general model implementation by Core Data and NSFetchedResultsController - ios

Question

I am newbie in Core Data. I put all model requirements (in respect of MVC) in a class of NSFetchedResultsController as below:
Header File :
#import <CoreData/CoreData.h>
#interface GeneralModel : NSFetchedResultsController
#property (nonatomic, strong) NSManagedObjectContext *context;
#property (nonatomic, strong) NSManagedObjectModel *model;
- (NSString *)storagePath;
- (void)removeStorage;
- (void)truncateAllEntity;
- (void)truncateEntity:(NSString *)entityName;
- (void)addGroups:(NSDictionary *)insertData;
- (NSArray *)getEntity:(NSString *)entityName sortBy:(NSString *)sortAttribute;
- (id)getMaxValue:(NSString *)entityName forProperty:(NSString *)propertyName;
- (NSArray *)getEntity:(NSString *)entityName predicateBy:(NSPredicate *)predicate sortBy:( NSString * )sortAttribute;
#end
Main File:
#import "GeneralModel.h"
#import "GeneralHelper.h"
#implementation GeneralModel
- (instancetype)init
{
self = [super init];
if (self) {
// Read in Model.xcdatamodeld
_model = [NSManagedObjectModel mergedModelFromBundles:nil];
NSPersistentStoreCoordinator *psc =
[[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:_model];
// Where does the SQLite file go?
NSString *path = self.storagePath;
NSURL *storeURL = [NSURL fileURLWithPath:path];
NSError *error = nil;
if (![psc addPersistentStoreWithType:NSSQLiteStoreType
configuration:nil
URL:storeURL
options:nil
error:&error]) {
#throw [NSException exceptionWithName:#"OpenFailure"
reason:[error localizedDescription]
userInfo:nil];
}
// Create the managed object context
_context = [[NSManagedObjectContext alloc] init];
_context.persistentStoreCoordinator = psc;
}
return self;
}
- (NSString *)storagePath
{
NSArray *documentDirectories =
NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,
NSUserDomainMask,
YES);
// Get one and only document directory from that list
NSString *documentDirectory = [documentDirectories firstObject];
return [documentDirectory stringByAppendingPathComponent:#"model.sqlite"];
}
- (void)removeStorage {
// NSError *error2;
// NSString *storagePath = [self storagePath];
//
// NSDictionary *options = #{NSPersistentStoreUbiquitousContentNameKey: #"model"};
// bool removeResult = [NSPersistentStoreCoordinator removeUbiquitousContentAndPersistentStoreAtURL:[NSURL URLWithString:storagePath] options:options error:&error2];
// if (removeResult == NO) {
// NSLog(#"Could not remove Storage. Reason: %#", error2.localizedFailureReason);
// }
NSPersistentStore *store = [self.context.persistentStoreCoordinator.persistentStores lastObject];
NSError *error = nil;
NSURL *storeURL = store.URL;
BOOL isRemovePersistentStore = [self.context.persistentStoreCoordinator removePersistentStore:store error:&error];
if (isRemovePersistentStore == NO) {
NSLog(#"NO RemovePersistentStore. Reason: %#", error.localizedFailureReason);
}
BOOL isRemoveItemAtURL = [[NSFileManager defaultManager] removeItemAtURL:storeURL error:&error];
if (isRemoveItemAtURL == NO) {
NSLog(#"NO RemoveItemAtURL. Reason: %#", error.localizedFailureReason);
}
}
- (void)truncateAllEntity {
NSArray *entities = self.model.entities;
for (NSEntityDescription *entityDescription in entities) {
[self truncateEntity:entityDescription.name];
}
}
- (void)truncateEntity:(NSString *)entityName {
// delete all database
if (IOS_VERSION >= 9) {
NSFetchRequest *request = [[NSFetchRequest alloc] initWithEntityName:entityName];
NSBatchDeleteRequest *delete = [[NSBatchDeleteRequest alloc] initWithFetchRequest:request];
NSError *deleteError = nil;
[self.context.persistentStoreCoordinator executeRequest:delete withContext:self.context error:&deleteError];
} else {
NSFetchRequest *allItems = [[NSFetchRequest alloc] init];
[allItems setEntity:[NSEntityDescription entityForName:entityName inManagedObjectContext:self.context]];
[allItems setIncludesPropertyValues:NO]; //only fetch the managedObjectID
NSError *error = nil;
NSArray *items = [self.context executeFetchRequest:allItems error:&error];
//error handling goes here
for (NSManagedObject *item in items) {
[self.context deleteObject:item];
}
NSError *saveError = nil;
[self.context save:&saveError];
//more error handling here
}
[self.context rollback];
}
- (void)addGroups:(NSDictionary *)insertData {
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setDateFormat:#"yyyy-MM-dd HH:mm:ss"];
for (NSDictionary *response in insertData) {
NSManagedObject *existingGroup = [self getEntity:#"Group" AtValue:response[#"groupId"] forProperty:#"group_id" ];
if (existingGroup) {
NSLog(#"existingGroup");
continue;
}
// for (int i; i<=20; i++) {
// set bullet
NSManagedObject *object_bullet = [NSEntityDescription insertNewObjectForEntityForName:#"Group_bullet"
inManagedObjectContext:self.context];
NSData *bulletData = [[NSData alloc] initWithBase64EncodedString:response[#"bullet"] options:NSDataBase64DecodingIgnoreUnknownCharacters];
[object_bullet setValue:bulletData forKey:#"bullet"];
[object_bullet setValue:response[#"groupId"] forKey:#"group_id"];
// set group
NSManagedObject *object = [NSEntityDescription insertNewObjectForEntityForName:#"Group"
inManagedObjectContext:self.context];
[object setValue:object_bullet forKey:#"relatedBullet"];
// NSDate *createDate = [dateFormatter dateFromString:response[#"createDate"]];
// [object setValue:createDate forKey:#"group_created_date"];
NSNumber *createDate = response[#"createDate"];
[object setValue:[NSDate dateWithTimeIntervalSince1970:[createDate doubleValue]] forKey:#"group_created_date"];
[object setValue:response[#"groupId"] forKey:#"group_id"];
[object setValue:[GeneralHelper getInteger:response[#"memberNumber"]] forKey:#"group_members_count"];
[object setValue:response[#"name"] forKey:#"group_name"];
[object setValue:[GeneralHelper getInteger:response[#"privacy"]] forKey:#"group_privacy"];
[object setValue:#1 forKey:#"group_status"];
[object setValue:response[#"imageName"] forKey:#"image_name"];
[object setValue:response[#"lastMessageId"] forKey:#"last_message_id"];
[object setValue:response[#"lastMessageText"] forKey:#"last_message_text"];
//
// NSDate *lastMessageDate = [dateFormatter dateFromString:response[#"lastMessageDate"]];
// [object setValue:lastMessageDate forKey:#"last_time"];
NSNumber *lastMessageDate = response[#"lastMessageDate"];
[object setValue:[NSDate dateWithTimeIntervalSince1970:[lastMessageDate doubleValue]] forKey:#"last_time"];
[object setValue:response[#"machineId"] forKey:#"machine_id"];
NSNumber *timestamp = response[#"timestamp"];
[object setValue:[NSDate dateWithTimeIntervalSince1970:[timestamp doubleValue]] forKey:#"timestamp"];
[object setValue:[GeneralHelper getInteger:response[#"unreadCount"]] forKey:#"unread_count"];
// }
}
NSError *errorInsert;
if (![self.context save:&errorInsert]) {
NSLog(#"Failed to save - error: %#", [errorInsert localizedDescription]);
}
[self.context rollback];
}
- (id)getMaxValue:(NSString *)entityName forProperty:(NSString *)propertyName {
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:entityName];
fetchRequest.fetchLimit = 1;
fetchRequest.sortDescriptors = #[[NSSortDescriptor sortDescriptorWithKey:propertyName ascending:NO]];
NSError *error = nil;
id maxValue = [self.context executeFetchRequest:fetchRequest error:&error].firstObject;
if (maxValue == nil) {
maxValue = #{propertyName : #0};
}
[self.context rollback];
return [maxValue valueForKey:propertyName];
}
- (id)getEntity:(NSString *)entityName AtValue:(NSString *)indexValue forProperty:(NSString *)propertyName {
NSFetchRequest *request = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:entityName inManagedObjectContext:self.context];
NSPredicate *predicate = [NSPredicate predicateWithFormat:#"%K == %#", propertyName, indexValue];
// NSLog(#"predicate: %#", predicate);
[request setEntity:entity];
[request setPredicate:predicate];
[request setFetchLimit:1];
NSError *error;
NSArray *results = [self.context executeFetchRequest:request error:&error];
NSManagedObject *object = [results firstObject];
// if (object) {
// // there is at least one object matching you predicate
// } else {
// // there is no object matching your predicate in the moc
// }
// NSLog(#"results member %#", results);
// [self.context rollback];
return object;
}
Then I use this general model in my UITableViewController.
Is there any better way to do this? Or my implementation is buggy and useless?

Answer 1

It think this is a nice approach, but it has some drawbacks.
An NSFetchedResultsController is responsible, as the name implies for controlling results that were fetched. If you add functionality like deleting and modifying things, you are making an object with too many responsibilities.
Imagine there is one viewController showing results in a tableview. It would only need the fetchedresultscontroller functionality, nothing more. If by tapping on a cell you go into a new view controller where edits are possible, you don't need a fetchedresultscontroller: you need a managedObject to edit. This editViewController could use a child context of the tableViewController so that on cancel, all edits are discarded by simply discarding the child context.
I would suggest to no subclass the NSFetchedResultsController, but instead implement a subclass of NSObject that you initialize with an NSNMagedObjectContext that can do the things that apply to the entire context. Like adding groups, removing all data etc.
Then use composition: this NSObject subclass can be a property of a viewController so the viewCOntroller has a way to do those things without having this operation being specific to this viewController.
In general, instead of subclassing, consider composition first. This is because transforming a small components of a composed object to an object that is a certain subclass is usually easier then the other way around.
Feel free to disagree :), but this is what I've learned from experience of working with many people for a long time on a very large codebase.