iPhone - How to detect if a key exists in NSUserDefaults standardUserDefaults

问题

Using NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];,

I use calls like BOOL boolFromPrefs = [defaults boolForKey:@"theBoolKey"]; To get a saved BOOL value.

If the key is not found, NO is returned (default behaviour of boolForKey).
But... NO can be a saved setting. Same thing for intForKey

So how can I test if a key exists before trying to get its value ?

回答1:

Do it the right way and register default values.

NSDictionary *userDefaultsDefaults = @{
    @"SomeKey": @NO,
    @"AnotherKey": @"FooBar",
    @"NumberKey": @0,
};
[NSUserDefaults.standardUserDefaults registerDefaults:userDefaultsDefaults];

do this before you use anything from NSUserDefaults. The beginning of application:didFinishLaunchingWithOptions: is a safe place.

You have to register the defaults each time the app launches. NSUserDefaults only stores values that have been explicitly set.

If you use default values you don't have to use a check for a "isFirstLaunch" key, like suggested in other answers.
This will help you when you roll out an update and you want to change the default value for a NSUserDefaults item.

回答2:

Check if the object exists before conversion to a BOOL.

if ([defaults objectForKey:@"theBoolKey"] != nil) {
    boolFromPrefs = [defaults boolForKey:@"theBoolKey"];
} else {
    boolFromPrefs = DEFAULT_BOOL_VALUE;
}

回答3:

I did it this way:

NSUserDefaults *prefs = NSUserDefaults.standardUserDefaults;
if ([[prefs dictionaryRepresentation].allKeys containsObject:@"yourKey"]) {
  float yourValue = [prefs floatForKey:@"yourKey"];
}

回答4:

You can test by using objectForKey: and if that is nil then it is not set. All boolForKey does it takes the NSNumber returned if any and returns a BOOL value.

回答5:

I would recommend setting default values for any key that your application might use. You could do this in the application:didFinishLaunchingWithOptions: method. That way you will know that each value has been set.

Hint, set a key called "defaultsSet" to YES so that you only do this once. Also, remember to call [[NSUserDefaults standardUserDefaults] synchronize] to save the values.

回答6:

Swift Example based on MarkPowell's answer

if (NSUserDefaults.standardUserDefaults().objectForKey("SomeBoolSetting") == nil) {
    NSUserDefaults.standardUserDefaults().setBool(true, forKey: "SomeBoolSetting")
    println("Bool WAS nil")
} else {
    var boolValue:Bool? = NSUserDefaults.standardUserDefaults().boolForKey("SomeBoolSetting")
    println("Bool WAS NOT nil \(boolValue)")
}

回答7:

swift version of @Tim Autin 's answer:

if contains(NSUserDefaults.standardUserDefaults().dictionaryRepresentation().keys.array, "chuiser_cook_orders_dirty") {
    println("exist")
} 

回答8:

Dont Complicate it :

if([NSUserDefaults.standardUserDefaults  objectForKey:@"yourBoolKey"])
{
  // Object Already Stored in User defaults
}
else
{
   //Object not stored
}

来源：https://stackoverflow.com/questions/5397364/iphone-how-to-detect-if-a-key-exists-in-nsuserdefaults-standarduserdefaults