Android: What is android.R.id.content used for?

Question

As Philipp Reichart commented:

android.R.id.content gives you the root element of a view, without having to know its actual name/type/ID. Check out http://stackoverflow.com/questions/4486034/android-how-to-get-root-view-from-current-activity

The android.R.id.content ID value indicates the ViewGroup of the entire content area of an Activity.

It can be used with Fragment:

public class MyActivity extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        if (savedInstanceState == null) {
            getSupportFragmentManager().beginTransaction()
                .add(android.R.id.content, MyFragment.newInstance())
                .commit();
        }
    }

    ...

}

The code above will insert the View created by Fragment into the ViewGroup identified by android.R.id.content.

Google designers develop Android UX with specific or recommended design guidelines. The layout android.R.id.content defines a linearlayout with a few attributes Android believes are a good standard.

Thus loading a Fragment Manager's root view with android.R.id.content ensures these guidelines are implemented.

NOTE: This layout has set the attribute: android:addStatesFromChildren="true" to allow child fragments to overwrite attributes in this rootview.

As of version 19, android.R.id.content is defined in a file: auto_complete_list.xml

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    android:id="@+id/content"
    android:orientation="vertical"
    android:layout_width="match_parent" 
    android:layout_height="wrap_content"
    android:background="@android:drawable/edit_text"
    android:divider="@android:drawable/divider_horizontal_textfield"
    android:addStatesFromChildren="true">