The following is the first few sections of a chapter from The Busy Coder's Guide to Android Development, plus headings for the remaining major sections, to give you an idea about the content of the chapter.
The action bar — that bar that runs across the top of your activity — is the backbone of your UI. Here, you can provide actions for the user to perform related to the current activity (e.g., “edit the contact that you are viewing”) or related to the application as a whole (e.g., “here is the documentation”). Sometimes, these actions will appear as toolbar buttons or other widgets in the action bar. Sometimes, these actions will appear in the “overflow”, which amounts to a menu.
This chapter introduces the concept of the action bar and how to add actions to it.
Android has had many patterns for various “bars” as part of its UI. So, to help explain what an action bar is, it helps if we review the history and role of Android’s various bars.
In the beginning, there was the status bar and the title bar.
The status bar was a thin strip across the top of the screen, used for things like the clock, signal strength, battery charge, and notification icons (for events like new unread email messages). This bar is technically part of the OS, not your app’s UI.
The title bar was a thin gray strip beneath the status bar that, by default, would hold the name of your application, much like the title bar of a browser might show the name of a Web site.
Figure 208: Status Bar and Title Bar
When official support for tablets arrived with Android 3.0 in February 2011, the story changed.
The status bar was replaced by the system bar, appearing at the bottom of the screen. This had all of the contents of the old status bar, but also had the soft keys for BACK, HOME, etc. Android 1.x and 2.x required that devices have off-screen affordances for those operations; now, device manufacturers could skip those and have the system bar offer them.
The action bar, by default, appears at the top of your activity, replacing the old title bar. You can define what goes in the action bar (icon, title, toolbar buttons, etc.).
Figure 209: Action Bar and System Bar
The icon on the far left of the action bar also serves as a toolbar button, if you wish. A common pattern for using this is take the user back to the “main” or “home” activity of your application.
Sometimes, the far right side of the action bar will contain a “…” affordance. This is known as the “action overflow” or “overflow menu”:
Figure 210: Action Bar with Open Overflow Menu
Tapping it will give the user access to actions that might have been toolbar buttons on a larger screen, but there was insufficient room. Also, low-priority actions may be tucked into the overflow, rather than clutter up the screen with too many toolbar buttons.
Phone-sized devices were not supported by Android 3.x. They jumped from Android 2.3 to 4.0, and along the way adopted some of the Android 3.x UI features:
Figure 211: Status Bar, Action Bar, and System Bar
The Nexus 7, introduced in the summer of 2012, was a 7” tablet that did not follow the tablet UI structure that all other standard Android tablets used. Instead, it looked a bit like a really large phone, having a top status bar along with a bottom system bar solely for the navigation buttons (BACK, HOME, etc.). Apps, as before, could have an action bar as well.
Initially, it was thought that the Nexus 7 was going to be distinctive in that regard. Instead, with Android 4.2, Google switched all tablets to this model, restoring the status bar and relegating the system bar purely for navigation buttons.
Figure 212: Status Bar, Action Bar, and System Bar, on Nexus 7 Emulator
Functionally, the action bar is much the same in Android 5.0 as it was in previous releases. However, aesthetically, it has dropped the icon and made other minor stylistic adjustments.
Figure 213: Action Bar on Android 5.0 Emulator
Back in the dawn of Android time, referred to by some as “the year 2007”, we had options menus. These would rise up from the bottom of the screen based on the user pressing a MENU key:
Figure 214: Legacy Options Menu
This is why you will see references to “options menu” scattered throughout the Android SDK.
The action bar pattern was first espoused by Google at the 2010 Google I|O conference. However, at the time, there was no actual implementation of this, except in scattered apps, and definitely not in the Android SDK.
Android 3.0 — a.k.a., API Level 11 — added the action bar to the SDK, and apps targeting that API level will get an action bar when running on such devices.
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