Opinionated Core Data: Hold On To Your Butts

This presentation looks at the:
1) Principles
2) Architecture
3) Classes
4) ViewControllers

for successfully using and *TESTING* Core Data in your App.

Transcript

1. Opinionated Core Data
   Hold On To Your Butts
   http://ParveenKaler.com
   pk@smartfulstudios.com
   @kaler
   

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2. 1.Principles
   2.Architecture
   3.Classes
   4.ViewControllers
   Hold On To Your Butts
   

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3. Goaly
   CoreData
   Stack
   Fitbit
   Jawbone
   Nike+
   Runkeeper
   Health and fitness journal and dashboard
   Timeline
   Dashboard
   

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4. 1. Principles
   

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5. The Law of Leaky Abstractions
   "All non-trivial abstractions, to some degree, are leaky.”
   — Joel Spolsky
   

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6. What does Core Data abstract?
   Class Abstraction
   NSPeristentStoreCoordinator SQLite file
   NSManagedObjectModel SQLite schema
   NSManagedObjectContext SQL statements, query engine
   NSManagedObject Row in a table
   

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7. Active Record
   Principles of Enterprise Application Architecture by Martin Fowler
   

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8. Active Record
   • Construct an instance of the Active Record from a SQL result set row
   • Construct a new instance for later insertion into the table
   • Static finder methods to wrap commonly used SQL queries and return
   Active Record objects
   • Update the database and insert into it the data in the Active Record
   • Get and set the fields
   • Implement some pieces of business logic
   Principles of Enterprise Application Architecture by Martin Fowler
   

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9. Active Record
   • NSManagedObjectContext does insert, delete, update
   • NSEntityDescription constructs a row
   • NSManagedObject does have fields though
   

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10. Magical Record?
    https://github.com/magicalpanda/MagicalRecord
    

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11. Datamapper
    Principles of Enterprise Application Architecture by Martin Fowler
    

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12. Datamapper
    @class NSEntityDescription : NSObject
    + (NSEntityDescription *)entityForName:(NSString *)entityName inManagedObjectContext:(NSManagedObjectContext *)context
    @end
    

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13. SQLite
    

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14. SQLite
    CoreData: annotation: Connecting to sqlite database file at "/var/mobile/Applications/79445866-27E6-437D-B98B-
    F4FC9951E64A/Documents/xxxx.sqlite"
    CoreData: sql: pragma journal_mode=wal
    CoreData: sql: pragma cache_size=200
    CoreData: sql: SELECT Z_VERSION, Z_UUID, Z_PLIST FROM Z_METADATA
    CoreData: sql: SELECT 0, t0.Z_PK, t0.Z_OPT, t0.ZDATE, t0.ZFROM, t0.ZMESSAGE, t0.ZSUBJECT, t0.ZTHREADID, t0.ZTO FROM
    ZMESSAGE t0 ORDER BY t0.ZTHREADID DESC
    CoreData: annotation: sql connection fetch time: 0.0087s
    CoreData: annotation: total fetch execution time: 0.0154s for 71 rows.
    CoreData: sql: SELECT 0, t0.Z_PK, t0.Z_OPT, t0.ZACCESSTOKEN, t0.ZANONYMOUSEMAIL, t0.ZANONYMOUSPHONENUMBER, t0.ZEMAIL,
    t0.ZFIRSTNAME, t0.ZLASTNAME, t0.ZMEMBERID, t0.ZPHONENUMBER, t0.ZVERIFICATIONCODE FROM ZACCOUNT t0
    CoreData: annotation: sql connection fetch time: 0.0041s
    CoreData: annotation: total fetch execution time: 0.0067s for 0 rows.
    CoreData: sql: SELECT 0, t0.Z_PK, t0.Z_OPT, t0.ZACCESSTOKEN, t0.ZANONYMOUSEMAIL, t0.ZANONYMOUSPHONENUMBER, t0.ZEMAIL,
    t0.ZFIRSTNAME, t0.ZLASTNAME, t0.ZMEMBERID, t0.ZPHONENUMBER, t0.ZVERIFICATIONCODE FROM ZACCOUNT t0
    CoreData: annotation: sql connection fetch time: 0.0051s
    CoreData: annotation: total fetch execution time: 0.0079s for 0 rows.
    

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15. 2. Architecture
    

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16. Construct The Whole Stack
    @interface GLYManagedObjectContext : NSManagedObjectContext
    + (instancetype)createAt:(NSURL *)storeURLorNil;
    @end
    

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17. Construct The Whole Stack
    + (instancetype)createAt:(NSURL *)storeURLorNil
    {
    NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"Goaly" withExtension:@"momd"];
    NSManagedObjectModel *mom = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
    !
    NSPersistentStoreCoordinator *coordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:mom];
    NSString *type = storeURLorNil ? NSSQLiteStoreType : NSInMemoryStoreType;
    NSDictionary *options =
    @{
    NSMigratePersistentStoresAutomaticallyOption : @YES,
    NSInferMappingModelAutomaticallyOption : @YES
    };
    NSError *error = nil;
    if (![coordinator addPersistentStoreWithType:type configuration:nil URL:storeURLorNil options:options error:&error])
    {
    NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
    abort();
    }
    !
    GLYManagedObjectContext *ctx = [[GLYManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
    [ctx setPersistentStoreCoordinator:coordinator];
    return ctx;
    }
    

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18. Don’t Use Fucking Singletons
    • How are you gonna unit test? You unit test, right?
    • Explicitly hand the NSManagedObjectModel to a list view
    • Explicitly hand a NSManagedObject to a detail view
    • Handle signout by releasing the NSManagedObjectContext
    

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19. Unit Testing
    @interface GoalyTests : XCTestCase
    @property (nonatomic, strong) GLYManagedObjectContext *context;
    @end
    !
    @implementation GoalyTests
    !
    - (void)setUp
    {
    [super setUp];
    self.context = [GLYManagedObjectContext createAt:nil];
    }
    !
    @end
    

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20. Unit Testing
    • Fat Model, Skinny Controller
    • Model shouldn’t import UIView or UIViewController
    • EVERYTHING else should be in the Model
    

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21. Multiple Stores
    • Per HTTP service
    • Static data like a food database
    • Per user like username.sqlite
    

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22. Multiple Contexts
    • UI NSManagedObjectContext with NSMainQueueConcurrencyType
    • Detail view NSManagedObjectContext for save/cancel operations
    • NSManagedObjectContext HTTP client for GET requests with lots of
    inserts
    

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23. Parent-Child Contexts
    NSManagedObjectContext *child;
    if (child)
    {
    if ([child hasChanges] && ![child save:&error])
    {
    NSLog(@"Unresolved error %@, %@", error, [error userInfo]);
    abort();
    }
    __block NSManagedObjectContext *parent;
    [parent performBlockAndWait:
    ^{
    NSError *parentError = nil;
    if (parent.persistentStoreCoordinator.persistentStores.count && [parent hasChanges] && ![parent save:&parentError])
    {
    NSLog(@"Unresolved error %@, %@", parentError, [parentError userInfo]);
    abort();
    }
    }];
    }
    

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24. 3. Classes
    

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25. NSManagedObject
    @interface GLYSleep : NSManagedObject
    !
    @property (nonatomic) int64_t logId;
    @property (nonatomic) NSTimeInterval startTime;
    @property (nonatomic) int64_t timeInBed;
    !
    + (NSString *)entityName;
    + (instancetype)instanceInManagedObjectContext:(NSManagedObjectContext*)ctx;
    !
    + (void)mapFromDictionary:(NSDictionary*)response intoManagedObjectContext:(NSManagedObjectContext*)ctx;
    - (NSDictionary*)mapToDictionary;
    !
    // Keep ugly NSDateFormatter bullshit internal
    - (NSString*)sleepTimeStringValue;
    !
    @end
    

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26. NSManagedObject
    TEST THAT SHIT! YOU CAN DO IT!
    

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27. Use the API Console
    

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28. NSManagedObject Unit Tests
    - (void)testSleep
    {
    NSString *json = @"....";
    NSData *responseObject = [json dataUsingEncoding:NSUTF8StringEncoding];
    NSError *error = nil;
    NSDictionary *d = [NSJSONSerialization JSONObjectWithData:responseObject options:0 error:&error];
    [GLYSleep mapFromDictionary:d intoManagedObjectContext:self.context];
    }
    

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29. NSFetchRequest
    • I usually implement Rails-style find, find_by, first, last, all, etc
    • Sometimes I implement it in NSManagedObjectModel, sometimes in
    NSFetchRequest
    • I’ve been putting it in NSFetchRequest recently
    

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30. NSFetchRequest
    @interface GLYSleepFetchRequest : NSFetchRequest
    + (GLYSleep*)find:(int64_t)logId;
    + (GLYSleep*)findOrCreate:(int64_t)logId;
    + (NSArray*)findOrCreate:(NSArray*)logIds;
    @end
    

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31. +(NSArray*)findOrCreate:(NSArray*)logIds
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *e = [NSEntityDescription entityForName:@"GLYSleep" inManagedObjectContext:ctx];
    fetchRequest setEntity:entity];
    !
    NSPredicate *p = [NSPredicate predicateWithFormat:@"(logId IN %@)", logIds]
    [fetchRequest setPredicate:p];
    !
    NSArray *sd = @[[[NSSortDescriptor alloc] initWithKey: @"logId" ascending:YES]];
    [fetchRequest setSortDescriptors:sd];
    !
    NSError *error = nil;
    NSArray *sleeps = [ctx executeFetchRequest:fetchRequest error:&error];
    

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32. +(NSArray*)findOrCreate:(NSArray*)logIds
    for (NSNumber *Id in logIds)
    {
    NSPredicate *p = [NSPredicate predicateWithFormat:@"logId = %@”, Id];
    NSArray *res = [sleeps filteredArrayUsingPredicate:p];
    if (res.count == 0)
    {
    // create
    }
    else
    {
    // find
    }
    }
    The above loop is O(n^2). In real code, I have an NSEnumerator that makes it O(n).
    

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33. 4. ViewControllers
    

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34. Pass in the minimalist thing possible
    • Pass in a NSManagedObject by default
    • Pass in a NSFetchRequest for list views
    • Pass in a child NSManagedObjectContext for edits
    • Pass in the entire NSManagedObjectContext only if you have to
    

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35. Creation/Deletion
    • Alloc/init your CoreData stack in the signin/signup view controller
    • Release your CoreData stack in the signout view controller
    

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36. TEST THAT SHIT! YOU CAN DO IT!
    

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37. Opinionated Core Data
    Hold On To Your Butts
    http://ParveenKaler.com
    pk@smartfulstudios.com
    @kaler
    

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