[This post is by Dan Galpin, who lives the Android Games lifestyle every day. — Tim Bray]
Making a game on Android is easy. Making a great game for a mobile, multitasking, often multi-core, multi-purpose system like Android is trickier. Even the best developers frequently make mistakes in the way they interact with the Android system and with other applications*ó*mistakes that donít affect the quality of gameplay, but which affect the quality of the userís experience in other ways.
A truly great Android game knows how to play nice: how to fit seamlessly into the system of apps, services, and UI features that run on Android devices. In this multi-part series of posts, Android Developer Relations engineers who specialize in games explain what it takes to make your game play nice.
II: Navigation and Lifecycle

Android users get used to using the back key. We expect the volume keys to work in some intuitive fashion. We expect that the home key behaves in a manner consistent with the Android navigation paradigm. Sometimes we even expect the menu key to do something.
1. Problem: Thereís no place like [Home]

Iím playing [insert favorite game here] and I accidentally hit the [Home] key or the [Back] key. This is probably happening because Iím furiously using the touchscreen to actually play the game. Whether Iíve been cutting ropes, controlling aircraft, cleaving fruit, or flinging birds, Iím almost certainly angry if Iíve suddenly lost all of my game progress.
What went wrong?

Lots of developers assume that pressing the Home key exits a game. Perhaps this is because on some mobile devices the Home key is a somewhat-difficult-to-press physical button. Depending on the device and Android release, it might be a physical,, capacitative, or soft button. This means that it is relatively easy to hit accidentally. Having progress lost by such an event as an incoming call is even worse.
How to avoid Angry Users

  1. Save as much about the status of the game into the Bundle in onSaveInstanceState() as you can. This helper function will get called whenever your application receives an onPause() callback. Note that you can save byte arrays into that bundle, so it can easily be used for raw data.
  2. If your game takes lots of native system resources, consider dumping large textures (or all textures and geometry) during onPause() or onStop(). (GLSurfaceView will do this automatically unless you tell it not to*ó*at least you can tell it not to do so starting in API level 11). This will help your title continue to reside in memory, which will typically speed task-switching back to your game for very large titles that might otherwise be swapped out of memory, but may slow things down for smaller titles that can more efficiently multitask if they donít bother to do this.
  3. When your game resumes, restore the state from the bundle in onRestoreInstanceState(). If there is any sort of time-consuming loading that has to be done, make sure that you notify the user of what is happening to give them the best possible experience.
  4. Test thoroughly!
2. Problem: [Back] I say!

Iím in the middle of playing a game and I hit the back key. One of several bad things can happen here:
  1. The game exits immediately, losing all state and leading to Angry User Syndrome. (see Problem 1).
  2. The game does nothing.
What went wrong?

We already know what is wrong with scenario 1. Itís essentially a data loss scenario, and itís worse than pigs stealing your eggs. What is wrong with scenario 2?
The [Back] key is an essential part of the Android navigation paradigm. If the back key doesnít return to the previous screen in the activity stack (or in the game hierarchy) there better be a very good reason, such as an active document with no capability to save a draft.
What to do about it

If the user is in the middle of gameplay It is customary to display some sort of dialog asking the user if they intended the action:
ďAre you sure you wish to exit now? That monster looks hungry
In an extreme action game, you might also wish to do something similar to what Replica Island (RI) did. RI assumed that any [Back] keypress that happened within 200ms of another touch event was invalid in order to make it a bit more challenging to accidentally press the key.
At the Main Menu of the game, you can decide whether it makes sense to prompt the user or not. If your game has very long load times, you might want to prompt the user.
3. Problem: Quiet [Down]!

Thereís nothing worse than wanting to settle down for a good session of [insert favorite game here] in some sort of public place with your volume turned up. Suddenly everyone has learned that you prefer pummelling produce to predicting present progressions and thatís practically profane in your profession.
What went wrong?

By default, volume keys in most Android devices will control the ringer volume, and your application must pass the volume keys through to the super class so this continues to work.
What to do about it

In order to make these keys control the music volume (which is the channel that your game will be using), you need to call setVolumeControlStream(AudioManager.STREAM_MUSIC). As stated previously, all you need to do is pass these keys through to the framework and youíll get control of the audio in the standard and proper way. Do it as early as possible so a user can start changing the volume far before you begin playing anything.