OCMock: how to stub -[NSString stringWithContentsOfURL:encoding:error:] - ios

Question

I've tried many things. This still does not work:
id stringMock = OCMClassMock([NSString class]);
[[[[stringMock stub] ignoringNonObjectArgs] stringWithContentsOfURL:[OCMArg anyPointer] encoding:NSUTF8StringEncoding error:(NSError * __autoreleasing *)[OCMArg anyPointer]] andReturn:stringFromFile];
How can I make -[NSString stringWithContentsOfURL:encoding:error:] return stringFromFile anywhere in my code?

Answer 1

Unfortunately, it's not possible.
From OCMock documentation about the limitations (section 10):
10.5 Class methods on NSString cannot be stubbed or verified
id stringMock
= OCMClassMock([NSString class]); // the following will not work OCMStub([stringMock stringWithContentsOfFile:[OCMArg any]
encoding:NSUTF8StringEncoding error:[OCMArg setTo:nil]]);
It is not
possible to stub or verify class methods on NSString. Trying to do so
has no effect.
You can, however, wrap NSString and mock the wrapper. It means that you'll need to replace all the calls in your code with this wrapper, but the tests would work..
Here's an example:
The wrapper:
#interface NSStringWrapper : NSObject
+ (nullable id)stringWithContentsOfURL:(NSURL *)url;
#end
#implementation NSStringWrapper
+ (nullable id)stringWithContentsOfURL:(NSURL *)url
{
return [NSString stringWithContentsOfURL:url];
}
#end
The test:
static NSString *stringFromFile = #"myStringFromFile";
-(void)testStringWrapper
{
id nsstringWrapperMock = OCMClassMock([NSStringWrapper class]);
OCMStub(ClassMethod([nsstringWrapperMock stringWithContentsOfURL:OCMOCK_ANY])).andDo(^(NSInvocation *invocation)
{
[invocation setReturnValue:&stringFromFile];
});
NSURL *fakeURL = [NSURL URLWithString:#"htttp://google.com"];
NSString *test = [NSStringWrapper stringWithContentsOfURL:fakeURL];
XCTAssertEqual(test, stringFromFile, #"Should always return stringFromFile");
}

Answer 2

Another option is to try OCMockito, which mocks class methods differently than OCMock.
__strong Class mockStringClass = mockClass([NSString class]);
[[given([mockStringClass stringWithContentsOfFile:anything()
encoding:NSUTF8StringEncoding
error:NULL])
withMatcher:anything() forArgument:2]
willReturn:stringFromFile];
Unlike OCMock which swizzles class methods, this requires you to inject the class. That is, instead of production code making a direct call like
str = [NSString stringWithContentsOfFile:path encoding:NSUTF8StringEncoding error:NULL];
you need to do something like
str = [self.stringClass stringWithContentsOfFile:path encoding:NSUTF8StringEncoding error:NULL];
You should be able to add OCMockito without disrupting your existing use of OCMock.