在ActionBar中，即便设置showAsAction="always"，items仍然在overflow中显示的问题

今天很是苦恼，明明设置了android:showAsAction="always"，但是所有的items全部都显示在overflow中，然后在官网发现了答案。

如果你为了兼容 Android 2.1 的版本使用了 Support 库，在 android 命名空间下showAsAction 属性是不可用的。Support 库会提供替代它的属性，你必须声明自己的 XML 命名空间，并且使用该命名空间作为属性前缀。（一个自定义 XML 命名空间需要以你的 app 名称为基础，但是可以取任何你想要的名称，它的作用域仅仅在你声明的文件之内。）

添加此命名空间 xmlns:app="http://schemas.android.com/apk/res-auto" ，使用app:showAsAction代替android:showAsAction。

例如：

 <?xml version="1.0" encoding="utf-8"?>
 <menu xmlns:android="http://schemas.android.com/apk/res/android"
     xmlns:app="http://schemas.android.com/apk/res-auto"
     >

     <item
         android:id="@+id/action_search"
         android:icon="@drawable/peasy"
         app:showAsAction="always"
         android:title="@string/action_search"/>

     <!-- 设置, 在溢出菜单中展示 -->
     <item
         android:id="@+id/action_settings"
         android:showAsAction="never"
         android:title="@string/action_settings"/>
     <item
         android:id="@+id/action_about"
         app:showAsAction="never"
         android:title="@string/action_about"/>

 </menu>

UI依然很丑，但是效果实现了，大家将就着看吧。

另外，意外发现在actionBar只显示了icon但是没有显示title，这是怎么回事呢？于是又在官网深挖了....

看到了这一段... https://developer.android.com/guide/topics/ui/actionbar.html#Adding

If your menu item supplies both a title and an icon—with the title and icon attributes—then the action item shows only the icon by default. If you want to display the text title, add "withText" to the showAsAction attribute. For example:

<item yourapp:showAsAction="ifRoom|withText" ... />
Note: The "withText" value is a hint to the action bar that the text title should appear. The action bar will show the title when possible, but might not if an icon is available and the action bar is constrained for space.

You should always define the title for each item even if you don't declare that the title appear with the action item, for the following reasons:

If there's not enough room in the action bar for the action item, the menu item appears in the overflow where only the title appears.
Screen readers for sight-impaired users read the menu item's title.
If the action item appears with only the icon, a user can long-press the item to reveal a tool-tip that displays the action title.
The icon is optional, but recommended. For icon design recommendations, see the Iconography design guide. You can also download a set of standard action bar icons (such as for Search or Discard) from the Downloads page.

You can also use "always" to declare that an item always appear as an action button. However, you should not force an item to appear in the action bar this way. Doing so can create layout problems on devices with a narrow screen. It's best to instead use "ifRoom" to request that an item appear in the action bar, but allow the system to move it into the overflow when there's not enough room. However, it might be necessary to use this value if the item includes an action view that cannot be collapsed and must always be visible to provide access to a critical feature.

简而言之，如果同时设置了icon和title，默认只会显示icon。

如果想同时显示title和icon，可以加入app:showAsAction="always|withText"，但是即便这样也不会一定生效，withText对actionBar的title来说只是一个hint，在条件允许的情况下actionBar会显示title，但是当设置了icon并由于空间限制也不会显示title。

不过，官方还是建议我们设置title的，在长按icon的情况下title就会出现，另外官方还建议showAsAction最好设置为ifRoom，如果设置为always可能会在比较窄的屏幕上带来布局的问题。