iOS CoreData更新實體對象

Question

我正在實施iPhone應用程序的汽車管理器，並且無法保存新車。所以我有一個「Car」實體（和每個加載的DB），包含多個屬性。我有2個布爾值「保存」和「選擇」來跟蹤用戶添加到他的列表（如果添加，保存= 1）以及當前選擇哪輛車的車輛。因此，當我創建一輛新車時，我取消選擇舊車（選擇= 0），並且想要修改新車以設置其屬性saved = 1和selected = 1。

這裏是我的功能：

- (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]; 
} 

而且我有這個記錄所有的時間：「保存錯誤車」在兩個功能上。所以肯定有一些錯誤。

此外，它是相當惱人的取車我想修改它，而不是立即做一個更新。如果有另一個請告訴我！

謝謝。

Answer 1

在你的頭文件中，你應該爲你的汽車設置一個可變數組。

NSMutableArray *carArray; 

和

@property (nonatomic, retain) NSMutableArray *carArray; 

然後確保合成它在你的實現文件。然後，當你從你的管理對象範圍內獲取，可以設置與內容的數組返回

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]; 
} 

這樣做將保留所有對象在管理對象方面，所以當你要修改的車，你可以在那裏找到它而不是再次獲取。如果您需要對其進行排序，則可以將使用您的某個屬性初始化的排序描述符作爲關鍵字應用。例如：

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

這將根據他們是否選擇與否的汽車進行排序。

至於爲什麼你得到一個錯誤，這可能是許多原因之一。通常情況下，如果您的某個屬性設置爲零，則MOC將無法保存，因此可能是原因。你可以從它得到一些細節，如果你設置如下的東西

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

這將返回您遇到的實際錯誤。設置日誌以確保您指定了Car也是一個不錯的主意。像NSLog(@"selected car %@", carToSave);只是爲了安全