Writing custom matchers

OCHamcrest comes bundled with lots of useful matchers, but you'll probably find that you need to create your own from time to time to fit your testing needs.

This commonly occurs when you find a fragment of code that tests the same set of properties over and over again (and in different tests), and you want to bundle the fragment into a single assertion. By writing your own matcher you'll eliminate code duplication and make your tests more readable!

Let's write our own matcher for testing if a calendar date falls on a Saturday. This is the test we want to write:

- (void)testDateIsOnASaturday
{
    NSCalendarDate* date = [NSCalendarDate dateWithString:@"26 Apr 2008" calendarFormat:@"%d %b %Y"];
    assertThat(date, is(onASaturday()))
}

Here's the interface:

#import <OCHamcrest/HCBaseMatcher.h>
#import <objc/objc-api.h>

@interface IsGivenDayOfWeek : HCBaseMatcher
{
    NSInteger day;      // Sunday is 0, Saturday is 6
}

+ (instancetype)isGivenDayOfWeek:(NSInteger)dayOfWeek;
- (instancetype)initWithDay:(NSInteger)dayOfWeek;

@end

OBJC_EXPORT id <HCMatcher> onASaturday();

The interface consists of two parts: a class definition, and a factory function (with C binding). Here's what the implementation looks like:

#import "IsGivenDayOfWeek.h"
#import <OCHamcrest/HCDescription.h>

@implementation IsGivenDayOfWeek

+ (instancetype)isGivenDayOfWeek:(NSInteger)dayOfWeek
{
    return [[self alloc] initWithDay:dayOfWeek];
}

- (instancetype)initWithDay:(NSInteger)dayOfWeek
{
    self = [super init];
    if (self != nil)
        day = dayOfWeek;
    return self;
}

// Test whether item matches.
- (BOOL)matches:(id)item
{
    if (![item respondsToSelector:@selector(dayOfWeek)])
        return NO;

    return [item dayOfWeek] == day;
}

// Describe the matcher.
- (void)describeTo:(id <HCDescription>)description
{
    NSString *dayAsString[] =
        { @"Sunday", @"Monday", @"Tuesday", @"Wednesday", @"Thursday", @"Friday", @"Saturday" };
    [[description appendText:@"calendar date falling on "] appendText:dayAsString[day]];
}

@end


id <HCMatcher> onASaturday()
{
    return [IsGivenDayOfWeek isGivenDayOfWeek:6];
}

For our Matcher implementation we implement the -matches: method (which calls -dayOfWeek after confirming that the argument has such a method) and the -describe_to: method (which is used to produce a failure message when a test fails). Here's an example of how the failure message looks:

NSCalendarDate* date = [NSCalendarDate dateWithString: @"6 April 2008"
                                       calendarFormat: @"%d %B %Y"];
assertThat(date, is(onASaturday()));

fails with the message

Expected: is calendar date falling on Saturday, got: <06 April 2008> 

and Xcode shows it as a build error. Clicking the error message takes you to the assertion that failed.

Even though the onASaturday function creates a new matcher each time it is called, you should not assume this is the only usage pattern for your matcher. Therefore you should make sure your matcher is stateless, so a single instance can be reused between matches.