iOS CoreData更新实体对象

Question

我正在实施iPhone应用程序的汽车管理器，并且无法保存新车。所以我有一个“Car”实体（和每个加载的DB），包含多个属性。我有2个布尔值“保存”和“选择”来跟踪用户添加到他的列表（如果添加，保存= 1）以及当前选择哪辆车的车辆。因此，当我创建一辆新车时，我取消选择旧车（选择= 0），并且想要修改新车以设置其属性saved = 1和selected = 1。

这里是我的功能：

- (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]; 
} 

而且我有这个记录所有的时间：“保存错误车”在两个功能上。所以肯定有一些错误。

此外，它是相当恼人的取车我想修改它，而不是立即做一个更新。如果有另一个请告诉我！

谢谢。

Answer 1

在你的头文件中，你应该为你的汽车设置一个可变数组。

NSMutableArray *carArray; 

和

@property (nonatomic, retain) NSMutableArray *carArray; 

然后确保合成它在你的实现文件。然后，当你从你的管理对象范围内获取，可以设置与内容的数组返回

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]; 
} 

这样做将保留所有对象在管理对象方面，所以当你要修改的车，你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，则可以将使用您的某个属性初始化的排序描述符作为关键字应用。例如：

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

这将根据他们是否选择与否的汽车进行排序。

至于为什么你得到一个错误，这可能是许多原因之一。通常情况下，如果您的某个属性设置为零，则MOC将无法保存，因此可能是原因。你可以从它得到一些细节，如果你设置如下的东西

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也是一个不错的主意。像NSLog(@"selected car %@", carToSave);只是为了安全