I've updated the game to fix a few bugs and also re-arrange the code to make it easier to read. I've split the main game loop code into updating, then drawing. This is a better way to organise the main game loops as it allows you to focus on the objects when they get updated and not worry about drawing them. You can also draw things in any order you like without worrying aboout which objects affect each other.
Anyway, the play store should give you an update today, the only things yoou'll notice in the game are changes in the button sizes on the main menu and the aliens now speed up slightly for each level you complete.
I also fixed a bug where the bases would start disintegrating even when you'd killed the bottom row of aliens.
I want to implement a highscore system next. I can write a highscore table ingame, but is there a good library I can use to upload to google play service or scoreloop?
Lastly, I have got a banner ad in the game, which I hope people don't mind. But when the game gets more popular, and I write more games, I want to know what adverts work best for people here? In terms of generating revenue?
The source code link is:
https://dl.dropboxusercontent.com/u/8673694/my games/Invaders-source.zip
And the play store link is:
https://play.google.com/store/apps/details?id=uk.co.coffeeinducedgames.invaders
People have recommended features such as Auto-fire, left and right hand controls, and a landscape mode option, I will look at these soon.