Posted by Daniel Sandler, a software engineer on the Android System UI team
I’ve always loved screen savers. Supposedly they exist for a practical purpose: protecting that big, expensive monitor from the ghosts of spreadsheets past.
But I’ve always imagined that your computer is secretly hoping you’ll stand up and walk away for a bit. Just long enough for that idle timer to expire…so it can run off and play for a little while. Draw a picture, set off fireworks, explore the aerodynamics of kitchen appliances, whatever—while always ready to get back to work at a keystroke or nudge of the mouse.
Daydream, new in Android 4.2, brings this kind of laid-back, whimsical experience to Android phones and tablets that would otherwise be sleeping. If you haven’t checked it out, you can turn it on in the Settings app, in Display > Daydream; touch When to Daydream to enable the feature when charging.
An attract mode for apps

Apps that support Daydream can take advantage of the full Android UI toolkit in this mode, which means it’s easy to take existing components of your app — including layouts, animations, 3D, and custom views—and remix them for a more ambient presentation. And since you can use touchscreen input in this mode as well, you can provide a richly interactive experience if you choose.
Daydream provides an opportunity for your app to show off a little bit. You can choose to hide some of your app’s complexity in favor of one or more visually compelling experiences that can entertain from across a room, possibly drawing the user into your full app, like a video game’s attract mode.
Figure 1. Google Currents scrolls stories past in a smooth, constantly-moving wall of news.

Google Currents is a great example of this approach: as a Daydream, it shows a sliding wall of visually-interesting stories selected from your editions. Touch a story, however, and Currents will show it to you full-screen; touch again to read it in the full Currents app.
The architecture of a Daydream

Each Daydream implementation is a subclass of android.service.dreams.DreamService. When you extend DreamService, you’ll have access to a simple Activity-like lifecycle API.
Key methods on DreamService to override in your subclass (don’t forget to call the superclass implementation):

Important methods on DreamService that you may want to call:

  • setContentView() — set the scene for your Daydream. Can be a layout XML resource ID or an instance of View, even a custom View you implement yourself.
  • setInteractive(boolean) — by default, your Daydream will exit if the user touches the screen, like a classic screen saver. If you want the user to be able to touch and interact with your Views, call setInteractive(true).
  • setFullscreen(boolean) — convenience method for hiding the status bar (see below).
  • setScreenBright(boolean) — by default, Daydreams keep the screen on at full brightness, which may not be appropriate for some situations (for example, dark rooms); setting this to false will reduce the display brightness to a very low level.

Finally, to advertise your Daydream to the system, create a for it in your AndroidManifest.xml:

The tag is optional; it allows you to point to an XML resource that specifies a settings Activity specific to your Daydream. The user can reach it by tapping the settings icon next to your Daydream’s name in the Settings app.