如何更改将对象保存到对象的不同索引的 NSFetchedResultsController 数组？

Question

我有一个方法可以用来保存表格视图中的单元格并将其移动到不同的索引，如下所示：

- (void)moveTableView:(FMMoveTableView *)tableView moveRowFromIndexPath:(NSIndexPath *)fromIndexPath toIndexPath:(NSIndexPath *)toIndexPath { 
NSArray *allObjects = [self.fetchedResultController fetchedObjects];
//array to hold the new order    
NSMutableArray * arrayCorectOrder = [NSMutableArray new];
//the target object that is in the index we are moving from
Target *target = [self.fetchedResultController objectAtIndexPath:fromIndexPath];
 }

我有一个 coreDataStack 类负责处理所有核心数据（创建一个单例），它看起来像这样：

#import "CoreDataStack.h"

@implementation CoreDataStack

#pragma mark - Core Data stack

@synthesize managedObjectContext = _managedObjectContext;
@synthesize managedObjectModel = _managedObjectModel;
@synthesize persistentStoreCoordinator = _persistentStoreCoordinator;

+ (instancetype)defaultStack {

    static CoreDataStack *defaultStack;
    static dispatch_once_t onceTocken;
    dispatch_once (&onceTocken, ^{
        defaultStack = [[self alloc] init];
    });

    return defaultStack;
}


- (NSURL *)applicationDocumentsDirectory {
    // The directory the application uses to store the Core Data store file. This code uses a directory named "digitalCrown.Lister" in the application's documents directory.
    return [[[NSFileManager defaultManager] URLsForDirectory:NSDocumentDirectory inDomains:NSUserDomainMask] lastObject];
}

- (NSManagedObjectModel *)managedObjectModel {
    // The managed object model for the application. It is a fatal error for the application not to be able to find and load its model.
    if (_managedObjectModel != nil) {
        return _managedObjectModel;
    }
    NSURL *modelURL = [[NSBundle mainBundle] URLForResource:@"Lister" withExtension:@"momd"];
    _managedObjectModel = [[NSManagedObjectModel alloc] initWithContentsOfURL:modelURL];
    return _managedObjectModel;
}

- (NSPersistentStoreCoordinator *)persistentStoreCoordinator {
    // The persistent store coordinator for the application. This implementation creates and return a coordinator, having added the store for the application to it.
    if (_persistentStoreCoordinator != nil) {
        return _persistentStoreCoordinator;
    }

    // Create the coordinator and store

    _persistentStoreCoordinator = [[NSPersistentStoreCoordinator alloc] initWithManagedObjectModel:[self managedObjectModel]];
    NSURL *storeURL = [[self applicationDocumentsDirectory] URLByAppendingPathComponent:@"Lister.sqlite"];
    NSError *error = nil;
    NSString *failureReason = @"There was an error creating or loading the application's saved data.";
    if (![_persistentStoreCoordinator addPersistentStoreWithType:NSSQLiteStoreType configuration:nil URL:storeURL options:nil error:&error]) {
        // Report any error we got.
        NSMutableDictionary *dict = [NSMutableDictionary dictionary];
        dict[NSLocalizedDescriptionKey] = @"Failed to initialize the application's saved data";
        dict[NSLocalizedFailureReasonErrorKey] = failureReason;
        dict[NSUnderlyingErrorKey] = error;
        error = [NSError errorWithDomain:@"YOUR_ERROR_DOMAIN" code:9999 userInfo:dict];
        // Replace this 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();
    }

    return _persistentStoreCoordinator;
}


- (NSManagedObjectContext *)managedObjectContext {
    // Returns the managed object context for the application (which is already bound to the persistent store coordinator for the application.)
    if (_managedObjectContext != nil) {
        return _managedObjectContext;
    }

    NSPersistentStoreCoordinator *coordinator = [self persistentStoreCoordinator];
    if (!coordinator) {
        return nil;
    }
    _managedObjectContext = [[NSManagedObjectContext alloc] init];
    [_managedObjectContext setPersistentStoreCoordinator:coordinator];
    return _managedObjectContext;
}

#pragma mark - Core Data Saving support

- (void)saveContext {
    NSManagedObjectContext *managedObjectContext = self.managedObjectContext;
    if (managedObjectContext != nil) {
        NSError *error = 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();
        }
    }
}


@end

每当我向核心数据添加新对象时，我都会这样做：

- (void)insertTeget {

    CoreDataStack *stack = [CoreDataStack defaultStack];
    Target *target = [NSEntityDescription insertNewObjectForEntityForName:@"Target" inManagedObjectContext:stack.managedObjectContext];
    if (self.myTextView.text != nil) {
        target.body = self.myTextView.text;
        target.time = [NSDate date];
    }

    [stack saveContext];

}

在表格视图中，当我获取数据时，我是这样做的：

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {


    static NSString *cellIdentifier = @"StackTableViewCell";

    Target *target = [self.fetchedResultController objectAtIndexPath:indexPath];

    StackTableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier];

    if (!cell)
    {
        NSArray *topLevelObjects = [[NSBundle mainBundle] loadNibNamed:@"StackTableViewCell" owner:self options:nil];
        cell = [topLevelObjects objectAtIndex:0];
    }

    cell.cellLabel.text = target.body;

    cell.cellLabel.font = [UIFont fontWithName:@"Candara-Bold" size:20];

    cell.showsReorderControl = YES;



    // Configure the cell...

    return cell;
}

这是我在表视图控制器类中的 fetchresultconroller/fetch 请求配置：

- (NSFetchRequest *)targetsFetchRequest {

    NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:@"Target"];
    NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"time" ascending:YES];
    NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
    [fetchRequest setSortDescriptors:sortDescriptors];
    return fetchRequest;
}


- (NSFetchedResultsController *)fetchedResultController {

    if (_fetchedResultController != nil) {
        return _fetchedResultController;
    }

    CoreDataStack *stack = [CoreDataStack defaultStack];

    NSFetchRequest *fetchRequest = [self targetsFetchRequest];

    _fetchedResultController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:stack.managedObjectContext sectionNameKeyPath:nil cacheName:nil];

    _fetchedResultController.delegate = self;

    return _fetchedResultController;

}

现在我想检查订单是否已更改，因此我需要更改数据库中的订单...

我该怎么做？

谢谢！

Answer 1

根据我的 cmets，获取的结果控制器将每个对象的 indexPath 基于底层获取的排序顺序 - 在您的情况下为 time。如果您希望用户能够覆盖它并重新排序行，则更改 FRC 以使用不同的排序顺序。向您的实体添加一个新属性（日期类型），名为 sortTime，并修改 targetsFetchRequest 方法以使用 sortTime 作为排序键。

每当您添加新的Target 时，请同时设置sortTime。在对行重新排序时，设置 sortTime 的值以确保 FRC 将 Target 放置在新的 indexPath 上。像这样的：

-(void)tableView:(UITableView *)tableView moveRowAtIndexPath:(NSIndexPath *)sourceIndexPath toIndexPath:(NSIndexPath *)destinationIndexPath {
    Target *targetToMove = [self.fetchedResultsController objectAtIndexPath:sourceIndexPath];
    if (destinationIndexPath.row == 0) {
        // to move to row 0, the new sortTime can be any value
        // that is  less than the sortTime of the Target currently at
        // row 0
        Target *targetAtRow0 = [self.fetchedResultsController objectAtIndexPath:destinationIndexPath];
        targetToMove.sortTime = [targetAtRow0.sortTime dateByAddingTimeInterval:-1];
    } else {
        // for any other row, set the sortTime to be half way 
        // between the sortTime for the row before the new position
        // and the sortTime for the row at the new position
        Target *targetBefore = [self.fetchedResultsController objectAtIndexPath:[NSIndexPath indexPathForRow:(destinationIndexPath.row-1) inSection:destinationIndexPath.section]];
        Target *targetAfter = [self.fetchedResultsController objectAtIndexPath:destinationIndexPath];
        double lowerTimeInterval = [targetBefore.sortTime timeIntervalSinceReferenceDate];
        double upperTimeInterval = [targetAfter.sortTime timeIntervalSinceReferenceDate];
        double newInterval = (lowerTimeInterval + upperTimeInterval)/2;
        NSDate *newSortTime = [NSDate dateWithTimeIntervalSinceReferenceDate:newInterval];
        targetToMove.sortTime = newSortTime;
    }
    NSError *error;
    if (![self.managedObjectContext save:&error]) {
        NSLog(@"SaveContext: error %@, %@", error, [error userInfo]);
        abort();
    }
}

这应该会触发您的 FRC 委托方法来执行必要的表视图更新。

Answer 2

我建议您考虑为您的实体Target 添加另一个属性，或者将其称为“sortOrder”？

当您获取结果时，请考虑改为按sortOrder 排序。

要完成这项工作，您需要在创建每个实体时设置 sortOrder 值。这应该相对简单...在创建实体之前完成fetchedResults 的计数，假设您的下一个sortOrder 值将等于计数值加1。

当您在表格视图中移动一行时，您可以使用代码将受影响单元格的sortOrder 值重新分配为等于indexPath.row 属性。

保存后，您的新排序顺序应保持不变。