Using method swizzling and blocks to test Class methods in Objective-C.

ExampleTest.m

#import "SenTestCase+MethodSwizzling.m"

@interface ExampleTest : SenTestCase {}
+ (BOOL)trueMethod;
+ (BOOL)falseMethod;
@end

@implementation ExampleTest

+ (BOOL)trueMethod  { return YES; }
+ (BOOL)falseMethod { return NO; }

- (void)testMethodSwizzlingShouldCallAlternativeImplementation {
    [self swizzleMethod:@selector(trueMethod)  inClass:[self class]
             withMethod:@selector(falseMethod) fromClass:[self class]
           executeBlock:^{
                assertThatBool([[self class] trueMethod], isEqualToBool(NO));
            }]
}

@end

SenTestCase+MethodSwizzling.m

#include <objc/runtime.h>

@interface SenTestCase (MethodSwizzling)
- (void)swizzleMethod:(SEL)aOriginalMethod
              inClass:(Class)aOriginalClass
           withMethod:(SEL)aNewMethod
            fromClass:(Class)aNewClass
         executeBlock:(void (^)(void))aBlock;
@end

@implementation SenTestCase (MethodSwizzling)
- (void)swizzleMethod:(SEL)aOriginalMethod
              inClass:(Class)aOriginalClass
           withMethod:(SEL)aNewMethod
            fromClass:(Class)aNewClass
         executeBlock:(void (^)(void))aBlock {
    Method originalMethod = class_getClassMethod(aOriginalClass, aOriginalMethod);
    Method mockMethod = class_getInstanceMethod(aNewClass, aNewMethod);
    method_exchangeImplementations(originalMethod, mockMethod);
    aBlock();
    method_exchangeImplementations(mockMethod, originalMethod);
}
@end

the problem with this is if the test case you put into the block fails, you will not "deswizzle" the method. this is likely to mess up your other test cases. that's why i prefer to do the swizzling and cleanup stuff in the -setUp and -tearDown...