Tag: Cocoa

August 12, 2006 Technology Post a Comment

Designing for Core Data performance

On the comp.sys.mac.programmer.help newsgroup, Florian Zschocke asked about improving the performance of his Core Data application. Here’s an adapted version of my reply to his post.

Core Data applications should scale quite well to large data sets when using an SQLite persistent store. That said, there are a couple implementation tactics that are critical to performance for pretty much any application using a technology like Core Data:

  1. Maintain a well-normalized data model.
  2. Don’t fetch or keep around more data than you need to.

Implementing these tactics will make it much easier to both create well-performing Core Data applications in the first plce, and to optimize the performance of applications already in progress.

Maintaining a normalized data model is critical for not fetching more data than you need from a persistent store, because for data consistency Core Data will fetch all of the attributes of an instance at once. For example, consider a Person entity that can have a binary data attribute containing a picture. Even if you’re just displaying a table of Person instances by name, Core Data will still fetch the picture because it’s an attribute of Person. Thus for performance in a situation like this, you’d normalize your data so that you have a separate entity, Picture, to represent the picture for a Person on the other side of a relationship. That way the image data will only be retrieved from the persistent store if the relationship is actually traversed; until it’s traversed, it will just be represented by a fault.

Similarly, if you have lots of to-many relationships and need to display summary information about them, de-normalizing your data model slightly and caching the summary information in the main entity can help.

For example, say your app works with Authors and Books. Author.books is a to-many relationship to Book instances and Book.authors is a to-many relationship to Author instances. You may want to show a table of Authors that includes the number of Books related to the Author. However, binding to books.@count for that column value will cause the relationship fault to fire for every Author displayed, which can generate a lot more traffic to the persistent store than you want.

One strategy would be to de-normalize your data model slightly so Author also contains a booksCount attribute, and maintains that whenever the Author.books relationship is maintained. This way you can avoid firing the Author.books relationship fault just because you want to display the number of Books an Author is related to, by binding the column value to booksCount instead of books.@count.

Another thing be careful of is entity inheritance. It’s an implementation detail, but inheritance in Core Data is single-table. Thus if you have every entity in your application inheriting from one abstract entity, it’ll all wind up in a single table, potentially increasing the amount of time fetches take etc. because they require scanning more data.

Retaining or copying the arrays containing fetch results will keep those results (and their associated row cache entries) in memory for as long as you retain the arrays or copies of them, because the arrays and any copies will be retaining the result objects from the fetch. And as long as the result objects are in memory, they’ll also be registered with a managed object context.

If you want to prune your in-memory object graph, you can use -[NSManagedObjectContext refreshObject:mergeChanges:] to effectively turn an object back into a fault, which can also prune its relationship faults. A more extreme measure would be to use -[NSManagedObjectContext reset] to return a context to a clean state with no changes or registered objects. Finally, you can of course just ensure that any managed objects that don’t have changes are properly released, following normal Cocoa memory management rules: So long as your managed object context isn’t set to retain registered objects, and you aren’t retaining objects that you’ve fetched, they’ll be released normally like any other autoreleased objects.

July 3, 2006 Technology Post a Comment

Unit testing Cocoa user interfaces: Target-Action

It’s really great to see that a lot of people are adopting unit testing for their projects and dramatically improving their quality. Test-driven development and agile development methodologies built around it are really taking off. However, a lot of people still feel that their user interface is difficult to test through code, and either requires a capture-playback tool or requires a different design approach based heavily on interfaces/protocols to get right.

In last year’s post Trust, but verify. I tried to dispel some of the mystery of testing your application’s user interface when using the Cocoa frameworks. However, I’ve still had a lot of (entirely well-justified!) requests for examples of how to put it into practice. So here’s a simple example of what I’d do to write a unit test for a button in a window that’s supposed to perform some action.

First, when implementing my window, I’d follow the standard Cocoa pattern of having a custom NSWindowController subclass to manage my window. This window controller will have an outlet connected to each of the views in the window, and will also wind up with a private accessor method — used only within the class and any subclasses, and in testing — for getting the value of each of its outlets. This design flows naturally from the test which I would write to specify that the window should contain a button. First, here’s the skeleton into which I’d put tests:

// TestMyWindow.h

#import <SenTestingKit/SenTestingKit.h>

@class MyWindowController;

@interface TestMyWindow : SenTestCase {
    MyWindowController *_windowController;
    NSWindow *_window;
}
@end

// TestMyWindow.m

#import "TestMyWindow.h"
#import "MyWindowController_Private.h"

@implementation TestMyWindow

- (void)setUp {
    // MyWindowController knows its nib name and
    // invokes -initWithWindowNibName: in -init
    _windowController = [[MyWindowController alloc] init];

    // Load the window, but don't show it.
    _window = [_windowController window];
}

- (void)tearDown {
    [_windowController release];
    _window = nil; // owned by _windowController
}

@end

That’s the infrastructure into which I’d put my other test methods for this window. For example, I’ll want to specify the nib name for the window controller and ensure that it actually knows its window: 

- (void)testNibName {
    STAssertEqualObjects([_windowController windowNibName], @"MyWindow",
      @"The nib for this window should be MyWindow.nib");
}

- (void)testWindowLoading {
    STAssertNotNil(_window,
      @"The window should be connected to the window controller.");
}

Now let’s check that I have a “Do Something” button in the window, and that it sends an action directly to the window controller.

- (void)testDoSomethingButton {
    // _doSomethingButton is a private method that returns the button
    // conected to the doSomethingButton outlet
    NSButton *doSomethingButton = [_windowController _doSomethingButton];
    
    STAssertNotNil(doSomethingButton,
      @"The window should have a 'Do something' button.");
    
    STAssertEqualObjects([doSomethingButton title], @"Do Something",
      @"The button should be titled accordingly.");

    STAssertEquals([doSomethingButton action], @selector(doSomething:),
      @"The button should send -doSomething: to its target.");

    STAssertEquals([doSomethingButton target], _windowController,
      @"The button should send its action to the window controller.");
}

You’ll notice something I’m not doing in the above: I’m not simulating interaction with the interface. This is the core of the trust, but verify approach to unit testing of your user interface.

I can trust that as long as I verify everything is hooked up properly that Cocoa will cause the button to send its action message to its target — whether it’s a specific object or, if the target is nil, the responder chain — whenever the button is clicked while it’s enabled and not hidden. I don’t need to simulate a user event, and I don’t even need to display the interface while running the unit tests. All I need to do is inspect, through code, that everything is wired up correctly.

Note that I can do way more than the above in testing my interface design, too. For example, I can ensure that the control layout is correct according to what my interface designer has specified, by checking bounding rectangles for example. But testing only the functionality of my interface has significant advantages, too. For example, it doesn’t matter if I wind up using a custom kind of button to achieve exactly the kind of look and feel or behavior I need. It doesn’t matter if I wind up changing the layout in response to feedback. No matter what I do, I’ll know that functionality won’t accidentally break while I’m messing around in Interface Builder — even if I completely rip out my interface and replace it with a new one!

This approach can also be used for testing Cocoa bindings using the -infoForBinding: method that was introduced in Mac OS X 10.4 Tiger. I hope to write up a post soon on how to approach Cocoa bindings using these same techniques, but it should be fairly straightforward given the above and the above documentation.

Update: I’ve struck through the check of the button’s title above, because you may or may not want to do that. For example, if you’re primarily running your unit tests against your development localization, you may want to put it in. But if you want to run your unit tests against a localized build of your application, you’ll probably want to avoid checking a localized title against an English string. A “have your cake and eat it too” strategy might be to keep a variable somewhere in your application that can be used to selectively disable checks of only localized strings.

Update July 7, 2007: I’ve finally written a post, Unit testing Cocoa user interfaces: Cocoa bindings, on how to write tests for Cocoa bindings. Now there’s no excuse for not doing test-driven development of your Cocoa user interfaces!

March 19, 2006 Business, Technology Post a Comment

“Enterprise” thought leadership?

David Heinemeier Hansson, creator of Rails at 37signals, takes James McGovern — some Java/J2EE author — to task for his über-lame rant against Ruby in the Enterprise in a great post titled Boy, is James McGovern enterprise or what!

> So by Enterprise, Architect, and Enterprise Architect standards, this gent must be the top of the pop. Thus, allow me to make this perfectly clear: I would be as happy as a clam never to write a single line of software that guys like James McGovern found worthy of The Enterprise.

> If Ruby, Rails, and the rest of the dynamic gang we’re lumped together to represent, is not now, nor ever, McGovern Enterprise Readyâ„¢, I say hallelujah! Heck, I’ll repeat that in slow motion just to underscore my excitement: HAL-LE-LU-JAH!

> With that out of the way, we’re faced with a more serious problem. How do we fork the word enterprise? The capitalized version has obviously been hijacked by McGovern and his like-minded to mean something that is synonymous with hurt and pain and torment.

Indeed, McGovern’s rant reads more like a parody of a rant than the real thing:

> 13\. Lets say there is a sixteen week project and the productivity stuff was true and Ruby could save me an entire three weeks which would be significant. Since Ruby is a new vendor and not represented by existing vendors I already do business with, do you think that I will spend more than three weeks in just negotiating the contract?

Yes, because there is some vendor out there named “Ruby that you need to sign a contract with before you can begin a project.

Despite his claims to be agile, McGovern obviously doesn’t know the first thing about agile development. People come first, sure, but agile development doesn’t say that tools aren’t important. Not using good tools makes it harder for good people to do good work.

That’s why I love developing software for Mac OS X and why I love helping people develop software on Mac OS X: We have great tools like Cocoa, Core Data, Interface Builder, OCUnit, WebObjects, and Xcode, and these can be used by great developers to do great things.

August 16, 2005 Technology Post a Comment

Xcode: Debugging Cocoa application unit tests

A couple weeks ago as part of my Unit Testing Series I talked about how to use Xcode to write unit tests for Cocoa frameworks, debug unit tests in Cocoa frameworks, and write unit tests for Cocoa applications. However, I haven’t yet described how to debug your unit tests in Objective-C Cocoa applications. I’ll take care of that tonight.

After you’ve set up unit testing in your Cocoa application, debugging your unit tests is similar to debugging them in a Cocoa framework. All you have to do is adjust the arguments and environment variables your application’s Executable is configured to use in Xcode. You don’t even have to create a new executable.

To start, bring up the Info window for your application’s executable (which is its entry in Xcode’s Executable smart group). In the Arguments tab, add the argument -SenTest All. This tells the unit testing infrastructure that you want to run all of the unit tests, not just the ones that are built in to the executable. (After all, you don’t have any unit tests in your executable itself.)

Now we’ll need to engage in a little bit of environment variable magic. When you test an application, what you’re really doing is injecting your unit test bundle into the application and telling it to run its tests at its first opportunity. This is accomplished through by telling dyld to insert a private framework, DevToolsBundleInjection.framework, into your application on launch, and telling that framework via an environment variable, XCInjectBundle, the path of the test bundle to inject.

You also have to tell the injection framework itself the full path to the application executable you want to inject the bundle into, via the XCInjectBundleInto environment variable. This is needed to avoid injecting your test bundle into other executables that are run by the application you’re testing, or that are run as a side-effect of running the application you’re testing. (For example, gdb generally wants to run applications from within a shell, so that environment variables are expanded in its environment and command-line parameters.)

In the Arguments tab of your application executable, first add an environment variable named DYLD_INSERT_LIBRARIES. Set its value to the path to the DevToolsBundleInjection.framework/DevToolsBundleInjection library in the framework of the same name that’s included in the developer tools. Prior to Xcode 2.5, this was in $(SYSTEM_LIBRARY_DIR)/PrivateFrameworks but as of Xcode 2.5 and Xcode 3.0, it has been moved to $(DEVELOPER_LIBRARY_DIR)/PrivateFrameworks

Then add a second environment variable, XCInjectBundle. Set its value to $(BUILT_PRODUCTS_DIR)/MyTestBundle.octest.

Add a third environment variable, XCInjectBundleInto. Set its value to the full path to your application’s executable — not just the application bundle — e.g. $(BUILT_PRODUCTS_DIR)/MyApplication.app/Contents/MacOS/MyApplication. This is the debugging equivalent of the Test Host build setting you used to tell Xcode what executable to inject your tests into when running them.

For Xcode 3.0 and later, add a final environment variable, DYLD_FALLBACK_FRAMEWORK_PATH to your executable. Set its value to $(DEVELOPER_LIBRARY_DIR)/Frameworks.

Why do you need to do this? In order to support moving and renaming the Developer folder, all of the frameworks within it — including OCUnit — use runpath search paths. This means that the internal name of the framework, including the one copied into your test bundle, will start with @rpath rather than an absolute path starting with /Developer/Library/Frameworks. Unfortunately this means that your unit tests won’t find SenTestingKit.framework without some extra help. That’s what DYLD_FALLBACK_FRAMEWORK_PATH does: It tells dyld to try an additional set of directories in place of @rpath when it can’t be resolved. (More information on runpath-relative install names can be found in the ld(1) man page.)

Make sure the check marks next to all three of these environment variables — and your -SenTest All argument, of course — are set.

Troubleshooting note: Troubleshooting note: If this doesn’t work — that is, if your test bundle isn’t found and run — change the executable’s working directory (in the General tab) to Built Products Directory and remove $(BUILT_PRODUCTS_DIR) above. Generally this is caused by $(BUILT_PRODUCTS_DIR) not being expanded to a full path, but rather to a partial path relative to your project directory.

Now if you choose Run Executable from the Debug menu, your application should launch, you should see the results of executing your unit tests in the Run Log, and as soon as your unit tests are complete your application should quit!

To debug a failing test, build your tests and set a breakpoint on the line where the failure occurs. Now choose Debug Executable from the Debug menu. As with a Cocoa framework, do not choose Build and Debug from the Build menu. You need to use Debug Executable because your build will fail due to the failing test. Debug Executable will work as long as your executable itself is actually present.

Having done all this, you should be stopped at the breakpoint!

Just as any other time you use OCUnit, instead of -SenTest All you can specify -SenTest MyTestCaseClassName to run just the tests in the test case class MyTestCaseClassName, or -SenTest MyTestCaseClassName/testMethodName to run just a single test.

Update July 7, 2007: Added the troubleshooting note about removing $(BUILT_PRODUCTS_DIR) if you get errors about not being able to load the bundle.

Update March 17, 2008: I’ve updated this a bit to handle some changes in the process introduced with Xcode 3.0.

Update September 5, 2008: I’ve updated this again to cover the changes that were made to bundle injection for Xcode 3.1; the change is the introduction of the XCInjectBundleInto environment variable.

July 24, 2005 Technology Post a Comment

Xcode: Unit Testing Cocoa Applications

Yesterday, I talked about how to add unit tests to Cocoa frameworks using Xcode. There’s only a little more set-up required to add tests to Objective-C Cocoa applications.

First, turn off ZeroLink for the application target you want to test. Just as with a framework, your unit tests will be built as a test bundle containing only the tests, completely separate from the code being tested. The test bundle will access the code in your application by linking against it, and you can’t link against something built with ZeroLink. Note: You only need to do this for Xcode 2.1 through Xcode 2.5. Xcode 3.0 removed support for ZeroLink, since the linker is now sufficiently fast as to obviate it.

Next, add a new Cocoa Unit Test Bundle target to your application project. This is the target that will actually contain your tests. When you add the test bundle target, Xcode shows its Info window. In the General tab of this window, press the + button and choose your application target from the sheet that appears. This will make your test bundle target depend on your application target, so you can just build your test bundle all the time and have your application automatically rebuild first.

Now you actually need to make your test bundle link against your application. You do this using the Bundle Loader property under Linker Options in your test bundle target’s configurations. You need to set the value of this option to the path of your application executable and not just its wrapper: $(BUILT_PRODUCTS_DIR)/MyApplication.app/Contents/MacOS/MyApplication. The $(BUILT_PRODUCTS_DIR) variable is expanded at build time to be the path to the build products for the currently-selected configuration, which lets you avoid using an absolute path.

The final step is to tell the unit testing infrastructure how to run your tests. For a framework, the dynamic loader does the heavy lifting; when the test rig loads your test bundle, the loader will automatically load the frameworks it depends on including the one you’re testing. However, for an application to be tested the application must be launched and the test bundle injected into it. You can specify that this should happen using the Test Host property under Unit Testing in your test bundle target’s configurations. Just as with the Bundle Loader property, you need to set the value to the full path of your application executable. Since you’ve already done that once, you can just re-use the value of the Bundle Loader setting by specifying $(BUNDLE_LOADER) — this is the underlying variable that the property is associated with.

Now just like with frameworks you can add test cases to your test bundle just by adding new files based on the Cocoa Objective-C test case class template. Your application code can remain in your application target and your unit testing code can remain in your test bundle target. Whenever you build your test bundle target, once everything is built your application will launch, its tests will be run, and it will quit — and the test results will be reported via the Build Results window just like compiler and linker errors.

July 23, 2005 Technology 1 Comment

Xcode: Debugging Cocoa framework unit tests

So you’ve set up unit testing for your Objective-C Cocoa framework and it’s been working great. But now you’ve written a test and it fails, and you can’t figure out why. It’s time to break out the debugger, but how, exactly, do you do that? Your unit tests are built as a bundle, and you can’t run a bundle.

It’s simple. All you have to do is set up an appropriate Executable in Xcode to run the test rig that runs your bundle, and then debug that. To get started, choose the Project > New Custom Executable menu item. For its name, specify otest — this is the name of the test rig used by OCUnit. Specify /Developer/Tools/otest as the path to your tool.

When you add the custom executable, Xcode will bring up its info window. Switch to the Arguments tab. Here you’ll need to enter some command-line arguments to pass to the test rig and some environment variables to affect how it’s run.

First, add the argument -SenTest Self to your executable. This tells otest to run all of the tests in your bundle. (It’s actually a pair of arguments, but you can add it as one as a convenience.) Then for your second argument specify $(BUILT_PRODUCTS_DIR)/MyTestBundle.octest where MyTestBundle is the name of your test bundle. This tells otest the path of the test bundle to load; $(BUILT_PRODUCTS_DIR) will be expanded to the appropriate build products directory for your project at run time. (If you get the order wrong, just drag the arguments around in the table.)

Troubleshooting note: If this doesn’t work — that is, if otest complains it can’t find your test bundle — change the executable’s working directory (in the General tab) to Built Products Directory and remove $(BUILT_PRODUCTS_DIR) above. Generally this is caused by $(BUILT_PRODUCTS_DIR) not being expanded to a full path, but rather to a partial path relative to your project directory.

Next add a pair of environment variables named DYLD_FRAMEWORK_PATH and DYLD_LIBRARY_PATH to your executable. These will tell the loader to check your build products directory first for frameworks and libraries whenever the executable is run from within Xcode. Specify $(BUILT_PRODUCTS_DIR) for the value of each variable.

Finally, from the Project > Set Active Executable menu choose your new otest executable. This way any time you run or debug within Xcode, it will run otest with the arguments and environment you’ve specified.

To actually debug a failing test, build your tests and set a breakpoint on the line where the failure occurs. Now choose Debug Executable from the Debug menu. Do not choose Build and Debug from the Build menu. You need to use Debug Executable because your build will fail due to the failing test; Debug Executable doesn’t try to build first, it only cares whether an executable is present.

You should successfully stop at your breakpoint!

If your tests take a long time to run — they shouldn’t, they’re unit tests after all, but it can still happen — you may want to just run the tests for one test case, or just one individual test. This is easy too. Rather than specifying -SenTest Self in your arguments to otest, you can specify -SenTest MyTestCaseClassName to run the all the tests in the specified test case. To run just a single test, use -SenTest MyTestCaseClassName/testMethodName instead.

Update July 7, 2007: Added the troubleshooting note about removing $(BUILT_PRODUCTS_DIR) if you get errors about not being able to load the bundle.

July 23, 2005 Technology Post a Comment

Xcode: Unit Testing Cocoa frameworks

It’s straightforward to write unit tests for Objective-C Cocoa frameworks with Xcode 2.1 and later.

First, turn off ZeroLink when building your framework. ZeroLink is a great technology, but you can’t link against something that’s built with ZeroLink, and that’s exactly what your unit tests are going to do. Note: You only need to do this for Xcode 2.1 through Xcode 2.5. Xcode 3.0 removed support for ZeroLink, since the linker is now sufficiently fast as to obviate it.

Next, add a new Cocoa Unit Test Bundle target to your framework project. This is the target that will actually contain your tests.

When you add the new test bundle target Xcode should bring up its Info window. Switch to its General tab and press the + button in the lower-left to add a new dependency. In the resulting sheet, choose your framework target. This tells the build system that your framework target needs to be built before your test bundle target.

Now that you’ve declared the dependency between the targets, you need to make sure your test bundle actually links against your framework. Make your test bundle target the active target by choosing it from the Project > Set Active Target submenu. Then highlight the Products group in the Groups & Files view at the left of the Xcode project window. In the detail view to the right, you’ll see all of the products that are built by the various targets in your project; one of the rows will be for your framework. Click the checkbox at the very right of its row, in the Target Membership column, to make your test bundle link against your framework.

Now you can add test cases to your test bundle by just adding new files based on the Cocoa Objective-C test case class template. You don’t need to add any of your framework code to your test bundle, and you don’t need to add any testing code to your framework. And you’re not restricted to using only header files with public visibility in tests, either; you should be able to use anything from your framework target, so long as it’s exported at link time.

July 17, 2005 Technology 1 Comment

Trust, but verify.

President Reagan, for all his faults, gave us a very useful aphorism in describing his approach to diplomacy with the Soviet Union: “Trust, but verify.” This is also a very useful approach to take when writing unit tests when you’re working with a framework, particularly when you’re developing a human interface.

For example, Cocoa uses the target-action pattern for controls. When you create an NSButton you can specify which object to send a message to when it’s clicked — the target — and what message to send — the action.

You can trust that when you click the button, Cocoa will cause the action message to be sent to the target object so long as they have been properly specified. Therefore you probably don’t need to write a unit test that simulates clicking on the button. However, you should verify that your button has had its target and action properly specified, and you can write a test for this.

You can also apply this principal in your own code. Let’s say you’re implementing a new type of control that also has to follow the target-action pattern. In the tests for your control itself, you probably will want to simulate the appropriate user-interface events and see that a testing instance of your control behaves appropriately. However, you don’t need to do this in the code that uses the control — you can trust that the control behaves correctly due to the tests you wrote for the control itself, and just verify that it’s been properly configured.

July 16, 2005 Technology Post a Comment

Xcode: Unit Testing

Xcode 2.1 introduced integrated unit testing to the Xcode IDE. Xcode includes two unit testing frameworks, target templates for setting up test bundle targets, and infrastructure for running unit tests every time you build your project and reporting their results in the Build Results window just like compilers and linkers do.

With Xcode unit testing, you group your test cases into test bundles which are built by separate targets. This means that you don’t need to build two versions of your software — one with tests and one without tests — and can run your tests against the actual software you want to deliver.

Xcode 2.1 and later include OCUnit for unit testing of Objective-C Cocoa software, and it includes a new framework called CPlusTest for unit testing of C++ software. Using either you should be able to test C code with relative ease. Corresponding target templates are included for creating unit test bundles appropriate for Cocoa and Carbon applications and frameworks, and file templates are included for creating OCUnit and CPlusTest test case classes.

The test bundle target templates have a shell script build phase at the very end that invokes /Developer/Tools/RunUnitTests. RunUnitTests looks at the build settings it’s passed via its environment and determines from that information how to run the tests in your test bundle.

If you’re testing a framework, RunUnitTests will run the appropriate test rig and tell it to load and run the tests in your bundle. Since your test bundle should link against your framework, your framework will be loaded when the test rig loads your bundle.

If you’re testing an application, you need to specify the application as the Test Host and Bundle Loader for your test bundle in its configuration’s build settings. The Bundle Loader setting tells the linker to link your bundle against the application that’s loading it as if the application were a framework, allowing you to refer to classes and other symbols within the application from your bundle without actually including them in the bundle. The Test Host setting tells RunUnitTests to launch the specified application and inject your test bundle into it in order to run its tests.

There’s even support in RunUnitTests for invoking a test rig of your own, rather than the test rig for one of the supplied frameworks. You just need to specify the Test Rig build setting for your test bundle; this should be the path to a tool to run. It will be passed the path to your test bundle as its first argument, and if it needs to generate failure information it can just generate it in a gcc/compiler-like format on stderr:

FailingTest.c: 10: error: (1 == 0) failed

This will cause it to show up in the Build Results window as an error, just like a compiler or linker error. You can see the RunUnitTests manpage for more information on the environment in which your test rig will be run.

There’s a great Unit Testing Guide on the Apple Developer Connection web site that has lots of information on getting started with Xcode unit testing. Check it out!

May 5, 2005 Technology Post a Comment

Unit testing and Core Data

Mike Zornek asks about unit testing and Core Data. I’ve been meaning to write about this, so this is the perfect opportunity to do so.

Writing unit tests against your model and code that uses Core Data is easy. For example, it’s trivial to load your compiled model in a unit test: 

NSManagedObjectModel *model = [NSManagedObjectModel mergedModelFromBundles:nil];

Not only that, but you can introspect it:

NSArray *entities = [model entities];

And you can do this all the way down to the property level. This means that it’s possible to assert that your entire model is set up the way you expect it to be. For example, you can make sure that your Employee entity has a mandatory salary attribute with a minimum value of 1 and a type of NSDecimalAttributeType, and descends from a Person entity that has a mandatory name attribute with a minimum length of 1 and a default value of “name.”

But how do you test your use of Core Data? You just use Core Data in your tests as you would in your project. For example, to instantiate a complete Core Data “stack” (as it’s sometimes referred to):

NSManagedObjectModel *model;
NSPersistentStoreCoordinator *coordinator;
NSManagedObjectContext *context;

model = [[NSManagedObjectModel alloc] initWithContentsOfURL:urlToModel];
coordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:model];
context = [[NSManagedObjectContext alloc] init];
[context setPersistentStoreCoordinator:coordinator];

To instantiate managed objects associated with that context from entities in your model:

NSManagedObject *employee;

employee = [NSEntityDescription insertNewObjectForEntityForName:@”Employee”
inManagedObjectContext:context];

This gives you an autoreleased Employee (assuming your context’s coordinator’s model has an Employee entity, of course). It’s that easy.

You can then do things like check that this object was created with the correct defaults (e.g. the ones specified in your model), that it posts KVO notifications properly for properties where you care about such things, and so on. You can even add a persistent store to the coordinator and test that saving and loading work (and don’t work when they’re supposed to fail, of course) just by using normal Core Data and Foundation APIs.

You can multiply the full power of data modeling with the full power of unit testing and test driven development. It kicks ass.