ActionBar官方教程(4)给ActionBar添加操作项及它们的事件处理

Adding Action Items

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　　The action bar provides users access to the most important action items relating to the app's current context. Those that appear directly in the action bar with an icon and/or text are known as action buttons. Actions that can't fit in the action bar or aren't important enough are hidden in the action overflow. The user can reveal a list of the other actions by pressing the overflow button on the right side (or the device Menu button, if available).

　　When your activity starts, the system populates the action items by calling your activity's onCreateOptionsMenu() method. Use this method to inflate a menu resource that defines all the action items. For example, here's a menu resource defining a couple of menu items:

res/menu/main_activity_actions.xml 操作项资源文件如下:

 <menu xmlns:android="http://schemas.android.com/apk/res/android" >
     <item android:id="@+id/action_search"
           android:icon="@drawable/ic_action_search"
           android:title="@string/action_search"/>
     <item android:id="@+id/action_compose"
           android:icon="@drawable/ic_action_compose"
           android:title="@string/action_compose" />
 </menu>

　　Then in your activity's onCreateOptionsMenu() method, inflate the menu resource into the given Menu to add each item to the action bar:

 @Override
 public boolean onCreateOptionsMenu(Menu menu) {
     // Inflate the menu items for use in the action bar
     MenuInflater inflater = getMenuInflater();
     inflater.inflate(R.menu.main_activity_actions, menu);
     return super.onCreateOptionsMenu(menu);
 }

　在 activity或fragment的 onCreateOptionsMenu() 函数中加载操作项资源文件.

　　To request that an item appear directly in the action bar as an action button, include showAsAction="ifRoom"in the <item> tag. For example:

 <menu xmlns:android="http://schemas.android.com/apk/res/android"
       xmlns:yourapp="http://schemas.android.com/apk/res-auto" >
     <item android:id="@+id/action_search"
           android:icon="@drawable/ic_action_search"
           android:title="@string/action_search"
           yourapp:showAsAction="ifRoom"  />
     ...
 </menu>

　　If there's not enough room for the item in the action bar, it will appear in the action overflow.

Using XML attributes from the support library

　　Notice that the showAsAction attribute above uses a custom namespace defined in the <menu> tag. This is necessary when using any XML attributes defined by the support library, because these attributes do not exist in the Android framework on older devices. So you must use your own namespace as a prefix for all attributes defined by the support library.

　　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 showAsActionattribute. For example:

<item yourapp:showAsAction="ifRoom|withText" ... />

　　标题与图标同时存在时默认只显示图标

　　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 Downloadspage.

　　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.

　　ifRoom:如果空间足就显示,否则在溢出菜单里,　　withText:显示操作项的标题,　　always:问题显示,　　never:只在溢出菜单里显示.它们可以用 |  

Handling clicks on action items 处理操作项事件

　　When the user presses an action, the system calls your activity's onOptionsItemSelected() method. Using the MenuItem passed to this method, you can identify the action by calling getItemId(). This returns the unique ID provided by the <item> tag's id attribute so you can perform the appropriate action. For example:

 @Override
 public boolean onOptionsItemSelected(MenuItem item) {
     // Handle presses on the action bar items
     switch (item.getItemId()) {
         case R.id.action_search:
             openSearch();
             return true;
         case R.id.action_compose:
             composeMessage();
             return true;
         default:
             return super.onOptionsItemSelected(item);
     }
 }

　　Note: If you inflate menu items from a fragment, via the Fragment class's onCreateOptionsMenu() callback, the system calls onOptionsItemSelected() for that fragment when the user selects one of those items. However, the activity gets a chance to handle the event first, so the system first calls onOptionsItemSelected() on the activity, before calling the same callback for the fragment. To ensure that any fragments in the activity also have a chance to handle the callback, always pass the call to the superclass as the default behavior instead of returning false when you do not handle the item.

　　注意: 如果是在fragment的处理操作项,那么fragment宿主会先于它收到事件,虽然在宿主中返回false事件也会传到fragment中,但最好的方式是用 

　　　　　　return super.onOptionsItemSelected(item);