Android学习路线（十）如何将Action Bar叠放在你的布局上

为了第六篇的完整性，这里先贴出原文，下次再来翻译；:p

原文地址：http://developer.android.com/training/basics/actionbar/overlaying.html

By default, the action bar appears at the top of your activity window, slightly reducing the amount of space available for the rest of your activity‘s layout. If, during the course of user interaction, you want to hide and show the action bar, you can do so
by calling hide() and show() on
the ActionBar. However,
this causes your activity to recompute and redraw the layout based on its new size.

Figure 1. Gallery‘s action bar in overlay mode.

To avoid resizing your layout when the action bar hides and shows, you can enable overlay mode for the action bar. When in overlay mode, your activity layout uses all the space available as if the action bar is not there and the system draws the action
bar in front of your layout. This obscures some of the layout at the top, but now when the action bar hides or appears, the system does not need to resize your layout and the transition is seamless.

Tip: If you want your layout to be partially visible behind the action bar, create a custom style for the action bar with a partially transparent background, such as the one shown in figure 1. For information about how to define the action
bar background, read Styling the Action Bar.

Enable Overlay Mode

For Android 3.0 and higher only

If your minSdkVersion is
set to 11 or higher, your custom theme should use Theme.Holo theme
(or one of its descendants) as your parent theme. For example:

<resources>
    <!-- the theme applied to the application or activity -->
    <style name="CustomActionBarTheme"
           parent="@android:style/Theme.Holo">
        <item name="android:windowActionBarOverlay">true</item>
    </style>
</resources>

For Android 2.1 and higher

If your app is using the Support Library for compatibility on devices running versions lower than Android 3.0, your custom theme should use Theme.AppCompat theme
(or one of its descendants) as your parent theme. For example:

<resources>
    <!-- the theme applied to the application or activity -->
    <style name="CustomActionBarTheme"
           parent="@android:style/Theme.AppCompat">
        <item name="android:windowActionBarOverlay">true</item>

        <!-- Support library compatibility -->
        <item name="windowActionBarOverlay">true</item>
    </style>
</resources>

Also notice that this theme includes two definitions for the windowActionBarOverlay style: one with the android:prefix
and one without. The one with the android: prefix is for versions of Android that include the style in the platform and the one without the prefix is for older versions that read the
style from the Support Library.

Specify Layout Top-margin

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingTop="?android:attr/actionBarSize">
    ...
</RelativeLayout>

If you‘re using the Support Library for the action bar, you need to remove the android: prefix. For example:

<!-- Support library compatibility -->
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingTop="?attr/actionBarSize">
    ...
</RelativeLayout>

In this case, the ?attr/actionBarSize value without the prefix works on all versions, including Android 3.0 and higher.

Android学习路线（十）如何将Action Bar叠放在你的布局上