To use NSNotificationCenter to detect changes to NSUserDefaults, you have to observe NSUserDefaultsDidChangeNotification, but there's no way to tell which of the keys was changed. See RootViewController.m in the AppPrefs sample code.
Apparently in recent iOS & OS X you can indeed use KVO to observe [NSUserDefaults standardUserDefaults] (see
this StackOverflow answer). Then to solve your original question and your objection quoted above, I can't recommend highly enough the KVO-wrapper by Mike Ash, MAKVONotificationCenter. It lets you provide a custom selector (or block!) for each observation and also allows passing an array of observation key strings. Plus it eliminates the need to manually remove the observation, they get removed automatically when the either the observed or observing object is deallocated. The github repo is
here, the articles about it are
here and
here.
Those articles don't give you a step-by-step guide for using MAKVONotificationCenter though (they're more about the details of its implementation). Here's what your setup code would be:
Code:
[[MAKVONotificationCenter defaultCenter] observeTarget:[NSUserDefaults standardUserDefaults]
keyPath:userdefaultsKey
selector:@selector(userdefaultsKey1Changed:ofObject:change:userInfo:)
userInfo:NULL options:NSKeyValueObservingOptionNew];
or with an array of keys in place of the single key string:
Code:
[[MAKVONotificationCenter defaultCenter] observeTarget:[NSUserDefaults standardUserDefaults]
keyPath:@[userdefaultsKey1, userdefaultsKey2]
selector:@selector(userdefaultsKey1or2Changed:ofObject:change:userInfo:)
userInfo:NULL options:NSKeyValueObservingOptionNew];
And the observation method:
Code:
- (void)userdefaultsKey1or2Changed(NSString *)keyPath ofObject:(id)target change:(NSDictionary *)change userInfo:(id)userInfo {
// do something with [[NSUserDefaults standardUserDefaults] objectForKey:keyPath]
// or [change objectForKey:NSKeyValueChangeNewKey]
}
I'll let you figure out the blocks API yourself, except for this gotcha: Although I rarely find myself needing either option flags NSKeyValueChangeNewKey or NSKeyValueChangeOldKey, note that the block doesn't get access to the change dictionary itself. Instead, to access the NSKeyValueChangeNewKey and NSKeyValueChangeOldKey values you instead use notification.newValue and notification.oldValue, where notification is the block's parameter.
"nameofnotification" object: nil];