Google Cloud Messaging (GCM) is a free service for sending messages to Android devices. GCM messaging can greatly enhance the user experience. GCM can support up to 1,000 recipients for a single message. This capability makes it much easier to send out important messages to your entire user base. Often, GCM messages are meant to be a tickle, or indication to the device that there's fresh data waiting on a server somewhere. However, a GCM message can be up to 4kb in size, so sometimes it makes sense to simply send the data within the GCM message itself, so that the device doesn't need to contact the server at all.

When using GCM as an indicator to the device that data needs to be downloaded from the server, remember you have 4kb of metadata you can send along to help your application be smart about it.

Depending on the layout that your application is currently showing, the UI flow may be different. For example, if your application is in the dual-pane mode, clicking on an item on the left pane will simply display the content on the right pane; if it is in single-pane mode, the content should be displayed on its own.

Some actions may have a different result depending on the current layout. For example, in the News Reader sample, clicking on a headline from the headlines list opens the article in the right hand-side pane if the UI is in dual pane mode, but will launch a separate activity if the UI is in single-pane mode. Likewise, if the app is in dual-pane mode, it should set up the action bar with tabs for navigation, whereas if the app is in single-pane mode, it should set up navigation with a spinner widget.