RCTestCase.h

#import <XCTest/XCTest.h>

typedef id (^OperationToExchangeForClass)(void);

@interface RCTestCase : XCTestCase

- (void) exchangeClassMethodForClass:(Class)clazz selector:(SEL)selector operation:(OperationToExchangeForClass)operation;

@property (nonatomic) NSBundle *unitTestBundle;

@end

RCTestCase.m

#import "RCTestCase.h"
#import <objc/runtime.h>

static NSMutableArray *classMethodExchanges;
static NSMutableArray *classMethodBlocks;

@implementation RCTestCase

-(void) setUp
{
    _unitTestBundle = [NSBundle bundleForClass:[self class]];
}

-(void) tearDown
{
    [super tearDown];
    [self restoreSwizzles];
}

- (void) exchangeClassMethodForClass:(Class)clazz selector:(SEL)selector operation:(OperationToExchangeForClass)operation
{
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        classMethodExchanges = [NSMutableArray array];
        classMethodBlocks = [NSMutableArray array];
    });
    
    NSAssert(classMethodExchanges.count<=1, @"");
    NSAssert(classMethodBlocks.count<=1, @"");
    
	Method originalClassMethod = class_getClassMethod(clazz, selector);
    NSAssert(originalClassMethod, @"Method not found");
	Method mockMethod = class_getInstanceMethod([self class], @selector(classMethodExchangeCallBlock));
    NSAssert(mockMethod, @"Method not found");
    
    //Swizzle
    method_exchangeImplementations(originalClassMethod, mockMethod);
    
    //Store for de-swizzle
    {
        NSValue *originalClassMethodValue = [NSValue valueWithBytes:&originalClassMethod objCType:@encode(Method)];
        NSValue *mockMethodValue = [NSValue valueWithBytes:&mockMethod objCType:@encode(Method)];
        [classMethodExchanges addObject:@[originalClassMethodValue,mockMethodValue]];
    }
    
    //Store callback
    [classMethodBlocks addObject:operation];
}

- (void) restoreSwizzles
{
    for (NSArray *exchanges in classMethodExchanges) {
        NSValue *originalClassMethodValue = exchanges[0];
        NSValue *mockMethodValue = exchanges[1];
        Method originalClassMethod;
        Method mockMethod;

        [originalClassMethodValue getValue:&originalClassMethod];
        [mockMethodValue getValue:&mockMethod];        
        
        method_exchangeImplementations(mockMethod, originalClassMethod);
    }
    [classMethodExchanges removeAllObjects];
    [classMethodBlocks removeAllObjects];
}

-(id) classMethodExchangeCallBlock
{
    NSAssert(classMethodBlocks.count == 1, @"");
    OperationToExchangeForClass op = (OperationToExchangeForClass)classMethodBlocks[0];
    return op();
}

@end