Back button behavior

Android users rely on the back button on their navigation. Usually apps don't provide, and neither should they, a back button in the UI unlike their iOS counterparts.

Android Activity API provides developers a way to hook into back button functionality and even overwrite it. This power should be used with great care. Making back button behave in unexpected ways can easily destroy your application's user experience.


What back button does, technically

From technical point of view the back button manipulates the app's Activity stack. Pressing back calls finish on the active activity and removes it from the stack. If the app only has one activity or the current activity is the only one in the stack (ie. user has closed all the other ones) back button will close the application.

Behavior after pressing back or pressing home look very similar to the user. However, from technical point of view they are very different. Back will terminate the app but home will move the app to background.


Problem

Because the back button has the power of terminating the application it can also cause problems to the user.  If the app manages data changes poorly or fails to save application's state on exit user can lose data when pressing back.

In some applications it doesn't make sense to store the application's full state on exit. A good example of this would be a file manager application where user will probably expect the application to start again from the sdcard root every time.


How to prevent accidental app exit

Note: All of the following should be used only if saving the app state on exit is not possible or if closing the application can otherwise make the user to lose work. If neither of these conditions are true the back button should just exit the app.

Double press back


Some apps prevent accidental app exit by requiring users to press back twice when the app is exiting. Common pattern is to display a toast to the user when he or she presses the back button on activity stacks last activity.

ASTRO file manager app exit toast. 

Postcard Express app exit toast.

This functionality can easily be implemented by overriding main activity's onBackPressed() method. In this example when user presses back button the app will display a toast for 4 seconds on which time a new back press terminates the app immediately:

private Toast toast;
private long lastBackPressTime = 0;

@Override
public void onBackPressed() {
  if (this.lastBackPressTime < System.currentTimeMillis() - 4000) {
    toast = Toast.makeText(this, "Press back again to close this app", 4000);
    toast.show();
    this.lastBackPressTime = System.currentTimeMillis();
  } else {
    if (toast != null) {
    toast.cancel();
  }
  super.onBackPressed();
 }
}

Confirmation dialog

Some applications prompt user when it is about to exit. This seems to be particularly common with game apps. This practice is not recommended in normal applications. A confirmation prompt would disrupt user's normal workflow. Even in games using this option should be very carefully considered.

Angry Birds app exit confirmation

Grave app exit confirmation

Run in background

Some apps, like Spotify here, don't close when back is pressed if there's a process going on. In this case Spotify keeps running on background if a song is playing. This is indicated by a notification bar icon.

Spotify doesn't close the app when pressing back

Back long press for app home screen

Roman Nurik, a Android developer advocate from Google, tweeted an excellent idea for an extension to the normal back button behavior:

romannurik Roman Nurik Android UI idea: Long press BACK to get to the home screen of your app.

Generally hiding actions behind long press is not very good UI practice (it has some good uses like long pressing a list item to get context menu). Google has already replaced home long press as multi-tasking control in the latest Android version (3.0). In this case, however, I feel like long pressing back to bring user to the app's home screen is a good idea as the functionality is available to the user by other means also. Back long press in this case is just a shortcut for advanced users.