XML Namespace

xmlns

• prefix：前缀
• Namespace URI：所关联的Namespace位置

xmlns:app

为什么要使用xmlns:app

<?xml version="1.0" encoding="utf-8"?>
<resources>
    ...

    <!-- Base attributes that are available to all Item objects. -->
    <declare-styleable name="MenuItem">

        ...

        <!-- How this item should display in the Action Bar, if present. -->
        <attr name="showAsAction">
            <!-- Never show this item in an action bar, show it in the overflow menu instead.
                 Mutually exclusive with "ifRoom" and "always". -->
            <flag name="never" value="0" />
            <!-- Show this item in an action bar if there is room for it as determined
                 by the system. Favor this option over "always" where possible.
                 Mutually exclusive with "never" and "always". -->
            <flag name="ifRoom" value="1" />
            <!-- Always show this item in an actionbar, even if it would override
                 the system's limits of how much stuff to put there. This may make
                 your action bar look bad on some screens. In most cases you should
                 use "ifRoom" instead. Mutually exclusive with "ifRoom" and "never". -->
            <flag name="always" value="2" />
            <!-- When this item is shown as an action in the action bar, show a text
                 label with it even if it has an icon representation. -->
            <flag name="withText" value="4" />
            <!-- This item's action view collapses to a normal menu
                 item. When expanded, the action view takes over a
                 larger segment of its container. -->
            <flag name="collapseActionView" value="8" />
        </attr>

        ...

    </declare-styleable>
    
    ...

</resources>