Saving the DeviceToken for Later Use in Apple Push Notification Services

Question

In my iPhone app I am getting the device token from Apple which I am assigning a public property inside the Delegate file as shown below:

- (void)application:(UIApplication*)application didRegisterForRemoteNotificationsWithDeviceToken:(NSData*)deviceToken
{
   self.dToken = [[NSString alloc] initWithData:deviceToken encoding:NSUTF8StringEncoding]; 
}

The dToken property is declared as shown below:

NSString *dToken;

@property (nonatomic,retain) NSString *dToken;

But when I try to retrieve the device token from another file I get the null value.

+(NSString *) getDeviceToken
{
  NSString *deviceToken = [(MyAppDelegate *)[[UIApplication sharedApplication] delegate] dToken];

    NSLog(@" getDeviceToken = %@",deviceToken);  // This prints NULL

    return deviceToken; 

}

What am I doing wrong?

Solution 1

I suggest you to convert token to string in this way:

self.dToken = [[[deviceToken description]
                    stringByTrimmingCharactersInSet:[NSCharacterSet characterSetWithCharactersInString:@"<>"]] 
                    stringByReplacingOccurrencesOfString:@" " 
                    withString:@""];

UPDATED: As many people mentioned it is better to use next approach to convert NSData * to NSString *:

@implementation NSData (Conversion)
- (NSString *)hexadecimalString
{
  const unsigned char *dataBuffer = (const unsigned char *)[self bytes];

  if (!dataBuffer) {
    return [NSString string];
  }

  NSUInteger          dataLength  = [self length];
  NSMutableString     *hexString  = [NSMutableString stringWithCapacity:(dataLength * 2)];

  for (int i = 0; i < dataLength; ++i) {
    [hexString appendFormat:@"%02lx", (unsigned long)dataBuffer[i]];
  }

  return hexString;
}
@end

Solution 2

From the discussion at Best way to serialize an NSData into a hexadeximal string, here is a better way to do it. Is longer, but your code will be future-proof if Apple changes the way NSData emit debugger descriptions.

Extend NSData as follows:

@implementation NSData (Hex)
- (NSString*)hexString {
    unichar* hexChars = (unichar*)malloc(sizeof(unichar) * (self.length*2));
    unsigned char* bytes = (unsigned char*)self.bytes;
    for (NSUInteger i = 0; i < self.length; i++) {
        unichar c = bytes[i] / 16;
        if (c < 10) c += '0';
        else c += 'A' - 10;
        hexChars[i*2] = c;
        c = bytes[i] % 16;
        if (c < 10) c += '0';
        else c += 'A' - 10;
        hexChars[i*2+1] = c;
    }
    NSString* retVal = [[NSString alloc] initWithCharactersNoCopy:hexChars
                                                           length:self.length*2 
                                                     freeWhenDone:YES];
    return [retVal autorelease];
}
@end

Solution 3

I know that this is an old question and that this may be new information that has come up since then, but I'd just like to point something out to all of the people who are claiming that using the description method is a really bad idea. In most cases, you'd be exactly right. The description property is generally just used for debugging, but for the NSData class, it's specifically defined as returning a hexadecimal representation of the receivers contents which is exactly what is needed here. Since Apple has put it in their documentation, I think you're pretty safe as far as them changing it.

This can be found in the NSData Class Reference here: https://developer.apple.com/library/ios/documentation/Cocoa/Reference/Foundation/Classes/NSData_Class/Reference/Reference.html

Comments

[azamsharp]

Actually it is the getDeviceToken method which is not able to retrieve the value of dToken.

[Nekto]

Are you sure that didRegisterForRemoteNotificationsWithDeviceToken is called and dToken is initialized?

[azamsharp]

Actually, your code for deviceToken description did the trick. Thank you very much!

[Juan Carlos Méndez]

The code above works (at least for now) but is not good form. Reading the description property should not be considered an acceptable mechanism for HEX encoding the string, as it is meant for debugging description, not for encoding! See stackoverflow.com/questions/1305225/… for more correct ways to serialize the token

[Brian Sachetta]

"Since Apple has put it in their documentation, I think you're pretty safe as far as them changing it" ... Wishful thinking.

[art-divin]

@Nekto thanks for your answer. An improvement would be to return non-mutable copy of NSString , as signature suggests.

[Nekto]

@art-divin -- that is what a caller should care about otherwise you might have a useless copy operation. You can return mutable objects instead of immutable when designing APIs.