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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.
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.
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