iphone - iOS CoreData 更新实体对象

Question

我正在为一个 iPhone 应用程序实现一个汽车管理器，但我无法保存一辆新车。所以我有一个“汽车”实体(和每个加载的数据库)，包含多个属性。我有 2 个 bool 值“保存”和“选择”来跟踪用户添加到他的列表中的汽车(如果添加，保存 = 1)以及当前选择的汽车。所以当我创建一辆新车时，我取消选择旧车(selected=0)，并想修改新车以设置其属性 saved=1 和 selected=1。

这是我的功能:

- (IBAction)save
{
    // Disable the previous car selection.
    [self deselectCurrentSelectedCar];

    // Add new car as saved and selected.
    [self saveAndSelectNewCar];

    // Call the delegate to dismiss the modal view.
    [_delegate dismissAndSave];
}

- (void)deselectCurrentSelectedCar
{    
    // Fetched saved car.
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Car" inManagedObjectContext:[self managedObjectContext]];
    [fetchRequest setEntity:entity];

    // Set predicate and sort orderings...
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"selected = 1"];
    [fetchRequest setPredicate:predicate];

    // Execute the fetch -- create a mutable copy of the result.
    NSError *error = nil;
    NSMutableArray *mutableFetchResults = [[self.managedObjectContext executeFetchRequest:fetchRequest error:&error] mutableCopy];
    if (mutableFetchResults == nil) {
        // Handle the error.
        NSLog(@"[AddCarViewController] deselect car: car not found.");
    }
    else {
        // Get car and assign new selected value.
        Car *carToSave = (Car *)[mutableFetchResults objectAtIndex:0];
        [carToSave setSelected:[NSNumber numberWithInt:0]];

        // Save the car.
        NSError *error = nil;
        if (![self.managedObjectContext save:&error]) {
            // Handle the error.
            NSLog(@"[AddCarViewController] deselect car: error saving car.");
        }
        else {
            NSLog(@"[AddCarViewController] deselect car: car saved.");
        }
    }

    // Memory management.
    [fetchRequest release];
    [mutableFetchResults release];
}
- (void)saveAndSelectNewCar
{
    // Get the car, and pass to the delegate the new settings.
    NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
    NSEntityDescription *entity = [NSEntityDescription entityForName:@"Car" inManagedObjectContext:[self managedObjectContext]];
    [fetchRequest setEntity:entity];

    // Set predicate and sort orderings...
    NSPredicate *predicate = [NSPredicate predicateWithFormat:@"(year=%@) AND (make=%@) AND (model=%@) AND (d=%@) AND (n=%@)", _car.year, _car.make, _car.model, _car.d, _car.n];
    [fetchRequest setPredicate:predicate];

    // Execute the fetch -- create a mutable copy of the result.
    NSError *error = nil;
    NSMutableArray *mutableFetchResults = [[self.managedObjectContext executeFetchRequest:fetchRequest error:&error] mutableCopy];
    if (mutableFetchResults == nil) {
        // Handle error.
        NSLog(@"[AddCarViewController] save and select: can't save the new car");
    }
    else {
        // Get the car selected.
        Car *selectedCar = (Car *)[mutableFetchResults objectAtIndex:0];
        [selectedCar setValue:[NSNumber numberWithInt:1] forKey:@"selected"];
        [selectedCar setValue:[NSNumber numberWithInt:1] forKey:@"saved"];

        // Save the car.
        NSError *error = nil;
        if (![self.managedObjectContext save:&error]) {
            // Handle the error.
            NSLog(@"[AddCarViewController] save and select: error saving car.");
        }
        else {
            NSLog(@"[AddCarViewController] save and select: car saved.");
        }

        // Add car to delegate.
        [EcoAppAppDelegate setUserCar:selectedCar];
    }

    // Memory management.
    [fetchRequest release];
    [mutableFetchResults release];
}

而且我一直都有这个日志:“救车时出错。”在这两个功能上。所以肯定有问题。

此外，获取我想要修改的汽车而不是立即进行更新非常烦人。如果还有其他请告诉我!

谢谢。

Answer 1

在您的头文件中，您应该为您的汽车设置一个可变数组。

NSMutableArray *carArray;

和

@property (nonatomic, retain) NSMutableArray *carArray;

然后确保在您的实现文件中综合它。然后当你从你的托管对象上下文中获取时，你可以用返回的内容设置你的数组

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSEntityDescription *entity = [NSEntityDescription entityForName:@"Car" inManagedObjectContext:[self managedObjectContext]];
[fetchRequest setEntity:entity];

// Execute the fetch -- create a mutable copy of the result.
NSError *error = nil;
NSMutableArray *mutableFetchResults = [[self.managedObjectContext executeFetchRequest:fetchRequest error:&error] mutableCopy];
if (mutableFetchResults == nil) {
    // Handle the error.
    NSLog(@"[AddCarViewController] deselect car: car not found.");
} else {
    [self setCarArray:mutableFetchResults];
}

这样做会保留托管对象上下文中的所有对象，因此当您想要修改汽车时，您可以在那里找到它而不是再次获取。如果您需要对其进行排序，您可以应用一个排序描述符，该描述符使用您的一个属性作为键进行了初始化。例如:

NSSortDescriptor *sorter = [NSSortDescriptor sortDescriptorWithKey:@"selected" ascending:YES];
[carArray sortUsingDescriptors:[NSArray arrayWithObject:sorter]];

这将根据汽车是否被选中对汽车进行排序。

至于为什么会出现错误，这可能是多种原因之一。通常，如果您的某个属性设置为 nil，MOC 将无法保存，所以这可能就是原因。如果您设置类似以下内容，您可以从中获取一些详细信息

if (![self.managedObjectContext save:&error]) {
    NSLog(@"failed with error %@", error);
}

这将返回您遇到的实际错误。设置日志以确保您指定了 Car 也是一个好主意。像 NSLog(@"selected car %@", carToSave); 只是为了安全起见