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An attempt to make unit testing of singleton classes easier.
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An attempt to make unit testing of singleton classes easier.

Testing Singletons

The Singleton Design Pattern is a great one when applied following a few best practices. However, unit testing singleton classes is a pain in the ass. I have seen a few decent approaches, such as property injection and overriding using a category, but I still find it a hassle to test singletons.

A New Approach

KMSwizzleton is an attempt to make testing of singletons easier. But beware! It is not thoroughly tested and lacks a few things (see TODO). Here's a sample:

// singleton class to test
@interface MySingleton : NSObject
+ (MySingleton *)sharedInstance;

// a classes that uses the singleton
@implementation SomeClassTotest 
- (void)someMethod {
	[[MySingleton sharedInstance] somethingAwesome];

// test class
- (void)testSingletonSomething {
	id fakeSingleton = … // use favorite mock lib here
	[fakeSingleton expect:somethingAwesome] // behavior that before was very difficult to verify
	[KMSwizzleton stubSingleton:[MySingleton class] andReturnFakeInstance:fakeSingleton];
	SomeClassToTest *cut = [[SomeClassToTest alloc] init];
	[cut someMethod];
	// we can now verify whether cut called somethingAwesome on MySingleton or not
	[fakeSingleton verify];

NB: It is assumed that singleton classes follow the sharedInstance pattern.


  • Revert swizzling if +sharedInstance is not invoked in a test case (to maintain state across all test cases).
  • Ability to specify the name of the singleton's instance method when it doesn't follow the sharedInstance-pattern, such as [NSNotificationCenter defaultcenter].
  • Support for inheritance.
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