Change system date for Cocoa unit testing

Question

I'm testing a method which calls -[NSDate date], and uses system time as a reference to do some logic. I need to test if the logic is right on some specific days in the year, but I can't wait till that day. So can I change the system date programmatically when unit testing?

Answer 1

You need to use Mock object and in Objective C you can do it easily with categories by overriding only methods that you need. For example you could use the following category:

// ------------- File: NSDate+NSDateMock.h
@interface NSDate (NSDateMock)

 +(void)setMockDate:(NSString *)mockDate;
 +(NSDate *) mockCurrentDate;

@end

// -------------- File: NSDate+NSDateMock.m

#import "NSDate+NSDateMock.h"

@implementation NSDate (NSDateMock)

static NSDate *_mockDate;

+(NSDate *)mockCurrentDate
{
    return _mockDate;
}

+(void)setMockDate:(NSString *)mockDate
{
    _mockDate = [NSDate dateWithString:mockDate];
}

@end

and in addition you will need SwizzleClassMethod

void SwizzleClassMethod(Class c, SEL orig, SEL new) {

    Method origMethod = class_getClassMethod(c, orig);
    Method newMethod = class_getClassMethod(c, new);

    c = object_getClass((id)c);

    if(class_addMethod(c, orig, method_getImplementation(newMethod), method_getTypeEncoding(newMethod)))
        class_replaceMethod(c, new, method_getImplementation(origMethod), method_getTypeEncoding(origMethod));
    else
        method_exchangeImplementations(origMethod, newMethod);
}

and then in your unit test you will be able to use it like this

SwizzleClassMethod([NSDate class], @selector(date), @selector(mockCurrentDate));
[NSDate setMockDate:@"2007-03-24 10:45:32 +0200"];
NSLog(@"Date is: %@", [NSDate date]);