Stub current date and test date calculations which use `NSDate#timeIntervalSinceNow` internally

gistfile1.m

#import <objc/runtime.h>

@implementation NSDate (CZTesting)

- (NSTimeInterval)cz_timeIntervalSinceNow
{
  return (self.timeIntervalSince1970 - [NSDate date].timeIntervalSince1970);
}

@end

SPEC_BEGIN(SomeSpec)

describe(@"Something", ^{
  __block Method originalMethod;
  __block Method swizzledMethod;

  beforeAll(^{
    SEL originalSelector = @selector(timeIntervalSinceNow);
    SEL swizzledSelector = @selector(cz_timeIntervalSinceNow);
    Class class = [NSDate class];

    originalMethod = class_getInstanceMethod(class, originalSelector);
    swizzledMethod = class_getInstanceMethod(class, swizzledSelector);
    method_exchangeImplementations(originalMethod, swizzledMethod);
  });

  afterAll(^{
    method_exchangeImplementations(originalMethod, swizzledMethod);
  });

  it(@"does something", ^{
    [NSDate stub:@selector(date) andReturn:[NSDate mt_dateFromISOString:@"2014-04-09 22:12:00"]];

    // test production code which uses `NSDate#timeIntervalSinceNow` internally
  });
});

SPEC_END