How to test with NSUserDefaults? - cocoa

Question

In most of my classes that work with defaults I make the defaults object settable:
#property(retain) NSUserDefaults *defaults;
This is supposed to make testing easier:
// In a nearby test class:
- (void) setUp {
[super setUp];
NSUserDefaults *isolatedDefaults = [[NSUserDefaults alloc] init];
[someObjectBeingTested setDefaults:isolatedDefaults];
}
But now I have found out then when I create a fresh defaults object, there are already some values in it. Is that possible? I thought I could create an empty, isolated defaults object by calling -init. Do I have a bug somewhere in the testing code, or do I really have to do something more complex (like stubbing or mocking) if I want to test my defaults-based code?

Answer 1

In the end I have created a simple NSUserDefaults replacement that can be used to control the defaults environment in tests. The code is available on GitHub.

Answer 2

From the NSUserDefaults documentation:
init: Returns an NSUserDefaults object initialized with the defaults for the current user account.
This should normally not be empty. I am not really sure what you want to test here, since it would be a waste of time to test NSUserDefaults functionality.
But say you need some keys to be not registered yet for your test to always have the same initial point: then just remove them in setUp (and restore them later in tearDown if you want to).
Something like:
- (void) setUp {
[super setUp];
[[NSUserDefaults standardUserDefaults] removeObjectForKey:#"myTestKey"];
// synchronize the change, or just use resetStandardUserDefaults:
[someObjectBeingTested setDefaults:[NSUserDefaults resetStandardUserDefaults]];
}
If you don't have a specific list of keys but need to wipe out everything, you will have to use the CoreFoundation Preferences Utilities, see CFPreferencesCopyKeyList.

Answer 3

We also needed to override NSUserDefaults for testing, but didn't want to change any of the application code.
So we wrote a category on NSUserDefaults that allows us to override values returned by objectForKey: at runtime using method swizzling.
It looks like this in Objective C:
NSLog(#"Value before: %d", [[NSUserDefaults standardUserDefaults] boolForKey:#"Example"]);
// Value before: 0
[[NSUserDefaults standardUserDefaults] overrideValue:#(YES) forKey:#"Example"];
NSLog(#"Value after: %d", [[NSUserDefaults standardUserDefaults] boolForKey:#"Example"]);
// Value after: 1
And like this in Swift:
print(UserDefaults.standard.bool(forKey: "ExampleKey")) // false
UserDefaults.standard.overrideValue(true, forKey: "ExampleKey")
print(UserDefaults.standard.bool(forKey: "ExampleKey")) // true
You can find our code on Github: https://github.com/jakob/NSUserDefaultsOverride