I have published a totally free, no ads, WYSISYG commonmark editor for android. The link to
it on the google app store is https://play.google.com/store/apps/details?id=com.ratfreedesserts.commonmarkeditor.
I wrote it because all the existing editors I found required you to type raw markdown then
view the layout in a separate preview pane. Typing lots of # and * and _ characters on a phone
keyboard is slow (for me anyway) so I wrote my own editor.
I have attempted to follow the commonmark specification however if there is anything I have
missed or got wrong please let me know and I will happily fix it.
If there is any interest I will separate out the editor and rendering code into a self contained
android widget that you can use in your own apps.