flutter multiple letter selection by sliding ( motion event )

I need to create words by dragging letters. I searched but couldn't find enough code to detect motion event. I want to select multiple letters by swiping with my finger and create word from selected letters. I need to make a word puzzle game like the one below. how can i make a game like this?  

You should experiment with the GestureDetector and use its pan drags gesture in Flutter

For that you simply have to put a GestureDetector as a parent of your round Widget holding the letters and detect what is getting dragged.