Check if UserDefault exists - Swift

问题:

I'm trying to check if the a user default exists, seen below:

func userAlreadyExist() -> Bool {
    var userDefaults : NSUserDefaults = NSUserDefaults.standardUserDefaults()

    if userDefaults.objectForKey(kUSERID) {
        return true
    }

    return false
}

However, no mater what it will always return true even when the object doesn't exist yet? Is this the right way for checking existence ?

回答1:

Astun has a great answer. See below for the Swift 3 version.

func isKeyPresentInUserDefaults(key: String) -> Bool {
    return UserDefaults.standard.object(forKey: key) != nil
}

回答2:

I copy/pasted your code but Xcode 6.1.1 was throwing some errors my way, it ended up looking like this and it works like a charm. Thanks!

func userAlreadyExist(kUsernameKey: String) -> Bool {
    return NSUserDefaults.standardUserDefaults().objectForKey(kUsernameKey) != nil
}

回答3:

Yes this is right way to check the optional have nil or any value objectForKey method returns AnyObject? which is Implicit optional.

So if userDefaults.objectForKey(kUSERID) have any value than it evaluates to true. if userDefaults.objectForKey(kUSERID) has nil value than it evaluates to false.

From swift programming guide

If Statements and Forced Unwrapping You can use an if statement to find out whether an optional contains a value. If an optional does have a value, it evaluates to true; if it has no value at all, it evaluates to false.

Now there is a bug in simulators than after setting key in userDefaults they always remain set no matter you delete your app.You need to reset simulator.

Reset your Simulator check this method before setting key in userDefaults or remove key userDefaults.removeObjectForKey(kUSERID) from userDefaults and it will return NO.On devices it is resolved in iOS8 beta4.

回答4:

This is essentially the same as suggested in other answers but in a more convenient way (Swift 3+):

extension UserDefaults {
    func contains(key: String) -> Bool {
        return UserDefaults.standard.object(forKey: key) != nil
    }
}

回答5:

Simple Code to check whether value stored in UserDefault.

let userdefaults = UserDefaults.standard
if let savedValue = userdefaults.string(forKey: "key"){
   print("Here you will get saved value")       
} else {
   print("No value in Userdefault,Either you can save value here or perform other operation")
   userdefaults.set("Here you can save value", forKey: "key")
}

回答6:

for swift 3.2

func userAlreadyExist(kUsernameKey: String) -> Bool {
    return UserDefaults.standard.object(forKey: kUsernameKey) != nil
}