添加核心數據對象從一個segue

Question

在熟悉核心數據時，我發現自己在嘗試添加數據時要通過各種視圖控制器（VC）的問題感到困惑。

例如，在CoreDataRecipes項目，蘋果提供作爲一個例子（http://developer.apple.com/library/ios/#samplecode/iPhoneCoreDataRecipes/Introduction/Intro.html）它們用下面的辦法

當用戶想要向配方添加到在主表視圖呈現食譜的列表，並點擊Add按鈕，主表視圖控制器（稱爲RecipeListTableViewController）創建如下新的管理對象（配方）：被傳遞到RecipeAddViewController

- (void)add:(id)sender { 
// To add a new recipe, create a RecipeAddViewController. Present it as a modal view so that the user's focus is on the task of adding the recipe; wrap the controller in a navigation controller to provide a navigation bar for the Done and Save buttons (added by the RecipeAddViewController in its viewDidLoad method). 
RecipeAddViewController *addController = [[RecipeAddViewController alloc] initWithNibName:@"RecipeAddView" bundle:nil]; 
addController.delegate = self; 

Recipe *newRecipe = [NSEntityDescription insertNewObjectForEntityForName:@"Recipe" inManagedObjectContext:self.managedObjectContext]; 
addController.recipe = newRecipe; 

UINavigationController *navigationController = [[UINavigationController alloc] initWithRootViewController:addController]; 
[self presentModalViewController:navigationController animated:YES]; 

[navigationController release]; 
[addController release]; 
} 

這個新創建的對象（配方）。該RecipeAddViewController有兩種方法，保存和取消，具體如下：

- (void)save { 

recipe.name = nameTextField.text; 

NSError *error = nil; 
if (![recipe.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. If it is not possible to recover from the error, display an alert panel that instructs the user to quit the application by pressing the Home button. 
    */ 
    NSLog(@"Unresolved error %@, %@", error, [error userInfo]); 
    abort(); 
}  

[self.delegate recipeAddViewController:self didAddRecipe:recipe]; 

}

- (void)cancel { 

[recipe.managedObjectContext deleteObject:recipe]; 

NSError *error = nil; 
if (![recipe.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. If it is not possible to recover from the error, display an alert panel that instructs the user to quit the application by pressing the Home button. 
    */ 
    NSLog(@"Unresolved error %@, %@", error, [error userInfo]); 
    abort(); 
}  

[self.delegate recipeAddViewController:self didAddRecipe:nil]; 

}

我疑惑這個設計方法。爲什麼RecipeListViewController會在我們知道用戶是否想要輸入新配方名稱並保存新對象之前創建對象？爲什麼不將managedObjectContext傳遞給addRecipeController，並等待用戶點擊保存以創建對象並使用數據填充其字段？這避免瞭如果沒有新的配方添加，必須刪除新的對象。或者爲什麼不在RecipeListViewController和RecipeAddController之間來回傳遞配方名稱（字符串）？

我這麼問是因爲我在努力瞭解何時塞格斯之間傳遞一個字符串，什麼時候該傳球的對象，而當通過managedObjectContexts ...

大加讚賞任何指導，含。任何有關討論設計理念的鏈接。

Answer 1

你的問題是，NSManagedObject s不能沒有上下文生活。因此，如果您不將配方添加到上下文中，則必須將該配方的所有屬性保存在「常規」實例變量中。當用戶點擊保存時，您可以從這些實例變量中創建一個配方。

這對於一個AddViewController來說不是一個大問題，但是你想用什麼viewController來編輯配方？你可以重用你的AddViewController。但是，如果將所有數據保存爲實例變量，則它會變得有點難看，因爲首先必須從配方中獲取所有數據，將其保存爲實例變量，並在完成後必須執行相反操作。

這就是爲什麼我通常使用不同的方法。我使用編輯上下文進行編輯（或添加，這基本上只是編輯）。

- (void)presentRecipeEditorForRecipe:(MBRecipe *)recipe { 
    NSManagedObjectContext *editingContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSMainQueueConcurrencyType]; 
    editingContext.parentContext = self.managedObjectContext; 
    MBRecipe *recipeForEditing; 
    if (recipe) { 
     // get same recipe inside of the editing context. 
     recipeForEditing = (MBRecipe *)[editingContext objectWithID:[recipe objectID]]; 
     NSParameterAssert(recipeForEditing); 
    }  
    else { 
     // no recipe for editing. create new one 
     recipeForEditing = [MBRecipe insertInManagedObjectContext:editingContext]; 
    } 

    // present editing view controller and set recipeForEditing and delegate 
} 

非常直接的代碼。它創建一個用於編輯的新的兒童上下文。並從該上下文獲取編輯配方。

您必須不是將上下文保存在您的EditViewController中！只需設置食譜的所有需要​​的屬性，但只保留上下文。

用戶點擊「取消」或「完成」後，調用此委託方法。要麼保存editingContext和我們的上下文，要麼什麼也不做。

- (void)recipeEditViewController:(MBRecipeEditViewController *)editViewController didFinishWithSave:(BOOL)didSave { 
    NSManagedObjectContext *editingContext = editViewController.managedObjectContext; 
    if (didSave) { 
     NSError *error; 
     // save editingContext. this will put the changes into self.managedObjectContext 
     if (![editingContext save:&error]) { 
      NSLog(@"Couldn't save editing context %@", error); 
      abort(); 
     } 

     // save again to save changes to disk 
     if (![self.managedObjectContext save:&error]) { 
      NSLog(@"Couldn't save parent context %@", error); 
      abort(); 
     } 
    } 
    else { 
     // do nothing. the changes will disappear when the editingContext gets deallocated 
    } 
    [self dismissViewControllerAnimated:YES completion:nil]; 
    // reload your UI in `viewWillAppear:` 
}