Action bar 行动条

The action bar is a window feature that identifies the application and user location, and provides user actions and navigation modes. You should use the action bar in most activities that need to prominently present user actions or global navigation, because the action bar offers users a consistent interface across applications and the system gracefully adapts the action bar's appearance for different screen configurations. You can control the behaviors and visibility of the action bar with the ActionBarAPIs, which were added in Android 3.0 (API level 11).

Action bar 是以个window特性，它根据应用和用户所在位置，提供用户操作和导航。应该在绝大多数Acitvity中使用Action bar来明显的提供用户操作和全局导航，由于action bar提供整个应用的一致接口，而且系统为不同的屏幕配置自动调整Action bar，你可以控制action bar的行为和是否可见，action bar是Android3.0增加的特性。

The primary goals of the action bar are to:

action bar的主要目标：

• Provide a dedicated space for identifying the application brand and user location.

  提供一个专用空间来标识应用商标和用户位置。

  This is accomplished with the app icon or logo on the left side and the activity title. You might choose to remove the activity title, however, if the current view is identified by a navigation label, such as the currently selected tab.

  这是通过在activity的标题左侧放置应用图标来实现的。你可以去除activity的标题，然而，如果当前view可以通过导航标签识别，例如当前选中的tab页。

• Provide consistent navigation and view refinement across different applications.

  跨不同应用，提供一致的导航和view

  The action bar provides built-in tab navigation for switching between fragments. It also offers a drop-down list you can use as an alternative navigation mode or to refine the current view (such as to sort a list by different criteria).

  action bar提供内置的标签导航，用于切换不同的fragment. 提供下拉列表作为可选的导航模式或优化当前view.

• Make key actions for the activity (such as "search", "create", "share", etc.) prominent and accessible to the user in a predictable way.

  对于Activity的操作显著和可用。

  You can provide instant access to key user actions by placing items from the options menu directly in the action bar, as "action items." Action items can also provide an "action view," which provides an embedded widget for even more immediate action behaviors. Menu items that are not promoted to an action item are available in the overflow menu, revealed by either the device Menu button (when available) or by an "overflow menu" button in the action bar (when the device does not include aMenu button).

  
  

Figure 1. Action bar from the Honeycomb Gallery app (on a landscape handset), showing the logo on the left, navigation tabs, and an action item on the right (plus the overflow menu button).

Note: If you're looking for information about the contextual action bar for displaying contextual action items, see the Menuguide.

Action Bar Design

For design guidelines, read Android Design's Action Bar guide.

Remaining backward-compatible

If you want to provide an action bar in your application and remain compatible with versions of Android older than 3.0, you need to create the action bar in your activity's layout (because the ActionBarclass is not available on older versions).

To help you, the Action Bar Compatibility sample app provides an API layer and action bar layout that allows your app to use some of the ActionBar APIs and also support older versions of Android by replacing the traditional title bar with a custom action bar layout.

Adding the Action Bar

Beginning with Android 3.0 (API level 11), the action bar is included in all activities that use the Theme.Holo theme (or one of its descendants), which is the default theme when either the targetSdkVersion or minSdkVersion attribute is set to "11" or greater. For example:

 ... >     android:minSdkVersion="4"              android:targetSdkVersion="11" />    ...

In this example, the application requires a minimum version of API Level 4 (Android 1.6), but it also targets API level 11 (Android 3.0). This way, when the application runs on Android 3.0 or greater, the system applies the holographic theme to each activity, and thus, each activity includes the action bar.

If you want to use ActionBar APIs, such as to add navigation modes and modify action bar styles, you should set theminSdkVersion to "11" or greater. If you want your app to support older versions of Android, there are ways to use a limited set of ActionBar APIs on devices that support API level 11 or higher, while still running on older versions. See the sidebox for information about remaining backward-compatible.

Removing the action bar

If you don't want the action bar for a particular activity, set the activity theme to Theme.Holo.NoActionBar. For example:

 android:theme="@android:style/Theme.Holo.NoActionBar">

You can also hide the action bar at runtime by calling hide(). For example:

ActionBar actionBar = getActionBar();actionBar.hide();

When the action bar hides, the system adjusts your activity layout to fill all the screen space now available. You can bring the action bar back with show().

Beware that hiding and removing the action bar causes your activity to re-layout in order to account for the space consumed by the action bar. If your activity regularly hides and shows the action bar (such as in the Android Gallery app), you might want to use overlay mode. Overlay mode draws the action bar on top of your activity layout rather than in its own area of the screen. This way, your layout remains fixed when the action bar hides and re-appears. To enable overlay mode, create a theme for your activity and set android:windowActionBarOverlayto true. For more information, see the section about Styling the Action Bar.

Tip: If you have a custom activity theme in which you'd like to remove the action bar, set the android:windowActionBar style property to false. However, if you remove the action bar using a theme, then the window will not allow the action bar at all, so you cannot add it later—calling getActionBar() will return null.

Adding Action Items 增加操作项

Sometimes you might want to give users immediate access to an item from the options menu. To do this, you can declare that the menu item should appear in the action bar as an "action item." An action item can include an icon and/or a text title. If a menu item does not appear as an action item, then the system places it in the overflow menu. The overflow menu is revealed either by the device Menu button (if provided by the device) or an additional button in the action bar (if the device does not provide the Menu button).

有时，你想通过可选菜单给用户一些直接操作的操作项。你可以声明菜单项作为操作项显示在action bar上。操作项包括一个图标和一个文本标题。如果菜单项不能显示在action bar上，系统会把它们放到溢出菜单中。溢出菜单可以通过菜单按钮或action bar上一个附件按钮调出。

Figure 2. Two action items with icon and text titles, and the overflow menu button.

When the activity first starts, the system populates the action bar and overflow menu by callingonCreateOptionsMenu() for your activity. As discussed in the Menus developer guide, it's in this callback method that you should inflate an XML menu resource that defines the menu items. For example:

当activity初次启动，系统通过调用onCreateOptionMenu()生成action bar和溢出菜单。正如菜单开发指南中描述的，在该方法中可以由菜单资源生成菜单项。

@Overridepublic boolean onCreateOptionsMenu(Menu menu) {    MenuInflater inflater = getMenuInflater();    inflater.inflate(R.menu.main_activity, menu);    return true;}

In the XML file, you can request a menu item to appear as an action item by declaring android:showAsAction="ifRoom" for the  element. This way, the menu item appears in the action bar for quick access only if there is room available. If there's not enough room, the item appears in the overflow menu.

在xml文件中，你可以要求菜单项出现在action bar上，为菜单定义android:showAsAction ="ifRoom"即可。这样如果有空间，菜单项将变成动作项。如果没有空间，菜单项会出现在溢出菜单中。

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

如果菜单项支持标题和图标--属性android:title和android:icon，动作项缺省只显示图标，如果想显示标题，需给android:showAsAction增加属性withText.例如：

<?xml version="1.0" encoding="utf-8"?> xmlns:android="http://schemas.android.com/apk/res/android">     android:id="@+id/menu_save"          android:icon="@drawable/ic_menu_save"          android:title="@string/menu_save"          android: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.

When the user selects an action item, your activity receives a call to onOptionsItemSelected(), passing the ID supplied by the android:id attribute—the same callback received for all items in the options menu.

当用户选择了操作项，activity接收调用onOptionsItemSelected()传递菜单项的ID,和可选菜单一样。

It's important that you always define android:title for each menu item—even if you don't declare that the title appear with the action item—for three reasons:

定义菜单项的android:title是重要的，即使你不把他们显示在操作项上。三个理由：

• If there's not enough room in the action bar for the action item, the menu item appears in the overflow menu and only the title appears.
• 如果没有操作项的空间，菜单项会出现在溢出菜单中，此时只显示标题。
• Screen readers for sight-impaired users read the menu item's title.
• 屏幕阅读器会阅读菜单项标题。
• 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 item's title.
• 如果操作项只显示图标，用户可以长按显示菜单提示，这是显示的是菜单标题。

The android:icon is always optional, but recommended. For icon design recommendations, see the Action Bar Icon design guidelines.

Note: If you added the menu item from a fragment, via the Fragment class's onCreateOptionsMenu callback, then the system calls the respective onOptionsItemSelected() method for that fragment when the user selects one of the fragment's items. However the activity gets a chance to handle the event first, so the system calls onOptionsItemSelected() on the activity before calling the same callback for the fragment.

You can also declare an item to "always" appear as an action item, instead of being placed in the overflow menu when space is limited. In most cases, you should not force an item to appear in the action bar by using the "always" value. However, you might need an item to always appear when it provides an action view that does not offer a default action for the overflow menu. Beware that too many action items can create a cluttered UI and cause 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 menu when there's not enough room.

你可以声明菜单项为“always”,使其出现在操作项上，而不是放到溢出菜单中。

For more information about creating the options menu that defines your action items, see the Menus developer guide.

Choosing your action items 改变操作项

Menu items vs. other app controls

As a general rule, all items in the options menu (let alone action items) should have a global impact on the app, rather than affect only a small portion of the interface. For example, if you have a multi-pane layout and one pane shows a video while another lists all videos, the video player controls should appear within the pane containing the video (not in the action bar), while the action bar might provide action items to share the video or save the video to a favorites list.

So, even before deciding whether a menu item should appear as an action item, be sure that the item has a global scope for the current activity. If it doesn't, then you should place it as a button in the appropriate context of the activity layout.

You should carefully choose which items from your options menu should appear as action items by assessing a few key traits. In general, each action item should be at least one of the following:

你需要仔细选择那些菜单项成为操作项。一般，每个操作项应该具有下列特性：

1. Frequently used: It's an action that your users need seven out of ten visits or they use it several times in a row.

   经常使用

   Example frequent actions: "New message" in the Messaging app and "Search" in Android Market.

2. Important: It's an action that you need users to easily discover or, if it's not frequently used, it's important that it be effortless to perform in the few cases that users do need it.

   重要操作

   Example important actions: "Add network" in Wi-Fi settings and "Switch to camera" in the Gallery app.

3. Typical: It's an action that is typically provided in the action bar in similar apps, so your users expect to find it in yours.

   典型操作

   Example typical actions: "Refresh" in an email or social app, and "New contact" in the People app.

If you believe that more than four of your menu items can be justified as action items, then you should carefully consider their relative level of importance and try to set no more than four as action items (and do so using the "ifRoom" value to allow the system to put some back in the overflow menu when space is limited on smaller screens). Even if space is available on a wide screen, you should not create a long stream of action items that clutter the UI and appear like a desktop toolbar, so keep the number of action items to a minimum.

Additionally, the following actions should never appear as action items: Settings, Help, Feedback, or similar. Always keep them in the overflow menu.

另外，下列操作不应该出现在操作项：设置、帮助、反馈等等。应该放到溢出菜单中。

Note: Remember that not all devices provide a dedicated hardware button for Search, so if it's an important feature in your app, it should always appear as an action item (and usually as the first item, especially if you offer it with an action view).

注意：不是搜有的设备都提供专用的搜索键，如果那时应用的重要特性，它应该出现在操作项。

Using split action bar 分离的操作条

When your application is running on Android 4.0 (API level 14) and higher, there's an extra mode available for the action bar called "split action bar." When you enable split action bar, a separate bar appears at the bottom of the screen to display all action items when the activity is running on a narrow screen (such as a portrait-oriented handset). Splitting the action bar to separate the action items ensures that a reasonable amount of space is available to display all your action items on a narrow screen, while leaving room for navigation and title elements at the top.

当你的应用运行在Android 4.0以上时，有一个action bar的附加模式--分离的操作条。当使用分离的操作条时，在屏幕下方出现独立的操作条，显示所有的操作项。

To enable split action bar, simply add uiOptions="splitActionBarWhenNarrow" to your  or  manifest element.

通过给或设置uiOptions="splitActionBarWhenNarrow"，使用分离的操作条。

Be aware that Android adjusts the action bar's appearance in a variety of ways, based on the current screen size. Using split action bar is just one option that you can enable to allow the action bar to further optimize the user experience for different screen sizes. In doing so, you may also allow the action bar to collapse navigation tabs into the main action bar. That is, if you use navigation tabs in your action bar, once the action items are separated on a narrow screen, the navigation tabs may be able to fit into the main action bar rather than be separated into the "stacked action bar." Specifically, if you've disabled the action bar icon and title (with setDisplayShowHomeEnabled(false) and setDisplayShowTitleEnabled(false)), then the navigation tabs collapse into the main action bar, as shown by the second device in figure 3.

Figure 3. Mock-ups of split action bar with navigation tabs on the left; with the app icon and title disabled on the right.

Note: Although the android:uiOptions attribute was added in Android 4.0 (API level 14), you can safely include it in your application even if your minSdkVersion is set to a value lower than "14" to remain compatible with older versions of Android. When running on older versions, the system simply ignores the XML attribute because it doesn't understand it. The only condition to including it in your manifest is that you must compile your application against a platform version that supports API level 14 or higher. Just be sure that you don't openly use other APIs in your application code that aren't supported by the version declared by your minSdkVersion attribute—only XML attributes are safely ignored by older platforms.

Using the App Icon for Navigation 应用图标做导肮

Using a logo instead of icon

By default, the system uses your application icon in the action bar, as specified by the android:iconattribute in the  or element. However, if you also specify theandroid:logo attribute, then the action bar uses the logo image instead of the icon.

A logo should usually be wider than the icon, but should not include unnecessary text. You should generally use a logo only when it represents your brand in a traditional format that users recognize. A good example is the YouTube app's logo—the logo represents the expected user brand, whereas the app's icon is a modified version that conforms to the square requirement.

By default, your application icon appears in the action bar on the left side. If you'd like, you can enable the icon to behave as an action item. In response to user action on the icon, your application should do one of two things:

缺省情况下，应用图标出现在action bar的左边。它可以作为操作项。在图标上的操作，应该是2件事：

• Go to the application "home" activity, or
• 返回应用的主activity
• Navigate "up" the application's structural hierarchy
• 导航到应用程序结构的“返回”。

When the user touches the icon, the system calls your activity's onOptionsItemSelected() method with theandroid.R.id.home ID. In response, you should either start the home activity or take the user one step up in your application's structural hierarchy.

当用户触摸应用图标，系统调用activity的onOptionsItemSelected()方法，此时传递android.R.id.home作为参数。也可以用来返回主activity或程序的上一步。

If you respond to the application icon by returning to the home activity, you should include the FLAG_ACTIVITY_CLEAR_TOP flag in the Intent. With this flag, if the activity you're starting already exists in the current task, then all activities on top of it are destroyed and it is brought to the front. Adding this flag is often important because going "home" is an action that's equivalent to "going back" and you should usually not create a new instance of the home activity. Otherwise, you might end up with a long stack of activities in the current task with multiple instances of the home activity.

如果应用图标是返回主activity,在intent中要包括FLAG_ACTIVITY_CLEAR_TOP标志。如果当前任务中已经启动了主activity，使用这个标志可以销毁所有主activity堆栈上面的activity。

For example, here's an implementation of onOptionsItemSelected() that returns to the application's "home" activity:

@Overridepublic boolean onOptionsItemSelected(MenuItem item) {    switch (item.getItemId()) {        case android.R.id.home:            // app icon in action bar clicked; go home            Intent intent = new Intent(this, HomeActivity.class);            intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);            startActivity(intent);            return true;        default:            return super.onOptionsItemSelected(item);    }}

In case the user can enter the current activity from another application, you might also want to add the FLAG_ACTIVITY_NEW_TASK flag. This flag ensures that, when the user navigates either "home" or "up", the new activity is not added to the current task, but instead started in a task that belongs to your application. For example, if the user starts an activity in your application through an intent invoked by another application, then selects the action bar icon to navigate home or up, the FLAG_ACTIVITY_CLEAR_TOP flag starts the activity in a task that belongs to your application (not the current task). The system either starts a new task with your new activity as the root activity or, if an existing task exists in the background with an instance of that activity, then that task is brought forward and the target activity receives onNewIntent(). So if your activity accepts intents from other applications (it declares any generic intent filters), you should usually add the FLAG_ACTIVITY_NEW_TASK flag to the intent:

使用FLAG_ACTIVITY_NEW_TASK标志，当用户导航到主acitivity或返回时，新的activity不添加到当前任务，而是开始一个属于自己的应用。如果Activity接受其他应用发出的intent，应该增加FLAG_ACTIVITY_NEW_TASK标志给intent:

intent.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP | Intent.FLAG_ACTIVITY_NEW_TASK);

For more information about these flags and other back stack behaviors, read the Tasks and Back Stack developer guide.

Note: If you're using the icon to navigate to the home activity, beware that beginning with Android 4.0 (API level 14), you must explicitly enable the icon as an action item by callingsetHomeButtonEnabled(true) (in previous versions, the icon was enabled as an action item by default).

注意：如果使用应用图表导航到主activity，是从Android 4.0 开始的，你必须调用setHomeButtonEnabled(true)明确图标作为一个操作项。

Navigating up 导航

Figure 4. The Email app's standard icon (left) and the "navigate up" icon (right). The system automatically adds the "up" indicator.

As a supplement to traditional "back" navigation—which takes the user to the previous screen in the task history—you can enable the action bar icon to offer "up" navigation, which should take the user one step up in your application's structural hierarchy. For instance, if the current screen is somewhere deep in the hierarchy of the application, touching the app icon should navigate upward one level, to the parent of the current screen.

作为传统"back"键的补充，把用户带回到先前的屏幕。

For example, figure 5 illustrates how the BACK button behaves when the user navigates from one application to an activity belonging to a different application (specifically, when composing an email to a person selected from the People app).

Figure 5. The BACK button behavior after entering the Email app from the People (or Contacts) app.

However, if the user wants to stay within the email application after composing the email, up navigation allows the user to navigate upward in the email application, rather than go back to the previous activity. Figure 6 illustrates this scenario, in which the user again comes into the email application, but presses the action bar icon to navigate up, rather than back.

Figure 6. Example behavior for UP navigation after entering the Email app from the People app.

Navigation Design

For more about how Up and Back navigation differ, read Android Design's Navigation guide.

To enable the icon for up navigation (which displays the "up" indicator next to the icon), call setDisplayHomeAsUpEnabled(true) on your ActionBar:

使应用图标当做返回导航，可以调用ActionBar的setDisplayHomeAsUpEnabled(true).

protected void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    setContentView(R.layout.main);    ActionBar actionBar = getActionBar();    actionBar.setDisplayHomeAsUpEnabled(true);    ...}

When the user touches the icon, the system calls your activity's onOptionsItemSelected() method with the android.R.id.home ID, as shown in the above section about Using the App Icon for Navigation.

Remember to use the FLAG_ACTIVITY_CLEAR_TOP flag in the Intent, so that you don't create a new instance of the parent activity if one already exists. For instance, if you don't use theFLAG_ACTIVITY_CLEAR_TOP flag, then after navigating up, the BACK button will actually take the user "forward", with respect to the application structure, which would be strange.

Note: If there are many paths that the user could have taken to reach the current activity within your application, the up icon should navigate backward along the path the user actually followed to get to the current activity.

Adding an Action View 增加操作视图

Figure 7. An action bar with a collapsed action view for Search (top), then expanded action view with the SearchView widget (bottom).

An action view is a widget that appears in the action bar as a substitute for an action item's button. For example, if you have an item in the options menu for "Search," you can add an action view that replaces the button with a SearchView widget, as shown in figure 7.

操作视图是出现在action bar上代替操作项按钮的控件。例如：有一个搜索的可选菜单，你可以增加的操作视图来代替按钮，如图7所示。

To declare an action view for an item in your menu resource, use either the android:actionLayout or android:actionViewClassattribute to specify either a layout resource or widget class to use, respectively. For example:

在菜单资源文件中声明操作视图，使用android:actionLayout指定layout资源或android:actionViewClass指定控件类。

<?xml version="1.0" encoding="utf-8"?> xmlns:android="http://schemas.android.com/apk/res/android">     android:id="@+id/menu_search"          android:title="@string/menu_search"          android:icon="@drawable/ic_menu_search"          android:showAsAction="ifRoom|collapseActionView"          android:actionViewClass="android.widget.SearchView" />

Notice that the android:showAsAction attribute also includes "collapseActionView". This is optional and declares that the action view should be collapsed into a button. When the user selects the button, the action view expands. Otherwise, the action view is visible by default and might consume valuable action bar space even when the user is not using it. For more information, see the next section about Handling collapsible action views.

菜单项的android:shwoAsAction属性可以包括collapseActionView。这个选项声明操作视图应该被压缩成一个按钮。当用户选择该按钮时，操作视图展开。否则，操作视图会占用大量的action bar空间。更多信息，在下一节。

If you need to add some event hooks to your action view, you can do so during the onCreateOptionsMenu() callback. You can acquire elements in an action view by calling findItem()with the ID of the menu item, then call getActionView(). For example, the search widget from the above sample is acquired like this:

如果要增加操作视图的事件调用，可在onCreateOptionMenu()中实现。通过菜单的findItem(ID)获取菜单项，然后调用getActionView()获取操作视图。例如：

@Overridepublic boolean onCreateOptionsMenu(Menu menu) {    getMenuInflater().inflate(R.menu.options, menu);    SearchView searchView = (SearchView) menu.findItem(R.id.menu_search).getActionView();    // Configure the search info and add any event listeners    ...    return super.onCreateOptionsMenu(menu);}

For more information about using the search widget, see Creating a Search Interface.

Handling collapsible action views 处理可折叠操作视图

Supporting Android 3.0 with an action view

The "collapseActionView" option was added with Android 4.0 (API level 14). However, if your application supports older versions, you should still declare "collapseActionView" in order to better support smaller screens. Devices running Android 4.0 and higher will show the action view collapsed, while older versions work as designed otherwise.

Adding this value requires that you set your build target to Android 4.0 or higher in order to compile. Older versions of Android ignore the"collapseActionView" value because they don't understand it. Just be sure not to use other APIs in your source code that are not supported in the version declared by your minSdkVersion, unless you add the appropriate version check at runtime.

Action views allow you to provide fast access to rich actions without changing activities or fragments, or replacing the action bar. However, it might not be appropriate to make an action view visible by default. To preserve the action bar space (especially when running on smaller screens), you can collapse your action view into an action item button. When the user selects the button, the action view appears in the action bar. When collapsed, the system might place the item into the overflow menu if you've definedandroid:showAsAction with "ifRoom", but the action view still appears in the action bar when the user selects the item. You can make your action view collapsible by adding "collapseActionView" to the android:showAsAction attribute, as shown in the XML above.

操作视图在不跟更换activity或fragment的情况下，可以提供快速处理。然后，缺省情况下操作视图可见也是不合适的。为了节省action bar空间，可以把操作视图缩成一个操作按钮。当用户选择该按钮时，操作视图才展开。当处于收缩状态时，如果定义了android:showAsAction="ifRoom"系统有可能把它放到溢出菜单中,然而如果用户选择给操作视图它会显示在action bar上。你可以给android:showAsAction增加collapseActionView来实现可收缩。

Because the system will expand the action view when the user selects the item, so you do not need to respond to the item in theonOptionsItemSelected callback. The system still calls onOptionsItemSelected() when the user selects it, but the system will always expand the action view unless you return true (indicating you've handled the event instead).

因为当用户选择按钮时，系统会扩展操作视图，所以不必再处理onOptionsItemSelected事件。除非该时间饭不会true，否则系统总是展开操作视图。

The system also collapses your action view when the user selects the "up" icon in the action bar or presses the BACK button.

当用户选择“up"图标或按返回键，系统会收缩操作视图。

If necessary, you can expand or collapse the action view in your own code by calling expandActionView() andcollapseActionView() on the MenuItem.

如果必要，可以通过代码扩展或收缩操作视图。在MenuItem上调用expandActionView()或collapseActionView().

Note: Although collapsing your action view is optional, we recommend that you always collapse your action view if it includesSearchView. Also be aware that some devices provide a dedicated SEARCH button and you should expand your search action view if the user presses the SEARCH button. Simply override your activity's onKeyUp() callback method, listen for theKEYCODE_SEARCH event, then call expandActionView().

If you need to update your activity based on the visibility of your action view, you can receive callbacks when it's expanded and collapsed by defining an OnActionExpandListener and registering it with setOnActionExpandListener(). For example:

如果在操作视图可见或不可见时，需要更新activity，可以定义一个OnActionExpandListener并通过setOnActionExpandListener()来注册。

@Overridepublic boolean onCreateOptionsMenu(Menu menu) {    getMenuInflater().inflate(R.menu.options, menu);    MenuItem menuItem = menu.findItem(R.id.actionItem);    ...    menuItem.setOnActionExpandListener(new OnActionExpandListener() {        @Override        public boolean onMenuItemActionCollapse(MenuItem item) {            // Do something when collapsed            return true;  // Return true to collapse action view        }        @Override        public boolean onMenuItemActionExpand(MenuItem item) {            // Do something when expanded            return true;  // Return true to expand action view        }    });}

Adding an Action Provider添加Action Provider

Figure 8. Screenshot from the Gallery app, with theShareActionProvider submenu expanded to show share targets.

Similar to an action view, an action provider (defined by the ActionProvider class) replaces an action item with a customized layout, but it also takes control of all the item's behaviors. When you declare an action provider for a menu item in the action bar, it not only controls the appearance of the item in the action bar with a custom layout, but also handles the default event for the menu item when it appears in the overflow menu. It can also provide a submenu from either the action bar or the overflow menu.

和操作视图类似，一个action provider可以用自定义布局替换操作项，同时负责控制所有项的行为。

也可以为action bar或溢出菜单提供子菜单。

For example, the ShareActionProvider is an extension of ActionProvider that facilitates a “share" action by showing a list of available share targets from the action bar. Instead of using a traditional action item that invokes the ACTION_SEND intent, you can declare an instance ofShareActionProvider to handle an action item. This action provider presents an action view with a drop-down list of applications that handle theACTION_SEND intent, even when the menu item appears in the overflow menu. Hence, when you use an action provider such as this one, you don't have to handle user events on the menu item.

To declare an action provider for an action item, define the android:actionProviderClass attribute for the appropriate the  element in yourmenu resource, using the fully-qualified class name of the action provider. For example:

<?xml version="1.0" encoding="utf-8"?> xmlns:android="http://schemas.android.com/apk/res/android">     android:id="@+id/menu_share"          android:title="@string/share"          android:showAsAction="ifRoom"          android:actionProviderClass="android.widget.ShareActionProvider" />    ...

In this example, the ShareActionProvider is used as the action provider. At this point, the action provider officially takes control of the menu item and handles both its appearance and behavior in the action bar and its behavior in the overflow menu. You must still provide a text title for the item to be used in the overflow menu.

Although the action provider can perform the default action for the menu item when it appears in the overflow menu, your activity (or fragment) can override that behavior by also handling the click event from the onOptionsItemSelected() callback method. If you do not handle the event in that callback, then the action provider receives the onPerformDefaultAction() callback to handle the event. However, if the action provider provides a submenu, then your activity will not receive the onOptionsItemSelected() callback, because the submenu is shown instead of invoking the default menu item behavior when selected.

Using the ShareActionProvider

If you want to provide a "share" action in your action bar by leveraging other applications installed on the device (for example, to share a photo using a messaging or social app), then usingShareActionProvider is an effective way to do so, rather than adding an action item that invokes the ACTION_SEND intent. When you use ShareActionProvider for an action item, it presents an action view with a drop-down list of applications that handle the ACTION_SEND intent (as shown in figure 8).

All the logic for creating the submenu, populating it with share targets, and handling click events (including when the item appears in the overflow menu) is implemented by theShareActionProvider—the only code you need to write is to declare the action provider for the menu item and specify the share intent.

By default, the ShareActionProvider retains a ranking for each share target based on how often the user selects each one. The share targets used more frequently appear at the top of the drop-down list and the target used most often appears directly in the action bar as the default share target. By default, the ranking information is saved in a private file with a name specified by DEFAULT_SHARE_HISTORY_FILE_NAME. If you use the ShareActionProvider or an extension of it for only one type of action, then you should continue to use this default history file and there's nothing you need to do. However, if you use ShareActionProvider or an extension of it for multiple actions with semantically different meanings, then each ShareActionProvidershould specify its own history file in order to maintain its own history. To specify a different history file for the ShareActionProvider, call setShareHistoryFileName() and provide an XML file name (for example, "custom_share_history.xml").

Note: Although the ShareActionProvider ranks share targets based on frequency of use, the behavior is extensible and extensions of ShareActionProvider can perform different behaviors and ranking based on the history file (if appropriate).

To add ShareActionProvider, simply define the android:actionProviderClass attribute with "android.widget.ShareActionProvider", as shown in the XML example above. The only thing left to do is define the Intent you want to use for sharing. To do so, you must call getActionProvider() to retrieve the ShareActionProvider that's associated with a MenuItem, then call setShareIntent().

If the format for the share intent depends on the selected item or other variables that change during the activity lifecycle, you should save the ShareActionProvider in a member field and update it by calling setShareIntent() as necessary. For example:

private ShareActionProvider mShareActionProvider;...@Overridepublic boolean onCreateOptionsMenu(Menu menu) {    mShareActionProvider = (ShareActionProvider) menu.findItem(R.id.menu_share).getActionProvider();    // If you use more than one ShareActionProvider, each for a different action,    // use the following line to specify a unique history file for each one.    // mShareActionProvider.setShareHistoryFileName("custom_share_history.xml");    // Set the default share intent    mShareActionProvider.setShareIntent(getDefaultShareIntent());    return true;}// When you need to update the share intent somewhere else in the app, call// mShareActionProvider.setShareIntent()

The ShareActionProvider now handles all user interaction with the item and you do not need to handle click events from the onOptionsItemSelected() callback method.

For a sample using the share action provider, see ActionBarShareActionProviderActivity.

Creating a custom action provider

When you want to create an action view that has dynamic behaviors and a default action in the overflow menu, extending ActionProvider to define those behaviors is a good solution. Creating your own action provider offers you an organized and reusable component, rather than handling the various action item transformations and behaviors in your fragment or activity code. As shown in the previous section, Android provides one implementation of ActionProvider for share actions: the ShareActionProvider.

To create your own, simply extend the ActionProvider class and implement its callback methods as appropriate. Most importantly, you should implement the following:

ActionProvider()

This constructor passes you the application Context, which you should save in a member field to use in the other callback methods.

onCreateActionView()

This is where you define the action view for the item. Use the Context acquired from the constructor to instantiate a LayoutInflater and inflate your action view layout from an XML resource, then hook up event listeners. For example:

public View onCreateActionView() {    // Inflate the action view to be shown on the action bar.    LayoutInflater layoutInflater = LayoutInflater.from(mContext);    View view = layoutInflater.inflate(R.layout.action_provider, null);    ImageButton button = (ImageButton) view.findViewById(R.id.button);    button.setOnClickListener(new View.OnClickListener() {        @Override        public void onClick(View v) {            // Do something...        }    });    return view;}

onPerformDefaultAction()

The system calls this when the menu item is selected from the overflow menu and the action provider should perform a default action for the menu item.

However, if your action provider provides a submenu, through the onPrepareSubMenu() callback, then the submenu appears even when the menu item is in the overflow menu. Thus,onPerformDefaultAction() is never called when there is a submenu.

Note: An activity or a fragment that implements onOptionsItemSelected() can override the action provider's default behavior by handling the item-selected event (and returning true), in which case, the system does not call onPerformDefaultAction().

For an example extension of ActionProvider, see ActionBarSettingsActionProviderActivity.

Adding Navigation Tabs 增加导航标签

Figure 9. Screenshot of action bar tabs from the Honeycomb Gallery app.

Figure 10. Screenshot of tabs in the stacked action bar on a narrow screen.

When you want to provide navigation tabs in an activity, using the action bar's tabs is a great option (instead of using TabWidget), because the system adapts the action bar tabs for different screen sizes—placing them in the main action bar when the screen is sufficiently wide, or in a separate bar (known as the "stacked action bar") when the screen is too narrow, as shown in figures 9 and 10.

当想要在activity中提供导航标签时，可以使用action bar的标签，系统自动调整action bar标签，屏幕足够大时，放在主action bar上，屏幕太窄时，放到单独的标签行上。

To switch between fragments using the tabs, you must perform a fragment transaction each time a tab is selected. If you're not familiar with how to change fragments using FragmentTransaction, first read theFragments developer guide.

使用标签切换不同的fragment，你每次选择标签时，必须执行fragement转换。如果不熟悉使用FragmentTrasaction切换fragment，请先阅读Fragments开发指南。

To get started, your layout must include a ViewGroup in which you place each Fragment associated with a tab. Be sure the ViewGrouphas a resource ID so you can reference it from your tab-swapping code. Alternatively, if the tab content will fill the activity layout (excluding the action bar), then your activity doesn't need a layout at all (you don't even need to call setContentView()). Instead, you can place each fragment in the default root ViewGroup, which you can refer to with the android.R.id.content ID (you can see this ID used in the sample code below, during fragment transactions).

首先，布局文件必须包括ViewGroup，在其中放置每个标签对应的Fragment。ViewGroup要有一个资源ID，可以在标签切换时引用它。如果标签内容充满真个activity，activity不需要布局文件。把每个fragment放到根ViewGroup,你可以使用android.R.id.content来引用ViewGroup.

Once you determine where the fragments appear in the layout, the basic procedure to add tabs is:

决定fragment出现在哪里之后，添加标签的步骤如下：

1. Implement the ActionBar.TabListener interface. Callbacks in this interface respond to user events on the tabs so you can swap fragments.实现ActionBar.TabListener接口，对用户在标签上的操作做出反应，可以用来切换fragment.
2. For each tab you want to add, instantiate an ActionBar.Tab and set the ActionBar.TabListener by calling setTabListener(). Also set the tab's title and/or icon with setText()and/or setIcon().每个要添加的页签，实例化ActionBar.Tab然后通过setTabListenter()设置ActionBar.TabListener.还要通过setText(),setIcon()设置标题和图标。
3. Add each tab to the action bar by calling addTab().通过addTab()将标签添加到action bar.

When looking at the ActionBar.TabListener interface, notice that the callback methods provide only the ActionBar.Tab that was selected and a FragmentTransaction for you to perform fragment transactions—it doesn't say anything about what fragment you should swap in or out. Thus, you must define your own association between each ActionBar.Tab and the appropriate Fragment that it represents (in order to perform the appropriate fragment transaction). There are several ways you can define the association, depending on your design. In the example below, the ActionBar.TabListener implementation provides a constructor such that each new tab uses its own instance of the listener. Each instance of the listener defines several fields that are necessary to later perform a transaction on the appropriate fragment.

观察一下ActionBar.TabListener接口，你会注意到该方法只提供了选中的ActionBar.Tab和执行fragment转换的FragmentTransaction两个参数。FratmentTransaction没有指明显示和消失的Fragment，因此，必须自己定义ActionBar.Tab和Fragment之间的联系。有几种定义的方法，可根据设计进行选择。在下面的例子中，ActionBar.TabListener的实现提供了一个构造器，每个新标签使用一个自己的监听器实例。每个监听器实例定义了几个必须的属性字段，在以后执行fragment转换时使用。

For example, here's how you might implement the ActionBar.TabListener such that each tab uses its own instance of the listener:

例子，如何实现ActionBar.TabListener,每个标签使用自己的监听器实例：

public static class TabListener<T extends Fragment> implements ActionBar.TabListener {    private Fragment mFragment;    private final Activity mActivity;    private final String mTag;    private final Class<T> mClass;    /** Constructor used each time a new tab is created. 构造器      * @param activity  The host Activity, used to instantiate the fragment       * @param tag  The identifier tag for the fragment       * @param clz  The fragment's Class, used to instantiate the fragment      */    public TabListener(Activity activity, String tag, Class<T> clz) {        mActivity = activity;        mTag = tag;        mClass = clz;    }    /* The following are each of the ActionBar.TabListener callbacks */    public void onTabSelected(Tab tab, FragmentTransaction ft) {        // Check if the fragment is already initialized        if (mFragment == null) {            // If not, instantiate and add it to the activity            mFragment = Fragment.instantiate(mActivity, mClass.getName());            ft.add(android.R.id.content, mFragment, mTag);        } else {            // If it exists, simply attach it in order to show it            ft.attach(mFragment);        }    }    public void onTabUnselected(Tab tab, FragmentTransaction ft) {        if (mFragment != null) {            // Detach the fragment, because another one is being attached            ft.detach(mFragment);        }    }    public void onTabReselected(Tab tab, FragmentTransaction ft) {        // User selected the already selected tab. Usually do nothing.    }}

Caution: You must not call commit() for the fragment transaction in each of these callbacks—the system calls it for you and it may throw an exception if you call it yourself. You also cannot add these fragment transactions to the back stack.

In this example, the listener simply attaches (attach()) a fragment to the activity layout—or if not instantiated, creates the fragment and adds (add()) it to the layout (as a child of theandroid.R.id.content view group)—when the respective tab is selected, and detaches (detach()) it when the tab is unselected.

在这个例子中，当标签选中时，监听器简单的添加fragment到activity--如果没有实例化，创建一个fragment然后添加到布局中(作为android.R.id.content视图组的一个子视图)。当标签未选中时拆卸。

The ActionBar.TabListener implementation is the bulk of the work. All that remains is to create each ActionBar.Tab and add it to the ActionBar. Additionally, you must callsetNavigationMode(NAVIGATION_MODE_TABS) to make the tabs visible. You might also want to disable the activity title by calling setDisplayShowTitleEnabled(false) if the tab titles actually indicate the current view.

ActionBar.TabListener实现是一大块工作。需要创建每个ActionBar.Tab然后添加到ActionBar。另外，必须调用setNavigationMode(NAVIGRTION_MODE_TABS)使标签可见。如果标签标题已经指明当前视图，也可以通过setDisplayShowTitleEnabled(false)隐藏activity标题。

For example, the following code adds two tabs using the listener defined above:

@Overrideprotected void onCreate(Bundle savedInstanceState) {    super.onCreate(savedInstanceState);    // Notice that setContentView() is not used, because we use the root    // android.R.id.content as the container for each fragment    // setup action bar for tabs    ActionBar actionBar = getActionBar();    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);    actionBar.setDisplayShowTitleEnabled(false);    Tab tab = actionBar.newTab()            .setText(R.string.artist)            .setTabListener(new TabListener<ArtistFragment>(                    this, "artist", ArtistFragment.class));    actionBar.addTab(tab);    tab = actionBar.newTab()        .setText(R.string.album)        .setTabListener(new TabListener<AlbumFragment>(                this, "album", AlbumFragment.class));    actionBar.addTab(tab);}

Note: The above implementation for ActionBar.TabListener is one of several possible techniques. You can see more of this style in the API Demos app.

注意：上面个的ActionBar.TabListener的实现是几种技术之一。你可以在API Demos中看到更多方法。

If your activity stops, you should retain the currently selected tab with the saved instance state so you can open the appropriate tab when the user returns. When it's time to save the state, you can query the currently selected tab with getSelectedNavigationIndex(). This returns the index position of the selected tab.

如果activity关闭，你应该保留当前选中的标签，以便用户回来时打开相应的标签。在保存状态时，可以通过getSelectedNavigationIndex()查询当前选中的标签，它返回选中标签的索引号。

Caution: It's important that you save the state of each fragment as necessary, so that when users switch fragments with the tabs and then return to a previous fragment, it looks the way it did when they left. For information about saving the state of your fragment, see the Fragments developer guide.

Note: In some cases, the Android system will show your action bar tabs as a drop-down list in order to ensure the best fit in the action bar.

Adding Drop-down Navigation增加下拉导航

As another mode of navigation (or filtering) within your activity, the action bar offers a built in drop-down list. For example, the drop-down list can offer different modes by which content in the activity is sorted.

在Activity中另一种导航，ActionBar提供内置的下拉列表。例如，下拉列表可以提供不同的模式的排序。

The basic procedure to enable drop-down navigation is:

使用下拉导航的基本过程：

1. Create a SpinnerAdapter that provides the list of selectable items for the drop-down and the layout to use when drawing each item in the list.  创建一个SpinnerAdapter,提供下拉可选项及列表布局。
2. Implement ActionBar.OnNavigationListener to define the behavior that occurs when the user selects an item from the list. 实现ActionBar.OnNavigationListener定义用户选择列表后的行为。
3. Enable navigation mode for the action bar with setNavigationMode(). For example: 使用setNavigationMode()设定ActionBar为下拉模式。例如：

4. ActionBar actionBar = getActionBar();actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_LIST);

   Note: You should perform this during your activity's onCreate() method.

5. Set the callback for the drop-down list with setListNavigationCallbacks(). For example: 调用setListNavigationCallbacks()设置下拉的回调方法。

   actionBar.setListNavigationCallbacks(mSpinnerAdapter, mNavigationCallback);

   This method takes your SpinnerAdapter and ActionBar.OnNavigationListener.

That's the basic setup. However, implementing the SpinnerAdapter and ActionBar.OnNavigationListener is where most of the work is done. There are many ways you can implement these to define the functionality for your drop-down navigation and implementing various types of SpinnerAdapter is beyond the scope of this document (you should refer to theSpinnerAdapter class reference for more information). However, below is a simple example for a SpinnerAdapter and ActionBar.OnNavigationListener to get you started (click the title to reveal the sample).

这是基本的设置。然而，实现SpinnerAdapter和ActionBar.OnNavigationListener有大量工作要做。下面是一个简单的例子。

 Example SpinnerAdapter and OnNavigationListener

Styling the Action Bar 风格化ActionBar

If you've implemented a custom design for the widgets in your application, you might also want to redesign some of the action bar to match your app design. To do so, you need to use Android's style and theme framework to restyle the action bar using special style properties.

如果曾经客户化控件，你也许想重新设计actionBar的一些属性来适用自己的应用。你需要适用style和theme框架，风格化actionBar的一些属性。

Note: In order for background images to change appearance depending on the current button state (selected, pressed, unselected), the drawable resource you use must be a state list drawable.

Caution: For all background drawables you provide, be sure to use Nine-Patch drawables to allow stretching. The Nine-Patch image should be smaller than 40px tall and 30px wide (for the mdpi asset).

General appearance 

android:windowActionBarOverlay

Declares whether the action bar should overlay the activity layout rather than offset the activity's layout position (for example, the Gallery app uses overlay mode). This is false by default.

Normally, the action bar requires its own space on the screen and your activity layout fills in what's left over. When the action bar is in overlay mode, your activity layout uses all the available space and the system draws the action bar on top. Overlay mode can be useful if you want your content to keep a fixed size and position when the action bar is hidden and shown. You might also like to use it purely as a visual effect, because you can use a semi-transparent background for the action bar so the user can still see some of your activity layout behind the action bar.

正常情况下，actionBar占用自己的空间，Activity占用剩余的空间。当actionBar在overlay模式，activity使用所有可用空间，系统把actionBar画在activity的上部。overlay模式可以是activity内容保持固定大小和位置，无论actionBar是隐藏还是显示。也可以看做是一种视觉效果，可以使actionbar的背景半透明，用户可以看到actionBar后面的窗体。

Note: The Holo theme families draw the action bar with a semi-transparent background by default. However, you can modify it with your own styles and the DeviceDefault theme on different devices might use an opaque background by default.

When overlay mode is enabled, your activity layout has no awareness of the action bar laying on top of it. So, you must be careful not to place any important information or UI components in the area overlayed by the action bar. If appropriate, you can refer to the platform's value for actionBarSize to determine the height of the action bar, by referencing it in your XML layout. For example:

使用overlay模式时，activity不会感知到actionBar的存在。所以，不要把重要的信息或UI控件放在actionBar区域。可以引用Android平台的值actionBarSize来决定actionbar的高度，可以在xml布局文件用引用。例如：

    ...    android:layout_marginTop="?android:attr/actionBarSize" />

You can also retrieve the action bar height at runtime with getHeight(). This reflects the height of the action bar at the time it's called, which might not include the stacked action bar (due to navigation tabs) if called during early activity lifecycle methods. To see how you can determine the total height at runtime, including the stacked action bar, see theTitlesFragment class in the Honeycomb Gallery sample app.

Action items 操作项

android:actionButtonStyle

Defines a style resource for the action item buttons. 

android:actionBarItemBackground

Defines a drawable resource for each action item's background. (Added in API level 14.)

android:itemBackground

Defines a drawable resource for each overflow menu item's background.

android:actionBarDivider

Defines a drawable resource for the divider between action items. (Added in API level 14.)

android:actionMenuTextColor

Defines a color for text that appears in an action item.

android:actionMenuTextAppearance

Defines a style resource for text that appears in an action item.

android:actionBarWidgetTheme

Defines a theme resource for widgets that are inflated into the action bar as action views. (Added in API level 14.)

Navigation tabs

android:actionBarTabStyle

Defines a style resource for tabs in the action bar.

android:actionBarTabBarStyle

Defines a style resource for the thin bar that appears below the navigation tabs.

android:actionBarTabTextStyle

Defines a style resource for text in the navigation tabs.

Drop-down lists

android:actionDropDownStyle

Defines a style for the drop-down navigation (such as the background and text styles).

For example, here's a file that defines a few custom styles for the action bar:

<?xml version="1.0" encoding="utf-8"?>         name="CustomActivityTheme" parent="@android:style/Theme.Holo">         name="android:actionBarTabTextStyle">@style/CustomTabTextStyle         name="android:actionBarDivider">@drawable/ab_divider         name="android:actionBarItemBackground">@drawable/ab_item_background             name="CustomTabTextStyle" parent="@android:style/TextAppearance.Holo">         name="android:textColor">#2456c2    

Note: Be certain that your theme declares a parent theme in the         name="MyActionBar" parent="@android:style/Widget.Holo.ActionBar">        name="android:background">@drawable/ab_background        name="android:backgroundStacked">@drawable/ab_background        name="android:backgroundSplit">@drawable/ab_split_background   

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