在具有强引用的另一个对象(BWMasterViewController中的dataController)的属性上设置的数组的键变为null.我不明白为什么.
BWMasterViewController
标题：
#import <UIKit/UIKit.h> @class BWBirdSightingDataController; @interface BWMasterViewController : UITableViewController @end
执行：
#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: %@",bird.name)
}
// ....
BWBirdSightingDataController
标题：
#import <Foundation/Foundation.h> @class Bird; @interface BWBirdSightingDataController : NSObject - (NSUInteger)countOfList; - (Bird *)objectInListAtIndex:(NSUInteger)theIndex; @end
执行：
#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
Bird是基于CoreData实体的NSManagedObject.
标题：
#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
执行：
#import "Bird.h" @implementation Bird @dynamic date; @dynamic name; @dynamic location; @dynamic image; @dynamic url; @end
产量
当我运行它时,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)
如您所见,在viewDidLoad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prepareForSegue中它将为null,我也需要它.
解决方法
您可以使用本地BWDataController * dataController在initializeDefaultDataList中创建Bird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.如果上下文被破坏,则恰好发生这种情况：访问所有属性将返回nil.
可能的解决方案：
>使dataController成为BWBirdSightingDataController的强属性,而不是局部变量.>在整个应用程序中使用共享的托管对象上下文.