How to deal with non-optional values in NSUserDefa

问题:

To get a value from NSUserDefaults I would do something like this:

let userDefaults = NSUserDefaults.standardUserDefaults()
if let value = userDefaults.objectForKey(key) {
    print(value)
}

However, these methods do not return optionals:

• boolForKey (defaults to false if not set)
• integerForKey (defaults to 0 if not set)
• floatForKey (defaults to 0.0 if not set)
• doubleForKey (defaults to 0.0 if not set)

In my app I want to used the saved value of an integer if it has been previously set. And if it wasn't previously set, I want to use my own default value. The problem is that 0 is a valid integer value for my app, so I have no way of knowing if 0 is a previously saved value or if it is the default unset value.

How would I deal with this?

回答1:

Register the user defaults in AppDelegate, the best place is awakeFromNib or even init. You can provide custom default values for every key.

 override init()
 {
   let defaults = NSUserDefaults.standardUserDefaults()
   let defaultValues = ["key1" : 12, "key2" : 12.6]
   defaults.registerDefaults(defaultValues)
   super.init()
 }

Then you can read the values from everywhere always as non-optionals

let defaults = NSUserDefaults.standardUserDefaults()
myVariable1 = defaults.integerForKey("key1")
myVariable2 = defaults.doubleForKey("key2")

回答2:

You can use objectForKey (which returns an optional) and use as? to optionally cast it to an Int.

if let myInteger = UserDefaults.standard.object(forKey: key) as? Int {
    print(myInteger)
} else {
    print("no value was set for this key")
}

The solutions for Bool, Float, and Double would work the same way.

Note:

I recommend going with the accepted answer and setting default values rather than casting a generic object if possible.

回答3:

You could register defaults for your app that aren't legitimate values using NSUserDefaults.standardUserDefaults.registerDefaults()

回答4:

This article describes very well how to "make" them Optional http://radex.io/swift/nsuserdefaults/

extension NSUserDefaults {
    class Proxy {
        var object: NSObject? {
            return defaults.objectForKey(key) as? NSObject
        }

        var number: NSNumber? {
            return object as? NSNumber
        }

        var int: Int? {
            return number?.integerValue
        }

        var bool: Bool? {
            return number?.boolValue
        }
    }
}

回答5:

Use this code

let userDefaults = NSUserDefaults.standardUserDefaults()
if var value : Int = userDefaults.objectForKey(key) {
    print(value)
}