Custom Constraints for OCMock

Posted on by in Development, Process

In my last post on unit testing iPhone development I introduced a couple helpful tools including OCMock, a mock objects implementation for Objective-C.

I recently came across a scenario where I needed to make an assertion on an argument passed to an expected method invocation but OCMock did not provide the constraint I needed. I was testing a FileDetailsViewController that pushes a new FileContentsViewController on to the view stack when a button is pressed. I mocked the UINavigationController so that I could add an assertion that pushViewController:animated: is called with the right arguments.

The meat of my test is:

@interface FileDetailsViewControllerTest : SenTestCase {}
    FileDetailsViewController *controller;
    id navigationController;

@implementation FileDetailsViewControllerTest

-(void) setUp {
    controller = [[FileDetailsViewController alloc] initWithNibName:@"FileDetailsView"
    navigationController = [OCMockObject mockForClass:[UINavigationController class]];
    controller.navigationController = navigationController;

-(void) testShowContentLoadsView {
    [[navigationController expect]
         pushViewController:[OCMConstraint isKindOfClass:[FileContentsViewController class]]
    [controller showContentsButtonWasPressed];

-(void) tearDown {
    [navigationController verify];


Note that in this example, I some fanciness is required to assign controller.navigationController = navigationController since it is a readonly property in the UIViewController API. I’ll explain what I did in a later post.

The OCMock feature I was missing is the OCMock.isKindOfClass constraint. So I wrote my own.

Following is the header file that declares the isKindOfClass assertion as a static method in an Objective-C category for OCMock. Categories are kind of like Ruby mixins for Objective-C. You can add static or instance methods to an existing class definition. With this approach I can follow the OCMock pattern of providing static factory methods on the OCMock class for the range of constraints available.

//File: OCMockConstraint+Extensions.h

@interface OCMConstraint (Extensions)

+ (id)isKindOfClass:(id)value;


Following is the implementation, both of my new constraint and the factory method to create it.

//File: OCMockConstraint+Extensions.m
#import "OCMockConstraint+Extensions.h"

@interface OCMKindOfClassConstraint : OCMConstraint
    id testClass;

@implementation OCMKindOfClassConstraint

- (BOOL)evaluate:(id)value
	return [value isKindOfClass:testClass];


// Static factory method
@implementation OCMConstraint (Extensions)

+ (id)isKindOfClass:(id)value {
	OCMKindOfClassConstraint *constraint = [OCMKindOfClassConstraint constraint];
	constraint->testClass = value;
	return constraint;


This implementation shows both how to write a custom constraint for OCMock and a use of Objective-C categories to provide a nice API for using it.

In many cases, you’ll want to write a custom constraint that is specific to your test scenario and not appropriate to be shared globally in this manner. In this case you’ll probably follow a similar strategy but implemented in your test class instead of mixed in to OCMockConstraint.