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OneSignal offer Push Notification as a service, an Android SDK and a Public API.

Is possible that your flow looks like:

1. Get push notification token from device
2. Send token to your backend
3. Backend uses token and OneSignal API to send pushes.
4. Device get the push notification

Ok, Show me code!

We will use the next dependencies:

compile 'com.onesignal:OneSignal:3.0.0@aar'
compile "com.google.android.gms:play-services-gcm:9.2.1"
compile "com.google.android.gms:play-services-analytics:9.2.1"
compile "com.google.android.gms:play-services-location:9.2.1"

Lets use OneSignal Android SDK method to get push ids:

OneSignal.idsAvailable((userId, registrationId) -> {
    // Use any of those ids to send the push from backend/server/something
});

1. userId: playerId from OneSignal
2. registrationId: Google push token

When you check OneSignal API you will see that you can use any of those to create pushes with include_android_reg_ids and include_player_ids,

But which one should you use?

Try 1 - Using registrationId

From your server you should do a POST to https://onesignal.com/api/v1/notifications with:

{
  "app_id": "...",
  "contents": { "en": "Custom Content" },
  "headings": { "en": "Custom Heading" },
  "data": { },
  
  "include_android_reg_ids": [ "registrationIdValue"]
}

But you will notice that after some attemps you will get this error:

{
  "id": "",
  "recipients": 0,
  "errors": { "invalid_android_reg_ids": [ "registrationIdValue" ] }
}

Looks like they are having issues since OneSignal.idsAvailable always returns the same but it becomes invalid based on OneSignal API responses.

Try 2 - using userId (Best Solution)

They said by chat:

We recommend storing userId and using include_player_ids instead. Our system attempts to keep our OneSignal player / user id the same even if the user fully uninstalls and reinstalls your app.

And from docs you will see include_player_ids param:

include_player_ids: array of stringsOptional. Targeting parameter. An array of OneSignal user IDs (also known as player IDs)

From your server you should do a POST to https://onesignal.com/api/v1/notifications with:

{
  "app_id": "...",
  "contents": { "en": "Custom Content" },
  "headings": { "en": "Custom Heading" },
  "data": { },
  
  "include_player_ids": [ "userIdValue"]
}

Thanks to

Barista Ventures and Sebastian Arcila for the support and help