具有强引用的对象属性变为null

Question

在具有强引用的另一个对象（dataController中）的属性上设置的数组的键变为BWMasterViewController。我不明白为什么。

BWMasterViewController

头：

null

实现：

#import <UIKit/UIKit.h>

@class BWBirdSightingDataController;

@interface BWMasterViewController : UITableViewController
@end

BWBirdSightingDataController

头：

#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)

}

// ....

实现：

#import <Foundation/Foundation.h>

@class Bird;

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

鸟

Bird是基于CoreData实体的#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 。

头：

NSManagedObject

实现：

#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

输出

当我运行它时，输出：

#import "Bird.h"


@implementation Bird

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

@end

正如您所看到的，在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) 函数中，bird.name已变为null。怎么会？我宣称该物业是一个强有力的参考。

我在viewDidLoad中定义self.dataController时遇到同样的问题，然后在viewDidLoad中它会为空，我也需要它。

Answer 1

您使用本地Bird在initializeDefaultDataList中创建BWDataController *dataController个对象。 dataController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创建它的上下文中。如果上下文被销毁，那么恰好会发生这种情况：访问所有属性将返回nil。

可能的解决方案：

• 使dataController成为BWBirdSightingDataController的强大属性，而不是本地变量。
• 在整个应用程序中使用共享的托管对象上下文。