Swift NSUserDefaults NSArray using objectForKey

问题

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

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
}()

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

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.

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
}()

The error has now moved to the return line.

回答1:

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()
}

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.

来源：https://stackoverflow.com/questions/24598991/swift-nsuserdefaults-nsarray-using-objectforkey