How to stub a class method in OCMock?

Question

I often find in my iPhone Objective-C unit tests that I want stub out a class method, e.g. NSUrlConnection's +sendSynchronousRequest:returningResponse:error: method.

Simplified example:

- (void)testClassMock
{
    id mock = [OCMockObject mockForClass:[NSURLConnection class]];
    [[[mock stub] andReturn:nil] sendSynchronousRequest:nil returningResponse:nil error:nil];
}

When running this, I get:

Test Case '-[WorklistTest testClassMock]' started.
Unknown.m:0: error: -[WorklistTest testClassMock] : *** -[NSProxy doesNotRecognizeSelector:sendSynchronousRequest:returningResponse:error:] called!
Test Case '-[WorklistTest testClassMock]' failed (0.000 seconds).

I've had a really hard time finding any documentation on this, but I assume that class methods aren't supported by OCMock.

I found this tip after a lot of Googling. It works, but is very cumbersome: http://thom.org.uk/2009/05/09/mocking-class-methods-in-objective-c/

Is there anyway to do this within OCMock? Or can someone think of a clever OCMock category object that could be written to accomplish this sort of thing?

Answer 1

Coming from the world of Ruby, I understand exactly what you're trying to accomplish. Apparently, you were literally three hours ahead of me trying to do exactly the same thing today (time zone thing? :-).

Anyway, I believe that this is not supported in the way one would desire in OCMock because stubbing a class method needs to literally reach into the class and changes its method implementation regardless of when, where, or who calls the method. This is in contrast to what OCMock seems to do which is to provide you a proxy object that you manipulate and otherwise operate on directly and in lieu of a "real" object of the specified class.

For example, it seems reasonable to want to stub NSURLConnection +sendSynchronousRequest:returningResponse:error: method. However, it is typical that the use of this call within our code is somewhat buried, thus making it very awkward to parameterize it and swap in a mock object for the NSURLConnection class.

For this reason, I think the "method swizzling" approach you've discovered, while not sexy, is exactly what you want to do for stubbing class methods. To say it's very cumbersome seems extreme -- how about we agree it's "inelegant" and maybe not as convenient as OCMock makes life for us. Nevertheless, it's a pretty concise solution to the problem.

Answer 2

Update for OCMock 3

OCMock has modernized its syntax for supporting class method stubbing:

id classMock = OCMClassMock([SomeClass class]);
OCMStub(ClassMethod([classMock aMethod])).andReturn(aValue);

Update

OCMock now supports class method stubbing out of the box. The OP's code should now work as posted. If there is an instance method with the same name as the class method, the syntax is:

[[[[mock stub] classMethod] andReturn:aValue] aMethod]

See OCMock's Features.

Original Answer

Sample code following Barry Wark's answer.

The fake class, just stubbing connectionWithRequest:delegate:

@interface FakeNSURLConnection : NSURLConnection
+ (id)sharedInstance;
+ (void)setSharedInstance:(id)sharedInstance;
+ (NSURLConnection *)connectionWithRequest:(NSURLRequest *)request delegate:(id<NSURLConnectionDelegate>)delegate;
- (NSURLConnection *)connectionWithRequest:(NSURLRequest *)request delegate:(id<NSURLConnectionDelegate>)delegate;
@end
@implementation FakeNSURLConnection
static id _sharedInstance;
+ (id)sharedInstance { if (!_sharedInstance) { _sharedInstance = [self init]; } return _sharedInstance; }
+ (void)setSharedInstance:(id)sharedInstance { _sharedInstance = sharedInstance; }
+ (NSURLConnection *)connectionWithRequest:(NSURLRequest *)request delegate:(id<NSURLConnectionDelegate>)delegate {
    return [FakeNSURLConnection.sharedInstance connectionWithRequest:request delegate:delegate];
}
- (NSURLConnection *)connectionWithRequest:(NSURLRequest *)request delegate:(id<NSURLConnectionDelegate>)delegate { return nil; }
@end

Switching to and from the mock:

{
    ...
    // Create the mock and swap it in
    id nsurlConnectionMock = [OCMockObject niceMockForClass:FakeNSURLConnection.class];
    [FakeNSURLConnection setSharedInstance:nsurlConnectionMock];
    Method urlOriginalMethod = class_getClassMethod(NSURLConnection.class, @selector(connectionWithRequest:delegate:));
    Method urlNewMethod = class_getClassMethod(FakeNSURLConnection.class, @selector(connectionWithRequest:delegate:));
    method_exchangeImplementations(urlOriginalMethod, urlNewMethod);

    [[nsurlConnectionMock expect] connectionWithRequest:OCMOCK_ANY delegate:OCMOCK_ANY];

    ...
    // Make the call which will do the connectionWithRequest:delegate call
    ...

    // Verify
    [nsurlConnectionMock verify];

    // Unmock
    method_exchangeImplementations(urlNewMethod, urlOriginalMethod);
}

Answer 3

Here is a nice 'gist' with a swizzle implementation for class methods: https://gist.github.com/314009

Answer 4

If you modify your method under test to take a parameter which injects the class of the NSURLConnection, then it's relatively easy to pass in a mock that responds to the given selector (you may have to create a dummy class in your test module which has the selector as an instance method and mock that class). Without this injection, you're using a class method, essentially using NSURLConnection (the class) as a singleton and hence have fallen into the anti-pattern of using singleton objects and the testability of your code has suffered.

Answer 5

Link to the blogpost in the question and RefuX gist inspired me to come up with block enabled implementation of their ideas: https://gist.github.com/1038034