Core Data Odds and Ends

I've been doing a fair bit of work with Core Data lately, and have picked up a few tricks to make my life easier, so I thought I'd share.

Mo' Power

Back in the EOF days, there was a great little piece of third-party software called EOGenerator that implemented something called the generation gap design pattern. Essentially, this little program would create your custom subclasses of EOGenericRecord (EOF's equivalent of NSManagedObject) for you. It wouldn't just create that, though, it would actually create two classes for each entity in your EOModel.

One of these two classes was yours to modify as you saw fit. It would never get overwritten even if you ran the EOGenerator script again, over and over. The other class contained all the generated code derived from your EOModel and was designed not to be touched by the programmer. You had your class, EOGenerator had its class, and you could both work without any fear of stepping on each other's toes. You could add attributes and entities to your data model and regenerate the machine-readable classes without any chance of losing your custom validation or methods. It was nice.

Wolf Rentzsch has re-implemented this idea for Core Data, and the result is called MOGenerator. If you spend any time with Core Data and aren't familiar with MOGenerator, go become familiar with it. It will make your life much, much, much better.

SQLite to Check Data

The default persistent store type for Core Data under iOS is a SQLite store, which means that Core Data keeps all of its data inside of a single SQLite database in your application's /Documents folder. Now, the format of the persistent store is undocumented and you should never, ever, ever, ever, never change data in the persistent store or make any assumptions about how Core Data stores the data. That's part of why we use Core Data: It's an abstraction layer that lets us not worry about the underlying storage details.

But, sometimes, it can be really helpful to know whether data is getting into the persistent store or not. I often work on my data model before my user interface, so being able to peek in the data store to make sure all is well is really helpful.

The first thing you might want to consider doing, if you plan to do that, is to create a SQLite config file. This is a file of commands that will execute whenever you launch SQLite. To do that, simply create a file called .sqliterc in your home directory. Here's what mine looks like:

.mode column
.header on
.table

In mine, the first line tells SQLite to print the column names when I issue a SELECT command. The second line tells SQLite to present the data in tabular form. The last line prints out a list of all the tables in the database before it shows me the SQL command prompt. That's handy given that the table names don't usually exactly match your entity or class names.

Once you have a config file in place, you can open up any persistent store using the sqlite3 command in Terminal.app, like so:

sqlite3 MyApplication.sqlite

You can't, of course, execute this directly against database files on your phone, but you can do it in the simulator as well as on files copied from your phone using Xcode's Organizer. If you're using the simulator, you can look for your SQLite databases at the following location:

/Users/[your account name]/Library/Application Support/iPhone Simulator/[SDK version #]/Applications/[application UUID]/Documents/[Application Name].sqlite

When I log into a database with my SQLite config file, this is what I see:

-- Loading resources from /Users/jeff/.sqliterc
ZHERO         ZPOWER        Z_METADATA    Z_PRIMARYKEY
SQLite version 3.6.12
Enter ".help" for instructions
Enter SQL statements terminated with a ";"
sqlite>

Notice that it has dumped the names of all my tables. It's pretty easy to figure out which table holds what data once they're right in front of you. The example above is the persistent store from the SuperDB application from More iPhone 3 Development. The Hero entity managed objects are stored in ZHERO and the Power entity managed objects are stored in ZPOWER. To look at the contents of a table, you just need to execute a SQL SELECT command. The simplest is just SELECT * FROM TABLENAME;, which prints the entire contents of the table to the console, like so:

sqlite> select * from ZPOWER;
Z_PK        Z_ENT       Z_OPT       ZHERO       ZSOURCE     ZNAME     
----------  ----------  ----------  ----------  ----------  ----------
1           2           2           1           Mutation    Fire      
sqlite> 

The format of the data is pretty easy to figure out but, again, you really should look and not touch. It'd be very easy to accidentally destroy a persistent store. A seemingly inconsequential change could get the metadata out of whack and prevent your app from using the persistent store any more.

Better Descrption

I noticed a few days ago that Wil Shipley was complaining about the fact that many of Apple's delivered UIKit and Foundation objects have not overridden the -description method. This method is important because it's the one that is used in format strings, NSLog(), and when you type the po command in GDB. Instead of getting a meaningful description of many delivered objects, you instead get the class name and memory address of the instance, which is what NSObject's implementation provides, which isn't very much help while debugging.

Unfortunately, NSManagedObject is one of the many, many classes that doesn't have its own meaningful implementation of -description. That doesn't mean you can't provide one, though.

One thing you can do is to create an abstract subclass of NSManagedObject and put the -description method there. If you then subclass this abstract class, all of your managed objects will inherit the method from the abstract class. If I have other common functionality, I'll often do exactly that. But if I don't (and often you won't) I personally hate to clutter up my object hierarchy to add debugging tools. So I actually will cheat and do something you're not really supposed to do, which is override a method using a category. The reason it's bad to do this is because the Objective-C language does not specify which method should be used, which can lead to unpredictable behavior. But, when a class inherits a method from a superclass, I've found that overriding it in a category always works. I'm not suggesting you should do this in a shipping app, however. I wrap my method in pre-compiler definitions so that it doesn't get compiled into the release version of my application.

My version of -description assumes you're generating properties for your managed objects (I always do) and also assumes you're using custom subclasses (but you are, because you're using MOGenerator, right??). The method iterates over all the properties of the class and all its superclasses up to NSManagedObject, then prints them to the console. The categories I use look like this:

#if DEBUG
@implementation NSObject(MCManagedObjectDebug)
+ (NSMutableArray *)MCproperties
{
    NSMutableArray *properties = nil;
    
    if ([self superclass] != [NSManagedObject class])
        properties = [[self superclass] MCproperties];
    else
        properties = [NSMutableArray array];
    
    
    unsigned int propCount;
    objc_property_t * propList = class_copyPropertyList([self class], &propCount);
    int i;
    
    for (i=0; i < propCount; i++)
    {
        objc_property_t oneProp = propList[i];
        NSString *propName = [NSString stringWithUTF8String:property_getName(oneProp)];
        if (![properties containsObject:propName])
            [properties addObject:propName];
    }
    return properties;
}
@end

@implementation NSManagedObject(MCManagedObjectDebug)
- (NSString *)description
{
    NSArray *properties = [[self class] MCproperties];
    NSMutableString *ret = [NSMutableString stringWithFormat:@"%@:", [self className]];
    NSDictionary *myAttributes = [[self entity] attributesByName];
    
    for (NSString *oneProperty in properties)
    {
        NSAttributeDescription *oneAttribute = [myAttributes valueForKey:oneProperty];
        if (oneAttribute != nil) // If not, it's a relationship or fetched property
        {
            id value = [self valueForKey:oneProperty];
            [ret appendFormat:@"\n\t%@ = %@", oneProperty, value];
        }

    }
    return ret;
}

@end
#endif

The +MCProperties method uses the Objective-C runtime to discover and iterate over properties in the class definition, including @dynamic properties that were created at runtime. Because the property runtime calls do not include properties inherited from parent classes, the method acts recursively, iterating up the object hierarchy until it gets to NSManagedObject. The actual -description method simply prints out each of the attributes and the value currently stored for that attribute for this object instance. Once you do this, if you use NSLog() or po, the result is much richer information about the objects getting printed to the console. This version ignores relationships and fetched properties, but it wouldn't be hard to add those in if you needed them.

Absolute Filenames

Well, I got bit today by something I should've known better than to do. It was a classic newbie mistake so, but it caused me enough grief that I feel it's worth embarrassing myself with a cautionary post. Here's what happened:

When you use Core Data, Apple recommends that except for relatively small chunks of binary data, you store binary data like images, sounds, and movies in the file system and not inside Core Data's persistent store. Now, in my experience, SQLite can handle even really large blobs without too much additional overhead, but when Apple makes a specific recommendation like this, I find it's usually a good idea to follow that recommendation unless I have a really good reason not to. In this case, I didn't.

In an app I'm working on for a client, I'm recording audio using AVAudioRecorder. I don't ask the user to choose a name for the file, I just store the recordings as caf files in the file system using a filename based on the date and time, like so:

NSString *audioPathString = [NSString stringWithFormat:@"%@/%@", DOCUMENTS_FOLDER, [item valueForKey:@"audioPath"]];

Then, later on, after the recording is done, I store audioPathString inside a managed object that represents audio recordings:

audioItem.audioPath = audioPathString;

Then, when I want to play the sound, I just pull the pathname from Core Data and load the data from the file system, sort of like this:

NSError *error;

NSURL *url = [NSURL fileURLWithPath:item.audioPath];
AVAudioPlayer *player =[[AVAudioPlayer alloc] initWithContentsOfURL:url error:&error];

And it worked great. For a while. I could quit and start back up and it would keep working. And then, at some point, it wouldn't work any more. It would report back OSStatus codes of -43 or -50 when I tried to create an instance of AVAudioPlayer, even though the audio file was still in the /Documents directory. I'm sure a few of you are chuckling to yourselves about my dumb error, but for those who don't see it, I'll explain.

Tip: If you find yourself in a situation where an NSError gives you nothing more than an OSStatus or OSErr error code, you can find out what that error code means by dropping to the terminal and typing macerror followed by the error code you're interested in, including the - (these old-style error codes are negative numbers).

The full pathname to an item in the Documents directory of an application's sandbox includes not the name of the application, but a UUID (unique identifier) instead. When you build and run your application, a new UUID is calculated if, and only if, something has changed since your last build. If you haven't made any changes, it keeps running with the previous UUID. Once you make some changes to code or a resource and re-compile, the UUID changes, and the stored absolute path no longer points to the right location. The portion of the URL that represents the Application's UUID has now changed. Xcode is smart enough to move the application's /Documents directory to the new location so you don't lose all of your data while developing, but all that data is now at a new location in the file system.

So, how do you handle this situation? The easiest way is to just store the filename and then re-build the absolute path at runtime by assuming the file is in your application's /Documents directory. So, instead of storing the path from recorderFilePath as I did in the earlier code sample, you would store just the last path component using NSString's lastPathComponent method, like so:

newItem.audioPath = [recorderFilePath lastPathComponent];

Then, when it's time to load the file, you just re-build the full path to the file based on the current UUID of the application, like so:

NSString *documentsDirectory = [NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES) objectAtIndex:0];
NSString *audioPathString = [NSString stringWithFormat:@"%@/%@", documentsDirectory, [item valueForKey:@"audioPath"]];

Using this approach, your code will continue to function even between development builds or application releases, and you won't waste an hour trying to figure out why your perfectly valid files won't load.