iOS/iPhone - 在視圖之間傳遞自定義對象。對象返回始終爲空

Question

我試圖在視圖之間傳遞一個自定義類。我已經設置屬性爲類，但是當我訪問屬性，這些屬性總是返回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視圖）：

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

所以當我登錄的一個屬性eventDTO返回null ..

任何幫助將是偉大的

編輯1（2。視圖）： 那我怎麼登錄：

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

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

當您啓動locView時，對象eventDTO不存在。你必須在那裏發起一個新的事件DTO，例如：

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

或直接在您的LocateEventViewController的-init方法通過增加

-(id)init { 
... 
self.eventDTO = [[YourClassHere alloc] init]; 
... 
} 

Answer 2

請看到這個帖子更全面的解釋。總之，你要它錯了：

Passing Data between View Controllers

Answer 3

你確定財產eventDTO正在訪問eventName和eventDate之前設置或分配的？

我的猜測是eventDTO本身是零。在Objective-C中允許調用方法或訪問nil對象的屬性。它不像在C++或Java中那樣崩潰，它只是不做任何事情，並且如果ht方法具有返回值，則返回nil。

Answer 4

我猜eventDTO屬性沒有正確初始化，它本身是nil。

將消息發送到nil在Objective-C中是合法的，它只是靜默地失敗並返回nil。在學習Objetive-C時，這是一個常見的陷阱。訪問屬性只是發送消息的語法糖。

順便說一句，在Objective-C 「空對象」 是由nil，不null，NULL或NO表示，雖然它們都是零。