This app was mentioned in 6 comments, with an average of 4.33 upvotes
As much as I love Chess Tempo I feel like CT-ART 4.0 deserves to be mentioned as well. It's a little different than most tactics trainers in that it's a complete, structured training course that teaches you each theme in turn while increasing the difficulty progressively (up to 2400 Elo). I like it, it's very pedagogical.
The crazy thing is that stronger players don't even calculate much when looking at a position like this. It's just, 'oh, this is the bishop pins the rook on g2 back rank mate pattern.' I really like the CT Art tactics app on Android and usually do a few problems every day. This pattern occurs several times in that problem set.
CT Art and other Chess King apps.
I've tried most of them, and I think CT-ART is the best.
https://play.google.com/store/apps/details?id=com.chessking.android.learn.ctart4
There's nothing better for training.