Question

我正在尝试在视图之间传递自定义类。我为该类设置了一个属性，但是当我访问该属性时，属性返回null：

firstview.m：

- (void)initNextEventView{
NSDate *eventDate = [eventDatePicker date];
NSString *eventName = eventNameTextfield.text;

LocateEventViewController *locView = [[LocateEventViewController alloc]init];
locView.eventDTO.name = eventName;
locView.eventDTO.date = eventDate;

[[self navigationController] pushViewController:locView animated:YES];
}

LocateEventVC.h（2。view）：

@interface LocateEventViewController : UIViewController<MKMapViewDelegate, UISearchBarDelegate, BSForwardGeocoderDelegate>
@property (strong, nonatomic) IBOutlet UIImageView *imageBackground;
@property (strong, nonatomic) EventDTO *eventDTO;

所以当我记录 eventDTO 的属性时，它会返回null ..

任何帮助都会很棒

编辑1（2.查看）： 多数民众赞成我如何记录：

NSLog(@"name on 2. view: %@", eventDTO.name);
self.eventDTO.place = tmpEventPlace;
// init friends view!

//passing to next view
EventMembersViewController *members = [[EventMembersViewController alloc]init];

members.eventDTO = self.eventDTO;

EventDTO.h

@interface EventDTO : NSObject

@property (nonatomic, retain) NSString * name;
@property (nonatomic, retain) UserDTO * creatorIsAppUser;
@property (nonatomic, retain) ContactDTO * creatorIsNotAppUser;
@property (nonatomic, retain) NSString * description;
@property (nonatomic, retain) NSDate * date;
@property (nonatomic, readwrite) NSInteger *eventID;
@property (nonatomic, retain) CustomPlacemark * place;
@property (nonatomic, retain) NSMutableArray *invitedMembers;
@property (nonatomic, retain) NSMutableArray *acceptedMembers;


- (NSNumber*)getLatidude;
- (NSNumber*)getLongitude;

EventDTO.m

@synthesize place, date, eventID, name, acceptedMembers, description, invitedMembers;


- (NSNumber*)getLatidude{
    return [NSNumber numberWithDouble:place.coordinate.latitude];
}

- (NSNumber*)getLongitude{
    return [NSNumber numberWithDouble:place.coordinate.longitude];
}

Answer 1

请参阅此帖子以获取完整说明。简而言之，你错了：

Passing Data between View Controllers

Answer 2

在访问eventDTO和eventName之前，您确定已设置或分配了属性eventDate吗？

我的猜测是eventDTO本身就是零。在Objective-C中允许调用方法或访问nil对象上的属性。它不像在C ++或Java中那样崩溃，它只是什么都不做，如果ht方法有返回值，则返回nil。

Answer 3

我认为属性eventDTO未正确初始化，而且nil本身就是。{/ p>

在Objective-C中向nil发送消息是合法的，它只是无声地失败并返回nil。学习Objetive-C时，这是一个常见的陷阱。访问属性只是发送消息的语法糖。

BTW，Objective-C中的“空对象”由nil表示，而不是null，NULL或NO，尽管它们都是零。

Answer 4

LocateEventViewController *locView = [[LocateEventViewController alloc]init];
locView.eventDTO.name = eventName;
locView.eventDTO.date = eventDate;

当您启动locView时，对象eventDTO不存在。你必须在那里发起一个新的eventDTO，例如：

LocateEventViewController *locView = [[LocateEventViewController alloc]init];
YourClass *eventDTO = [[YourClass alloc] init];
locView.eventDTO = eventDTO;
locView.eventDTO.name = eventName;
locView.eventDTO.date = eventDate;

或直接在您-init的{{1}}方法中添加

LocateEventViewController

iOS / iPhone - 在视图之间传递自定义对象。对象返回始终为null

4 个答案: