Hamcrest for Objective-C: Powerful, combinable, extensible matchers for verification
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What is OCHamcrest?

OCHamcrest is:

  • a library of "matcher" objects that let you declare rules for whether a given object matches the criteria or not.
  • a framework for writing your own matchers.

Matchers are useful for a variety of purposes, such as UI validation. But they're most commonly used for writing unit tests that are expressive and flexible.

OCHamcrest can be used for either Mac and iOS development with:

  • OCUnit (SenTestingKit) built in to Xcode
  • Google Toolbox for Mac (GTM)
  • GHUnit
  • Cedar BDD framework
  • OCMock
  • OCMockito

How do I add OCHamcrest to my project?


If you want to build OCHamcrest yourself, cd to the Source folder, then

$ ./MakeDistribution.sh

Or just use the pre-built release available in Downloads.

Mac Project Setup:

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.


#import <OCHamcrest/OCHamcrest.h>

Note: If your Console shows

otest[57510:203] *** NSTask: Task create for path '...' failed: 22, "Invalid argument".  Terminating temporary process.

double-check your Copy Files phase.

iOS Project Setup:

Add OCHamcrestIOS.framework to your project.


#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>

#import <OCHamcrest/OCHamcrest.h>

@interface BiscuitTest : SenTestCase

@implementation BiscuitTest

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


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

    • conformsTo - match object that conforms to protocol
    • 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 nil, or not nil
    • sameInstance - match same object
  • Number

    • closeTo - match number close to a given value
    • equalTo<TypeName> - 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
    • hasItem - match if given item appears in the collection
    • hasItems - match if all given items appear in the collection, in any order
    • hasKey - match dictionary with a key
    • hasValue - match dictionary with a value
    • 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;


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];

- (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]];


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.

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