具有强引用的对象属性变为空

Question

设置在另一个对象上的属性（dataController的BWMasterViewController）中的数组的关键字带有强引用变为null。我不明白为什么。

BWMasterViewController

头：

#import <UIKit/UIKit.h> 

@class BWBirdSightingDataController; 

@interface BWMasterViewController : UITableViewController 
@end 

实现：

#import "BWMasterViewController.h" 
#import "BWBirdSightingDataController.h" 
#import "Bird.h" 
#import "BWWebviewController.h" 

@interface BWMasterViewController() 
@property (strong, nonatomic) BWBirdSightingDataController *dataController; 
@property (copy, nonatomic) NSString *test; 
@end 

@implementation BWMasterViewController 

- (void)awakeFromNib 
{ 

    [super awakeFromNib]; 

    NSLog(@"awake from nib"); 
    self.dataController = [[BWBirdSightingDataController alloc] init]; 
    self.test = @"Test var"; 

    NSLog(@"test: %@", self.test); 
    Bird *bird = [self.dataController objectInListAtIndex:0]; 
    NSLog(@"bird object: %@", bird); 
    NSLog(@"bird name: %@", bird.name) 

} 

- (void)viewDidLoad 
{ 

    [super viewDidLoad]; 

    NSLog(@"view did load"); 

    NSLog(@"test: %@", self.test); 
    Bird *bird = [self.dataController objectInListAtIndex:0]; 
    NSLog(@"bird object: %@", bird); 
    NSLog(@"bird name: %@", bird.name) 

} 

// .... 

BWBirdSightingDataController

头：

#import <Foundation/Foundation.h> 

@class Bird; 

@interface BWBirdSightingDataController : NSObject 
- (NSUInteger)countOfList; 
- (Bird *)objectInListAtIndex:(NSUInteger)theIndex; 
@end 

实现：

#import "BWBirdSightingDataController.h" 
#import "BWDataController.h" 
#import "Bird.h" 

@interface BWBirdSightingDataController() 
-(void)initializeDefaultDataList; 
@property (nonatomic, copy) NSMutableArray *birds; 
@end 

@implementation BWBirdSightingDataController 

- (id)init 
{ 

    if (self = [super init]) { 
     [self initializeDefaultDataList]; 
     return self; 
    } 

    return nil; 

} 

- (void)initializeDefaultDataList 
{ 

    BWDataController *dataController = [[BWDataController alloc] init]; 

    NSEntityDescription *birdsEntity = [NSEntityDescription 
             entityForName:@"Bird" 
             inManagedObjectContext:dataController.managedObjectContext]; 
    NSFetchRequest *fetchBirds = [[NSFetchRequest alloc] init]; 
    [fetchBirds setEntity:birdsEntity]; 

    NSError *fetchError = nil; 
    NSArray *birdsObjects = [dataController.managedObjectContext 
          executeFetchRequest:fetchBirds 
          error:&fetchError]; 
    self.birds = [birdsObjects mutableCopy]; 

} 

- (NSUInteger)countOfList 
{ 
    return [self.birds count]; 
} 

- (Bird *)objectInListAtIndex:(NSUInteger)theIndex 
{ 
    return self.birds[theIndex]; 
} 

@end 

鸟

鸟是NSManagedObject根据CoreData实体。

头：

#import <Foundation/Foundation.h> 
#import <CoreData/CoreData.h> 


@interface Bird : NSManagedObject 

@property (nonatomic, retain) NSDate * date; 
@property (nonatomic, retain) NSString * name; 
@property (nonatomic, retain) NSString * location; 
@property (nonatomic, retain) NSString * image; 
@property (nonatomic, retain) NSString * url; 

@end 

实现：

#import "Bird.h" 


@implementation Bird 

@dynamic date; 
@dynamic name; 
@dynamic location; 
@dynamic image; 
@dynamic url; 

@end 

输出

当我运行它，这个输出：

2013-05-31 11:36:47.824 BirdWatching[69565:c07] awake from nib 
2013-05-31 11:36:47.834 BirdWatching[69565:c07] test: Test var 
2013-05-31 11:36:47.834 BirdWatching[69565:c07] bird object: <Bird: 0x8545ec0> (entity: Bird; id: 0x8545040 <x-coredata://D24A664F-8E8F-4AF0-891C-098C8A7DD860/Bird/p1> ; data: <fault>) 
2013-05-31 11:36:47.835 BirdWatching[69565:c07] bird name: Pigeon 
2013-05-31 11:36:47.839 BirdWatching[69565:c07] view did load 
2013-05-31 11:36:47.840 BirdWatching[69565:c07] test: Test var 
2013-05-31 11:36:47.840 BirdWatching[69565:c07] bird object: <Bird: 0x8545ec0> (entity: Bird; id: 0x8545040 <x-coredata://D24A664F-8E8F-4AF0-891C-098C8A7DD860/Bird/p1> ; data: <fault>) 
2013-05-31 11:36:47.840 BirdWatching[69565:c07] bird name: (null) 

因此，大家可以看到，在viewDidLoad函数bird.name已经变为null。怎么来的？我宣布财产是一个强有力的参考。

我有同样的问题，当我在viewDidLoad中定义self.dataController时，那么它将在prepareForSegue为空，在那里我也需要它。

Answer 1

使用本地BWDataController *dataController在initializeDefaultDataList中创建Bird对象。 dataController会在此方法结束时自动释放，这可能意味着托管对象上下文dataController.managedObjectContext也不再存在。

但是，托管对象只能存在于其创建的上下文中。如果上下文被销毁，那么恰好会发生这种情况：访问所有属性返回nil。

可能的解决方案：

• 让dataController的BWBirdSightingDataController而不是一个局部变量强大的属性。
• 跨应用程序使用共享托管对象上下文。