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See also:

• PyHamcrest - Python version.
• Quality Coding - Tools, tips and techniques for building quality in to your iOS programs.

Introduction

OCHamcrest is a framework for writing matcher objects, allowing you to declaratively define "match" rules. There are a number of situations where matchers are invaluable, such as UI validation, or data filtering, but it is in the area of writing flexible tests that matchers are most commonly used. This tutorial shows you how to use OCHamcrest for unit testing.

When writing tests it is sometimes difficult to get the balance right between overspecifying the test (and making it brittle to changes), and not specifying enough (making the test less valuable since it continues to pass even when the thing being tested is broken). Having a tool that allows you to pick out precisely the aspect under test and describe the values it should have, to a controlled level of precision, helps greatly in writing tests that are "just right." Such tests fail when the behavior of the aspect under test deviates from the expected behavior, yet continue to pass when minor, unrelated changes to the behavior are made.

Cocoa and iOS

OCHamcrest is supported for both Cocoa and iOS development.

Cocoa:

Add OCHamcrest.framework to your project.

Add a Copy Files build phase to copy OCHamcrest.framework to your Products Directory. For unit test bundles, make sure this Copy Files phase comes before the Run Script phase that executes tests.

Add:

#define HC_SHORTHAND
#import <OCHamcrest/OCHamcrest.h>

iOS:

Add OCHamcrestIOS.framework to your project.

Add "-lstdc++" and "-ObjC" to your "Other Linker Flags".

Add:

#define HC_SHORTHAND
#import <OCHamcrestIOS/OCHamcrestIOS.h>

My first OCHamcrest test

We'll start by writing a very simple Xcode unit test, but instead of using OCUnit's STAssertEqualObjects function, we'll use OCHamcrest's assertThat construct and a predefined matcher:

#import <SenTestingKit/SenTestingKit.h>

#define HC_SHORTHAND
#import <OCHamcrest/OCHamcrest.h>

@interface BiscuitTest : SenTestCase
@end

@implementation BiscuitTest

- (void) testEquals
{
    Biscuit* theBiscuit = [Biscuit biscuitNamed:@"Ginger"];
    Biscuit* myBiscuit = [Biscuit biscuitNamed:@"Ginger"];
    assertThat(theBiscuit, equalTo(myBiscuit));
}

@end

The assertThat function is a stylized sentence for making a test assertion. In this example, the subject of the assertion is the object theBiscuit, which is the first method parameter. The second method parameter is a matcher for Biscuit objects, here a matcher that checks one object is equal to another using the -isEqual: method. The test passes since the Biscuit class defines an -isEqual: method.

OCHamcrest's functions are actually declared with an "HC" package prefix (such as HC_assertThat and HC_equalTo) to avoid name clashes. To make test writing faster and test code more legible, shorthand macros are provided if HC_SHORTHAND is defined before including the OCHamcrest header. For example, instead of writing HC_assertThat, simply write assertThat.

Predefined matchers

OCHamcrest comes with a library of useful matchers:

• Object

  • equalTo - match equal object
  • hasDescription - match object's -description
  • hasProperty - match return value of method with given name
  • instanceOf - match object type
  • nilValue, notNilValue - match @c nil, or not @nil
  • sameInstance - match same object

• Number

  • closeTo - match number close to a given value
  • equalTo - match number equal to a primitive number (such as equalToInt for an int)
  • greaterThan, greaterThanOrEqualTo, lessThan, lessThanOrEqualTo - match numeric ordering

• Text

  • containsString - match part of a string
  • endsWith - match the end of a string
  • equalToIgnoringCase - match the complete string but ignore case
  • equalToIgnoringWhitespace - match the complete string but ignore extra whitespace
  • startsWith - match the beginning of a string
  • stringContainsInOrder - match parts of a string, in relative order

• Logical

  • allOf - "and" together all matchers
  • anyOf - "or" together all matchers
  • anything - match anything, useful in composite matchers when you don't care about a particular value
  • isNot - negate the matcher

• Collection

  • contains - exactly match the entire collection
  • containsInAnyOrder - match the entire collection, but in any order
  • empty - match empty collection
  • hasCount - match number of elements against another matcher
  • hasCountOf - match collection with given number of elements
  • hasEntries - match dictionary with list of key-value pairs
  • hasEntry - match dictionary containing a key-value pair
  • hasKey - match dictionary with a key
  • hasValue - match dictionary with a value
  • hasItem - match if given item appears in the collection
  • hasItems - match if all given items appear in the collection, in any order
  • onlyContains - match if collections's items appear in given list

• Decorator

  • describedAs - give the matcher a custom failure description
  • is - decorator to improve readability - see Syntactic sugar below

The arguments for many of these matchers accept not just a matching value, but another matcher, so matchers can be composed for greater flexibility. For example, only_contains(endsWith(@".")) will match any collection where every item is a string ending with period.

Syntactic sugar

OCHamcrest strives to make your tests as readable as possible. For example, the is matcher is a wrapper that doesn't add any extra behavior to the underlying matcher. The following assertions are all equivalent:

assertThat(theBiscuit, equalTo(myBiscuit));
assertThat(theBiscuit, is(equalTo(myBiscuit)));
assertThat(theBiscuit, is(myBiscuit));

The last form is allowed since is wraps non-matcher arguments with equalTo. Other matchers that take matchers as arguments provide similar shortcuts, wrapping non-matcher arguments in equalTo.

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;      // 0 indicates Sunday
}

+ (id) isGivenDayOfWeek:(NSInteger)dayOfWeek;
- (id) 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

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

- (id) 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 the -dayOfWeek method 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. Double 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.