Drag Tree 1 is made and published in 2015. It is written in Java in the Android SDK.
Since I did not know how to create the GUI in Java, I found a neat solution: the application is just swapping images.
For example, there’s an image with only the first yellow on and all the rest off. Then there’s another image with the first and second yellows on and the rest off. Therefore the process flow is basically changing the images. For text and timing an atlas is used: an image containing a grid of smaller images.