NSUserDefaults Contains Value Or Not?

Question

How to know whether NSUserDefaults contains any value?How to check whether its empty?

Answer 1

There isn't a way to check whether an object within NSUserDefaults is empty or not. However, you can check whether a value for particular key is nil or not.

NSUserDefaults *prefs = [NSUserDefaults standardUserDefaults];

NSObject * object = [prefs objectForKey:@"your_particular_key"];
if(object != nil){
   //object is there
}

Answer 2

NSUserDefaults *data = [NSUserDefaults standardUserDefaults];  
NSString *string = [data objectForKey:@"yourKey"];
if(string==nil)
NSlog(@"nil")  

Take a look at NSUserDefault documentation

// For saving the values
NSUserDefaults *userDefaults = [NSUserDefaults standardUserDefaults];
// saving an NSString
[userDefaults setObject:@"Ttest" forKey:@"key"];
// --- For Retrieving

NSUserDefaults * userDefaults = [NSUserDefaults standardUserDefaults];
// getting an NSString
NSString *myString = [userDefaults stringForKey:@"key"];

Answer 3

To check whether a specific value is set or not, no matter of its location (global or application's), check the returned value of -[NSUserDefaults objectForKey:]

id obj = [[NSUserDefaults standardUserDefaults] objectForKey:@"My-Key-Name"];
if (obj != nil) {...}

To check if the application (bundle) has any settings stored in user defaults:

NSUserDefaults* sdu = [NSUserDefaults standardUserDefaults];

NSString* bundleId = [[NSBundle mainBundle] bundleIdentifier];
NSDictionary* mainBundleSettings = [sdu persistentDomainForName:bundleId];

NSLog(@"%@", mainBundleSettings);

If you are interested in all possible values for which -[NSUserDefaults objectForKey:] will return something, including system global settings, simply call

NSDictionary* allPossibleSettings = [sdu dictionaryRepresentation];

NSUserDefaults is never empty. It combines global settings, bundle's settings, temporary data and maybe something else. For example, if you call:

[[NSUserDefaults standardUserDefaults] objectForKey:@"NSBoldSystemFont"]

you will get the @"LucidaGrande-Bold" string value which will be taken from global settings, even when your application has never set this value.