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_getClassMethod(aNewClass, aNewMethod);
    method_exchangeImplementations(originalMethod, mockMethod);
    aBlock();
    method_exchangeImplementations(mockMethod, originalMethod);
}
@end