I found two ways to display preview of the markdown files in markdown mode. One is C-c C-c p which requires a parser (currently I use pandoc) to preview using browser. The other is C-c C-c l which display preview using eww in emacs buffer. However, none of them display changes in a "real-time" way.
For the first one, I have to strike C-c C-c p every time after edited. It will open a new time in my browser each time which is very annoying.
If I use C-c C-c l I can use C-c C-c e or simply save file to update changes but still not convenient.
More over, I found some subtle differences between these two commands' outputs. Which one is the real one?