在AppDelegate中将数据保存在NSUserdefaults中，但在其他视图控制器上检索时却变得空白

Question

我试图在AppDelegate中用NSUserDefaults存储一个值。成功地存储了它，但是当我尝试从NSUserDefaults获取相同的值时，它返回为空。我已经尝试找到解决方案，并实现了我所能得到的一切，但仍然无法找到这个空白值的正确原因。请检查代码，并帮助我解决这个问题。提前感谢！

AppDelegate

-(void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string{
     if([self.element isEqualToString:@"client_login"]){
          self.currentValue = string;
         [[NSUserDefaults standardUserDefaults] setObject:self.currentValue forKey:@"client_login"];
        //  NSLog(@"defaults value %@",[defaults valueForKey:@"client_login"]);
          //[defaults synchronize];
     }
     else{
         NSLog(@"currentValue value is blank");
     }

}

ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
   // [[NSUserDefaults standardUserDefaults] valueForKey:@"client_login"];
    _loginLbl.text =[[NSUserDefaults standardUserDefaults] valueForKey:@"client_login"];

    NSLog(@"%@", [[NSUserDefaults standardUserDefaults] dictionaryRepresentation]);

    // Do any additional setup after loading the view, typically from a nib.
}

日志

XMLParsingDemo[3549:162060] {
    AKLastIDMSEnvironment = 0;
    AddingEmojiKeybordHandled = 1;
    AppleITunesStoreItemKinds =     (
        "itunes-u",
        movie,
        ringtone,
        album,
        "software-update",
        booklet,
        tone,
        "music-video",
        song,
        podcast,
        software,
        "podcast-episode",
        wemix,
        eBook,
        artist,
        mix,
        document
    );
    AppleKeyboards =     (
        "en_US@sw=QWERTY;hw=Automatic",
        "emoji@sw=Emoji",
        "en_US@sw=QWERTY;hw=Automatic"
    );
    AppleKeyboardsExpanded = 1;
    AppleLanguages =     (
        en
    );
    AppleLanguagesDidMigrate = 15B87;
    AppleLocale = "en_US";
    ApplePasscodeKeyboards =     (
        "en_US@sw=QWERTY;hw=Automatic",
        "emoji@sw=Emoji",
        "en_US@sw=QWERTY;hw=Automatic"
    );
    NSInterfaceStyle = macintosh;
    NSLanguages =     (
        en
    );
    PKKeychainVersionKey = 4;
    "client_login" = "\n    ";
}

Answer 1

造成此问题的可能原因是您的viewDidLoad()是在您获得-(void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string中的任何值之前被调用的--这个方法会被更新。因此，在设置值之前，您可能正在访问一个值。

可能的解决办法可以是使用通知。在您的appDelegate中，在调用-(void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string时发布通知，并在视图控制器中添加一个观察者。在通知被触发后，尝试访问该值并更新主线程上的标签。

在app委托中，将其写入您的-(void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string中

[[NSNotificationCenter defaultCenter] 
        postNotificationName:@"TestNotification" 
        object:self];

在viewController viewDidLoad()中：

[[NSNotificationCenter defaultCenter] addObserver:self
        selector:@selector(receiveTestNotification:) 
        name:@"TestNotification"
        object:nil];

在viewController中，只编写以下函数：

- (void) receiveTestNotification:(NSNotification *) notification
{
    // [notification name] should always be @"TestNotification"
    // unless you use this method for observation of other notifications
    // as well.

    if ([[notification name] isEqualToString:@"TestNotification"])
        NSLog (@"Successfully received the test notification!");
}

在此方法中，更新标签。

实际问题

问题在于：

-(void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string{
    NSLog(@"^^^^^^^^^ %@", _selectedElement);
    if([_selectedElement isEqualToString:@"client_login"]){
       NSLog(@"Came here: %@, %@",_selectedElement, string);
       [[NSUserDefaults standardUserDefaults]setValue:string forKey:@"login"];


    }
    else{
       [[NSUserDefaults standardUserDefaults]setValue:string forKey:@"login"];
     }
    NSLog(@"value stored in defaults is %@",[[NSUserDefaults standardUserDefaults]objectForKey:@"login"]);
}

在这里，if条件为真两次，第一次字符串变量有正确的值，第二次是带新行的空格。

因此，修改后的函数是：

-(void)parser:(NSXMLParser *)parser foundCharacters:(NSString *)string{
     if([_selectedElement isEqualToString:@"client_login"]){

          NSString *trimmedString = [string stringByTrimmingCharactersInSet:
                                     [NSCharacterSet whitespaceAndNewlineCharacterSet]];
          if ([trimmedString isEqualToString:@""]){
          }else{
               [[NSUserDefaults standardUserDefaults]setValue:string forKey:@"login"];
          }

     }

}