Google GMail callback when someone reply to thread - api

Question

I'm using Google GMail Api. All works fine with synchronous requests. However I would like to get notified when someone reply to given thread_id to a callback somehow.
I found this article https://developers.google.com/gmail/api/guides/push?hl=pl about push notifications and it's great but still I don't know how to setup Observer I have described above.
I'm using PHP btw, but I just need concept how it works and how I can setup that kind of communication between API and app.

Answer 1

There is currently no available API that detects/observes (something that behaves like a callback) if there is a reply for a specific thread_id.
You'll have to do the checking yourself, doing some polling (with users.threads.get) to see if there is a reply. Similar to what the OP was doing in this post (emphasis mine):
We have a web application that makes use of the Gmail API to automate certain emails on our users' behalf. When the application sends an email, it stores the threadId returned by the Gmail API. It then uses this threadId to poll the Gmail users.threads.get API, looking for replies. When a reply is detected, it pauses future emails.

Answer 2

You can use the Push notifications provided by Gmail API. The document is available here:
https://developers.google.com/gmail/api/guides/push?hl=pl#python
It's pretty much straight forward. Create a topic, then create a push subscription for the topic. It requires a webhook pointing to your web app. Grant necessary permission to the Gmail service account as mentioned in the document.
Use the below Python code to get notified:
request = {
'labelIds': ['INBOX'],
'topicName': 'projects/myproject/topics/mytopic'
}
gmail.users().watch(userId='me', body=request).execute()
We get a response with HistoryId. Tracking all the messages since the HistoryId can be done like this:
history = (service.users().history().list(userId=user_id, startHistoryId=start_history_id)
I found the below video helpful:
https://www.youtube.com/watch?v=wjHp9_NAEJo