You can create a Preview block using the Templatedescriptor or you can go with Tomas's suggestion.
When you do changes then auto save feature is working, right? If you are getting any exception in logs or console (js related), please share here for more context about the issue.
Yes, it's related to the autosave feature.
Actually the problem is that when we created a new page and added blocks on the page and see changes in preview mode which looks good, but after publishing the page and added new blocks again then it does not appear in preview mode. I have checked the logs for that issue but not found.
In EPI Server CMS, I have managed the content area through the cascading feature (which is custom). If the user edits the top-level page and placed the blocks on the top page then it automatically inherited up-to child-level using cascade features and we can see the change in edit preview mode before to publish, but once page published and then add/remove the block from page then it does not show in edit preview mode, however after re-publishing the page I can see the changes.
Any idea why preview mode not working after publishing the page for blocks?