Ran into a bizarre problem this morning where a singleton (yeah, I know…) was being created twice during the execution of an XCTestCase.

That is, with code similar to:

+ (MyClass *)shared
{
	static MyClass *instance;
	static dispatch_once_t onceToken;
	dispatch_once(&onceToken, ^{
		instance = [[MyClass alloc] init];
	});
	return instance;
}

During testing, if you set a breakpoint at the alloc/init line inside the dispatch_once block, you would see instance being created twice.

Which caused me all sorts of hair pulling this morning.

The solution? Well, the unit test code was including the main application during linking.

And the MyClass class was also explicitly included (through Target Membership, on the right hand side when selecting the MyClass.m file) in our unit tests as well.

What this means is that two instances of the MyClass class is being included. That means two sets of global variables, two sets of ‘onceToken’, two sets of ‘instance’. And two separate calls to initialize two separate instances, causing all sorts of confusion.

The answer?

Remove the MyClass.m class from Target Membership.

Well, I guess the real solution is to design the application without singletons, but that’s an exercise for another day.

Besides, there are times when you really want a singleton: you really want only one instance of a particular class to exist because it represents a common object shared across the entire application–and the semantics creates confusion if multiple objects are created. (This is also the case with NSNotificationCenter, for example.)