Core Data 概念

The NSFetchedResultsController is a type of controller provided by the Core Data framework that helps manage results from queries. 
NSFetchedResultsController幫我們管理我們要做的查詢
NSManagedObjectContext is a handle to the application’s persistent store that provides a context, or environment, in which the managed objects can exist. 
NSManagedObjectContext 幫我們產生一個存放managed objects的環境，並幫我們處理與persistent store之間的溝通。

//Sample code

NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:
self.managedObjectContext sectionNameKeyPath:nil cacheName:@"Master"]; 
aFetchedResultsController.delegate = self; 
self.fetchedResultsController = aFetchedResultsController; 
我們來了解一下cacheName這個參數的用法：
如果你不想要cache查詢結果，那直接傳nil給他就好。
如果你傳了cacheName給它，那代表它會在下次搜尋之前，先去檢查上一次的cache有沒有相同的值。
如果有找到相同的查詢，就不進行查詢直接取用它；如果不相同，就刪除上次的cache，然後存放這次的查詢結果供下次使用。

the persistent store 通常在操作 SQLite database
而The managed object model 是persistent store的抽象邏輯表現層。

Calling the getter for the delegate’s managedObjectContext starts a chain reaction in which
-(NSManagedObjectContext *)managedObjectContext, calls -(NSPersistentStoreCoordinator *)persistentStoreCoordinator and then in turns calls -(NSManagedObjectModel *)managedObjectModel. The call to managedObjectContext therefore initializes the entire Core Data stack and readies Core Data for use. 

//Delete時，有四個規則

1. No action 什麼都不做：Does nothing and lets the related objects think the parent object still exists.
2. Nullify 把每個相關連的物件的父物件設為nil：For each related object, sets the parent object property to null. 
3. Cascade 把所有相關的物件也一併刪除：Deletes each related object.
4. Deny 如果仍有相關物件，則禁止delete的動作：Prevents the parent object from being deleted if there is at least one related object. 

//NSFetchedResultsController是用來給UITableView使用的
You use a fetched results controller to efficiently manage the results returned from a Core Data fetch request to provide data for a UITableView object.

//讀取資料

- (NSArray *) readDataWithEntityName:(NSString *)entityName andPredicate:(NSPredicate *)predicate sortDescriptor:(NSString *)sortDescriptorString asceding:(BOOL)isAsceding{

    

    NSManagedObjectContext *context = [_coreData managedObjectContext];

    NSEntityDescription *entityDescription = [NSEntityDescription entityForName:entityName inManagedObjectContext:context];

    NSFetchRequest *request = [[NSFetchRequest alloc] init];

    [request setEntity:entityDescription];

    

    // Set example predicate and sort orderings...

    [request setPredicate:predicate];

    

    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc]

                                        initWithKey:sortDescriptorString ascending:isAsceding];

    [request setSortDescriptors:@[sortDescriptor]];

    

    NSError *error;

    NSArray *array = [context executeFetchRequest:request error:&error];

    if (array == nil)

    {

        // Deal with error...

    }

    return array;

}

#pragma mark Save
- (void)saveContext
{
    NSError *error = nil;
    NSManagedObjectContext *managedObjectContext = self.managedObjectContext;
    if (managedObjectContext != nil) {
        if ([managedObjectContext hasChanges] && ![managedObjectContext 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();
        }
    }
}

#pragma mark Add
- (void) addNewRecordIntoEntityByName:(NSString *)entityName newObjects:(NSArray *)objects{

    

    NSManagedObject *newManagedObject = [NSEntityDescription insertNewObjectForEntityForName:entityName inManagedObjectContext:_managedObjectContext];

    

    // If appropriate, configure the new managed object.
    // Normally you should use accessor methods, but using KVC here avoids the need to add a custom class to the template.
    for (NSDictionary *objectInfo in objects) {
        [objectInfo enumerateKeysAndObjectsUsingBlock:^(id key, id obj, BOOL *stop) {
            [newManagedObject setValue:obj forKey:key];
        }];
    }

    

    

    // Save the context.
    NSError *error = nil;
    if (![_managedObjectContext 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();
    }
}
#pragma mark Delete

//刪除指定物件

-(void) deleteRecord:(NSManagedObject *)managedObject{

    

    [_managedObjectContext deleteObject:managedObject];

    

    NSError *error = nil;
    if (![_managedObjectContext 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();
    }
}

//恢復上一步

-(void) undoRecord{

    

    [_managedObjectContext.undoManager undo];

    

    NSError *error = nil;
    if (![_managedObjectContext 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();
    }
}

//恢復下一步

-(void) redoRecord{

    

    [_managedObjectContext.undoManager redo];

    

    NSError *error = nil;
    if (![_managedObjectContext 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();
    }

}