Hello Meng,

I have not tried to do this through an API.  However, if you have the capability to enable read-only access to Tableau Server's PostgreSQL internal metadata Workgroup database, you can query for comments, etc:

Workgroup Database - Tableau

I hope this helps.

Thanks,

Will

Hello Meng,

I am not sure whether you can retrieve comments via the REST API.  Searching the documentation, I did not see an attribute listed for comments. 

We at least have obtained this information from the PostgreSQL metadata Workgroup database.  We enabled a readonly account and only allow access to a select few people.  Perhaps your team that administers the server can write a query to pull this data for you programmatically from the database.  In addition to Tableau's documentation, below is a great viz on Tableau Public:

Tableau Public

Thanks,

Will