Question

I have seen two ways to set background for Android App screen.

case A:: android:background=”?attr/actionBarItemBackground”

case B:: android:background=”@drawable/ic_launcher_settings”

How do android:background functions in the case A? I searched code, but not able to figure out where “actionBarItemBackground” exists. I understood case B

Answer 1

Case A is a reference to a style attribute. It allows you to reference a particular piece of style information without knowing its actual value. There are a lot of Android-defined attributes that are used to style the Android UI, but you can also define you own attributes.

actionBarItemBackground is one such attribute. There are two ways you can use this attribute:

1. You can reference it to make the look and feel of UI elements consistent with action bar items:

   <com.mycompany.MyView
       android:id="@+id/myView"
       android:background="?attr/actionBarItemBackground"/>
   

2. You can define a theme and set the value of this attribute to override the default L&F for your app:

   <style name="MyTheme" parent="@android:style/Theme">
       <item name="android:actionBarItemBackground">#383c44</item>
   </style>
   

Here are some pointers to the Android documentation:

For styles an themes see:
http://developer.android.com/guide/topics/ui/themes.html.

For a list of all attributes see:
http://developer.android.com/reference/android/R.attr.html

For defining your own attributes there is no documentation. Have a look at one of the attrs.xml files in the SDK (e.g. under platforms\android-17\data\res\values) to see how style attributes are declared. You can declare attributes in the same way by putting an attrs.xml file in your res\values folder. You can then use these attributes in your own styles and themes.