OCMock failure mocking 'initWithAttributedString' on NSMutableAttributedString - ios

Question

I'm trying to understand what is mockable and what isn't.
In an experiment with an NSMutableAttributedString, I don't appear to be able to mock initWithAttributedString.
- (void)test_mutableString_shouldWorkAsAMutableString {
NSMutableAttributedString *_mutable = [OCMockObject mockForClass:NSMutableAttributedString.class];
NSAttributedString *_string = [OCMockObject mockForClass:NSAttributedString.class];
[[[(id)_mutable expect] andReturnValue:nil] initWithAttributedString:_string];
[_mutable initWithAttributedString:_string];
}
This code won't run; for some reason the proxy for the mutable screen doesn't recognise the initWithAttributedString selector:
2013-03-12 11:25:30.725 UnitTests[11316:c07] TestItClass/test_4_mutableString_shouldWorkAsAMutableString ✘ 0.00s
Name: NSInvalidArgumentException
File: Unknown
Line: Unknown
Reason: *** -[NSProxy doesNotRecognizeSelector:initWithAttributedString:] called!
0 CoreFoundation 0x01c0602e __exceptionPreprocess + 206
1 libobjc.A.dylib 0x01948e7e objc_exception_throw + 44
2 CoreFoundation 0x01c05deb +[NSException raise:format:] + 139
3 Foundation 0x00862bcd -[NSProxy doesNotRecognizeSelector:] + 75
4 CoreFoundation 0x01bf5bbc ___forwarding___ + 588
5 CoreFoundation 0x01bf594e _CF_forwarding_prep_0 + 14
6 UnitTests 0x00349e0b -[TestItClass test_4_mutableString_shouldWorkAsAMutableString] + 283
I am trying to understand how I can reliably use OCMock, but this confuses my and I'm not sure which OCMock calls I can expect to work and which I shouldn't.
I would very much appreciate some clarification on this, and an hints as to why the above doesn't work.
Thanks,
Joe

Answer 1

I learned something about Objective-C trying to figure this one out.
Your basic problem is that the class of an object created by alloc'ing NSMutableAttributedString is not NSMutableAttributedString (always be wary of toll-free bridged classes). To get your code to work, try the following:
NSMutableAttributedString *realMutable = [[NSMutableAttributedString alloc] init];
id mutable = [OCMockObject niceMockForClass:[realMutable class]];
id string = [OCMockObject niceMockForClass:[NSAttributedString class]];
[[[mutable expect] andReturn:#"YO" ] initWithAttributedString:string];
NSLog(#"MOCK: %#", [mutable initWithAttributedString:string]);
[mutable verify];
// Outputs 'MOCK: YO' and passes