//  CoreDataStack.h

#import <Foundation/Foundation.h>
#import <CoreData/CoreData.h>

@interface CoreDataStack : NSObject

+ (instancetype)sharedInstance;

@end

//  CoreDataStack.m

#import "CoreDataStack.h"

@implementation CoreDataStack

+ (instancetype)sharedInstance {
    static CoreDataStack *stack;
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        stack = [[CoreDataStack alloc] init];
    });

    return stack;
}
@end

//  CoreDataStack.m

@interface CoreDataStack ()

@property (nonatomic, strong) NSManagedObjectModel *managedModel;

@end

......
- (NSManagedObjectModel *)managedModel {
    if (!_managedModel) {
        NSURL *momdURL = [[NSBundle mainBundle]URLForResource:@"Model" withExtension:@"momd"];
        _managedModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:momdURL];
    }
    return _managedModel;
}

//  CoreDataStack.m

@interface CoreDataStack ()

@property (nonatomic, strong) NSPersistentStoreCoordinator *psc;

@end

......

- (NSPersistentStoreCoordinator *)psc {
    if (!_psc) {
        _psc = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:self.managedModel];

        NSURL *URL = [[self documentURL] URLByAppendingPathComponent:@"Model.sqlite"];
        NSError *error;
        if (![_psc addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:URL options:nil error:&error]) {
            NSLog(@"addPersistentStoreWithType Error: %@",[error localizedDescription]);
        }
    }
    return _psc;
}

- (NSURL *)documentURL {
    return [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] firstObject];
}

//  CoreDataStack.h

@interface CoreDataStack : NSObject

@property (nonatomic, strong, readonly) NSManagedObjectContext *managedContext;

@end

//  CoreDataStack.m

@interface CoreDataStack ()

@property (nonatomic, strong, readwrite) NSManagedObjectContext *managedContext;

@end

- (NSManagedObjectContext *)managedContext {
    if (!_managedContext) {
        _managedContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType];
        _managedContext.persistentStoreCoordinator = self.psc;
    }
    return _managedContext;
}

- (void)saveContext {
    NSError *error;
    if ([self.managedContext hasChanges] && ![self.managedContext save:&error]) {
        NSLog(@"NSManagedObjectContext Save Error: %@",[error localizedDescription]);
    }
}

@property (readonly, strong) NSPersistentContainer *persistentContainer;

- (NSPersistentContainer *)persistentContainer {
    // The persistent container for the application. This implementation creates and returns a container, having loaded the store for the application to it.
    @synchronized (self) {
        if (_persistentContainer == nil) {
            _persistentContainer = [[NSPersistentContainer alloc] initWithName:@"testCoreData"];
            [_persistentContainer loadPersistentStoresWithCompletionHandler:^(NSPersistentStoreDescription *storeDescription, NSError *error) {
                if (error != nil) {
                    // Replace this implementation with code to handle the error appropriately.
                    // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.

                    /*
                     Typical reasons for an error here include:
                     * The parent directory does not exist, cannot be created, or disallows writing.
                     * The persistent store is not accessible, due to permissions or data protection when the device is locked.
                     * The device is out of space.
                     * The store could not be migrated to the current model version.
                     Check the error message to determine what the actual problem was.
                    */
                    NSLog(@"Unresolved error %@, %@", error, error.userInfo);
                    abort();
                }
            }];
        }
    }

    return _persistentContainer;
}

- (void)saveContext {
    NSManagedObjectContext *context = self.persistentContainer.viewContext;
    NSError *error = nil;
    if ([context hasChanges] && ![context save:&error]) {
        // Replace this implementation with code to handle the error appropriately.
        // abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
        NSLog(@"Unresolved error %@, %@", error, error.userInfo);
        abort();
    }
}

NSEntityDescription *entity = [NSEntityDescription entityForName:@"Doge" inManagedObjectContext:_context];
Doge *doge = [[Doge alloc] initWithEntity:entity insertIntoManagedObjectContext:_context];
doge.name = @"xxx";
...
...
//注意，这里并没有实际把对象存入数据库，实际存入需要调用NSManagedObjectContext 的save:方法
[_context save:nil];

Doge *doge = ...
 [_context deleteObject:doge];
 [_context save:nil]

NSFetchRequest *fetchRequest =  [NSFetchRequest fetchRequestWithEntityName:@"Doge"];
//默认就为NSManagedObjectResultType
//fetchRequest.resultType = NSManagedObjectResultType;
NSArray *result = [_context executeFetchRequest:fetchRequest error:nil];
//这里数组的元素为Doge对象

NSFetchRequest *fetchRequest =  [NSFetchRequest fetchRequestWithEntityName:@"Doge"];
fetchRequest.resultType = NSCountResultType;
NSArray *result = [_context executeFetchRequest:fetchRequest error:nil];
//这里数组的元素为 NSNumber ，一般数组只有一个元素
if ([result count] > 0){
    NSInteger count = [[results objectAtIndex:0] integerValue];
}

NSFetchRequest *fetchRequest =  [NSFetchRequest fetchRequestWithEntityName:@"Doge"];
fetchRequestWithEntityName:@"Doge"];
fetchRequest.predicate = [NSPredicate predicateWithFormat:@"age > 5"];
NSArray *result = [_context executeFetchRequest:fetchRequest error:nil];

NSFetchRequest *fetchRequest =  [NSFetchRequest fetchRequestWithEntityName:@"Doge"];
    NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:@"age" ascending:YES];
    fetchRequest.sortDescriptors = @[sortDescriptor];

    NSArray *result = [_context executeFetchRequest:fetchRequest error:nil];

NSFetchRequest *fetchRequest =  [NSFetchRequest fetchRequestWithEntityName:@"Doge"];
NSAsynchronousFetchRequest *asyncFetchRequest = [[NSAsynchronousFetchRequest alloc] initWithFetchRequest:fetchRequest completionBlock:^(NSAsynchronousFetchResult * _Nonnull results) {
    NSLog(@"%@",results);
}];
[_context executeRequest:asyncFetchRequest error:nil];

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    // Override point for customization after application launch.
    [self importSeedJsonSeedIfNeeded];
    return YES;
}
- (void)importSeedJsonSeedIfNeeded {

    NSManagedObjectContext *context = [[MTCoreDataStack sharedInstance] managedContext];

    NSFetchRequest * fetchRequest = [Master fetchRequest];
    fetchRequest.resultType = NSCountResultType;

    NSError *error;
    NSArray *results = [context executeFetchRequest:fetchRequest error:&error];
    if (!error) {
        if ([results count] > 0) {
            NSInteger masterCount = [[results objectAtIndex:0] integerValue];
            if (masterCount <= 0) {
                [self importJsonSeed];
            }
        } else {
            [self importJsonSeed];
        }

    }else {
        [self importJsonSeed];
    }

}

- (void)importJsonSeed {
    NSURL *jsonURL = [[NSBundle mainBundle] URLForResource:@"seed" withExtension:@"json"];

    NSError *error;
    NSData *jsonData = [NSData dataWithContentsOfURL:jsonURL options:0 error:&error];

    if (!error) {
        NSDictionary *jsonDict = [NSJSONSerialization JSONObjectWithData:jsonData options:0 error:&error];
        if (!error) {

            NSManagedObjectContext *context = [[MTCoreDataStack sharedInstance] managedContext];

            NSEntityDescription *masterEntity = [NSEntityDescription entityForName:@"Master" inManagedObjectContext:context];
            NSEntityDescription *dogeEntity = [NSEntityDescription entityForName:@"Doge" inManagedObjectContext:context];

            NSDictionary *masterDict = jsonDict[@"master"];
            if (![masterDict isKindOfClass:[NSDictionary class]]) {
                return;
            }

            Master *master = [[Master alloc] initWithEntity:masterEntity insertIntoManagedObjectContext:context];
            master.name = masterDict[@"name"];
            master.age = [masterDict[@"age"] integerValue];

            NSArray *doges = jsonDict[@"doges"];

            for (NSDictionary *dogeDict in doges) {
                if (![dogeDict isKindOfClass:[NSDictionary class]]) {
                    continue;
                }
                Doge *doge = [[Doge alloc] initWithEntity:dogeEntity insertIntoManagedObjectContext:context];
                doge.name = dogeDict[@"name"];
                doge.age = [dogeDict[@"age"] integerValue];
                doge.master = master; //因为在DataModel 里设置了 inverse， 所以master 和 doge可以相互关联上
            }
            //一定要调用 save 方法，才能真正的将文件存入数据库
            [[MTCoreDataStack sharedInstance] saveContext];
        }
    }
}