iOS app is receiving a new APNS device token every time - ios

Question

I'm using Azure push notifications and am noticing that APNS is sending me a new device token every time my app is launched. Per Apple's documentation, device tokens should generally remain constant:
The device token included in each request represents the identity of
the device receiving the notification. APNs uses device tokens to
identify each unique app and device combination. It also uses them to
authenticate the routing of remote notifications sent to a device.
Each time your app runs on a device, it fetches this token from APNs
and forwards it to your provider. Your provider stores the token and
uses it when sending notifications to that particular app and device.
The token itself is opaque and persistent, changing only when a
device’s data and settings are erased. Only APNs can decode and read a
device token.
Yet, based on the constant number of failed messages sent via Azure, I can deduce that my app is receiving a new device token from APNS each time I launch the app. Can somebody tell me:
Why I'm getting a new device token each time even though I'm not changing phone settings or deleting the app?
In Apple's excerpt above, what do they mean by "deleting device's data"? Exactly what data is used by APNS to determine a "unique app and device combination"?
Here's the code:
AppDelegate.m
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
if ([application respondsToSelector:#selector(registerUserNotificationSettings:)]) {
UIUserNotificationSettings *settings = [UIUserNotificationSettings settingsForTypes:(UIUserNotificationTypeBadge |
UIUserNotificationTypeAlert |
UIUserNotificationTypeSound)
categories:nil];
[application registerUserNotificationSettings:settings];
} else {
[application registerForRemoteNotifications];
}
}
- (void)application:(UIApplication *)application didRegisterUserNotificationSettings:(UIUserNotificationSettings *)notificationSettings
{
//initiate registration process with Apple Push Notification service
[application registerForRemoteNotifications];
}
- (void)application:(UIApplication *)application didRegisterForRemoteNotificationsWithDeviceToken:(NSData *) token {
// Setting token with Azure hub
[[MyAzureNotificationHub sharedInfo] setDeviceTokenData:token];
}

Answer 1

You can read this:
Never cache device tokens; always get them from the system when you need them. Although device tokens are unique to an app and device, they can change over time. The device token can change at any time but is guaranteed to be different when the user restores their device from a backup, when the user installs your app on a new device, and when the user reinstalls the operating system. Fetching the token from the system ensures that you always have the current token needed to communicate with APNs. In addition, if the token has not changed, fetching it is fast and does not incur any significant overhead.
(Source: https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/HandlingRemoteNotifications.html#//apple_ref/doc/uid/TP40008194-CH6-SW3)
From my experience with APNS, the token changes anytime you'll register for remote notifications (app is relaunched). And each time you should resend new token to your backend, so it sends push notifications using the latest token.

Answer 2

Short answer is that device token should NOT change each time an app registers with APNS. From Apple:
The device token included in each request represents the identity of
the device receiving the notification. APNs uses device tokens to
identify each unique app and device combination. It also uses them to
authenticate the routing of remote notifications sent to a device.
Each time your app runs on a device, it fetches this token from APNs
and forwards it to your provider. Your provider stores the token and
uses it when sending notifications to that particular app and device.
The token itself is opaque and persistent, changing only when a
device’s data and settings are erased. Only APNs can decode and read a
device token
My app was getting a new device token each time because I was not converting the datatype of the the deviceToken from Data to string properly. APNS sends the device token as type Data but to pass the it to Azure, I needed to first convert the token to a string with the following function:
private func convertDataToString(data:NSData) -> String {
let singleChar = UnsafePointer<CChar>(data.bytes)
var tokenbuilder = String()
if data.length > 0
{
for index in 0...data.length - 1
{
tokenbuilder += String(format: "%02.2hhx", arguments: [singleChar[index]])
}
return tokenbuilder
}
else
{
return ""
}
}
In my case, I had forgotten that the position of the first char starts at 0 (not 1) and thus, the original for loop went from 0 to data.length (instead of data.length - 1), causing the function to return a string with random extra characters. Even with a string that had extra characters, the app did receive push notifications; however, we also had a significant number of invalid tokens in the Azure log. This also caused the user to receive a new deviceToken every time he/she launched the app (and registered for a device token). Once I fixed the for loop, APNS started returning the same device token to the user as long as the app wasn't deleted (thus, deleting device data) and reinstalled onto my device; thus, creating a "unique app and device combination".