't actually set any data yet, but what I'm aiming for is that if there is no data, I want the array to be empty. I'll be using the data to populate a table view.

现在使用 var 而不是常量:

Now using a var instead of a constant:

@lazy var tasks: NSArray = {
    let def = NSUserDefaults.standardUserDefaults()
    var obj: AnyObject? = {
        return def.objectForKey("tasks")
    }()
    return obj as NSArray
}()

错误现在已移至返回行.

The error has now moved to the return line.

推荐答案

我认为这里的问题是您试图将 nil 转换为非可选类型并返回它.Swift 不允许这样做.解决此问题的最佳方法如下:

I think the problem here is that you are attempting to cast nil to a non-optional type and return it. Swift does not allow that. The best way to solve this would be the following:

@lazy tasks: NSArray = {
    let defaults = NSUserDefaults.standardUserDefaults()
    if let array = defaults.arrayForKey("tasks") as? NSArray {
        return array
    }
    return NSArray()
}

将 Swift 的 if let 语法与 as? 运算符结合使用，可以让您在一行中进行赋值和安全转换.由于您的方法不返回可选项，因此如果转换失败，您必须返回一个有效值.

Using Swift's if let syntax combined with the as? operator lets you assign and safe cast in one line. Since your method does not return an optional, you must return a valid value if that cast fails.

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