addSnapshotListener triggers twice when setting timestamp field with FieldValue.serverTimestamp()

[mourjan]

Step 2: Describe your environment

• Android device: _samsung J7 & emulator
• Android OS version: 23 & 26_
• Google Play Services version: 11.9.75_
• Firebase/Play Services SDK version: 11.8.0___

Step 3: Describe the problem:

the scenario goe like this:

I have a recyclerView setup with a firestore adapter setup along with a query fetching documents like CHAT/MESSAGES (for example). all is normal.

now whenever I choose to insert a new message or update it, I set {stamp: FieldValue.serverTimestamp()| on a new docRef or update {stamp:FieldValue.serverTimestamp()} on a docRef.
both of setting and updating cause the addSnapshotListener to trigger twice.
first time, the message object (snapshot) will return with stamp field set to null,
and the second time with stamp field set with the correct server timestamp value.

I also assure you, that my addSnapshotListener is well implemented and there is only one listener running.. no more.

Expected Results:

I should not get an update from snapshot listener if the document's stamp field <FieldValue.serverTimestamp()> is null

[mourjan]

After researching the matter, I found this explanation on stackoverflow which makes sense therefore I am closing this issue.

[samtstern]

@mourjan thanks for following up, glad you figured it out.