OCMock is a great framework for Objective-C (iOS or Mac OS) for test driven development.

It changes the behaviours of classes during testing.

This is a guide on how to use OCMock, and also raise some of the pitfalls.

1. Mock an instance method

If you have a class MyClass, an instance anObject, and an instance method foo, you can change the return value of foo during testing:

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// Create a mock for the class
id myClassMock = OCMClassMock([MyClass class]);

// Change the method's behaviour for MyClass with a stub
OCMStub([myClassMock foo]).andReturn(@"bar");

// If you only want to stub for a specific value eg Force to return Apple for Green fruit
OCMStub([myClassMock fruitWithColor:@"Green"]).andReturn(@"Apple");

// If you want to return Apple for all fruits
OCMStub([myClassMock fruitWithColor:[OCMArg any]]).andReturn(@"Apple");

2. Mock a class method

Mocking a class method is exactly the same as mocking for instance method.

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id myClassMock = OCMClassMock([MyClass class]);
OCMStub([myClassMock classMethod]).andReturn(@"bar");

3. Mock an object (Partial mock)

If you have the instance anObject, you can use OCMPartialMock to create the mock.

This mock in effect is the same as mocking the class.

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id partialMock = OCMPartialMock(anObject);
OCMStub([partialMock foo]).andReturn(@"bar");

Pitfall: You might assume partial mock applies to only that instance anObject. That’s wrong. If you have anotherObject2, it will be affected by whatever was mocked because partial mock actually mock the whole class.

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[anObject foo];     // Will return @"bar"
[anObject2 foo];    // Will return @"bar" too!

4. Stop mocking

You can stop mocking to return to the real object and reset it’s behaviours.

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[anObject foo];     // Will return @"bar"
[anObject stopMocking];
[anObject2 foo];    // Will return as per normal

We use the example of partial mock, but you could call [MyClass stopMocking] and it will have the same effects.

5. Stub methods and chaining

You can stub the methods with different actions: return object, return native values, throw, post notification, forward to real object or do nothing.

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OCMStub([mock someMethod]).andReturn(anObject);
OCMStub([mock aMethodReturningABoolean]).andReturn(YES);
OCMStub([mock someMethod]).andThrow(anException);
OCMStub([mock someMethod]).andPost(aNotification);
OCMStub([mock someMethod]).andForwardToRealObject();
OCMStub([mock someMethod]).andDo(anInvocation);

You can also chain andPost and andReturn with other actions.

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OCMStub([mock someMethod]).andPost(aNotification).andReturn(aValue);

Limitations – Especially Core Data

Not only those listed on OCMock, but there are other limitations.

The most severe is that you cannot mock a NSManagedObject.

There is a way to get around it, but it is better that you never, never mock Core Data.