Android(安卓)Option Menu

Creating an Options Menu

Figure 1. Options menu in the Browser, on Android 2.3.

The options menu is where you should include actions and other options that are relevant to the current activity context, such as "Search," "Compose email," and "Settings."

Where the items in your options menu appear on the screen depends on the version for which you've developed your application:

If you've developed your application for Android 2.3.x (API level 10) or lower, the contents of your options menu appear at the bottom of the screen when the user presses the Menu button, as shown in figure 1. When opened, the first visible portion is the icon menu, which holds up to six menu items. If your menu includes more than six items, Android places the sixth item and the rest into the overflow menu, which the user can open by selectingMore.
If you've developed your application for Android 3.0 (API level 11) and higher, items from the options menu are available in the action bar. By default, the system places all items in the action overflow, which the user can reveal with the action overflow icon on the right side of the action bar (or by pressing the device Menu button, if available). To enable quick access to important actions, you can promote a few items to appear in the action bar by addingandroid:showAsAction="ifRoom" to the corresponding <item> elements (see figure 2).
For more information about action items and other action bar behaviors, see the Action Bar guide.

Note: Even if you're not developing for Android 3.0 or higher, you can build your own action bar layout for a similar effect. For an example of how you can support older versions of Android with an action bar, see the Action Bar Compatibility sample.

Figure 2. Action bar from the Honeycomb Gallery app, showing navigation tabs and a camera action item (plus the action overflow button).

You can declare items for the options menu from either your Activity subclass or a Fragment subclass. If both your activity and fragment(s) declare items for the options menu, they are combined in the UI. The activity's items appear first, followed by those of each fragment in the order in which each fragment is added to the activity. If necessary, you can re-order the menu items with the android:orderInCategory attribute in each <item> you need to move.

To specify the options menu for an activity, override onCreateOptionsMenu() (fragments provide their own onCreateOptionsMenu() callback). In this method, you can inflate your menu resource (defined in XML) into the Menu provided in the callback. For example:

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

You can also add menu items using add() and retrieve items with findItem() to revise their properties with MenuItem APIs.

If you've developed your application for Android 2.3.x and lower, the system calls onCreateOptionsMenu() to create the options menu when the user opens the menu for the first time. If you've developed for Android 3.0 and higher, the system calls onCreateOptionsMenu() when starting the activity, in order to show items to the action bar.

Handling click events

When the user selects an item from the options menu (including action items in the action bar), the system calls your activity's onOptionsItemSelected() method. This method passes the MenuItem selected. You can identify the item by calling getItemId(), which returns the unique ID for the menu item (defined by the android:id attribute in the menu resource or with an integer given to the add() method). You can match this ID against known menu items to perform the appropriate action. For example:

@Override public boolean onOptionsItemSelected(MenuItem item) {   // Handle item selection   switch (item.getItemId()) {     case R.id.new_game:       newGame();       return true;     case R.id.help:       showHelp();       return true;     default:       return super.onOptionsItemSelected(item);   } }

When you successfully handle a menu item, return true. If you don't handle the menu item, you should call the superclass implementation of onOptionsItemSelected() (the default implementation returns false).

If your activity includes fragments, the system first calls onOptionsItemSelected() for the activity then for each fragment (in the order each fragment was added) until one returnstrue or all fragments have been called.

Tip: Android 3.0 adds the ability for you to define the on-click behavior for a menu item in XML, using the android:onClick attribute. The value for the attribute must be the name of a method defined by the activity using the menu. The method must be public and accept a single MenuItem parameter—when the system calls this method, it passes the menu item selected. For more information and an example, see the Menu Resource document.

Tip: If your application contains multiple activities and some of them provide the same options menu, consider creating an activity that implements nothing except the onCreateOptionsMenu() and onOptionsItemSelected() methods. Then extend this class for each activity that should share the same options menu. This way, you can manage one set of code for handling menu actions and each descendant class inherits the menu behaviors. If you want to add menu items to one of the descendant activities, override onCreateOptionsMenu() in that activity. Call super.onCreateOptionsMenu(menu) so the original menu items are created, then add new menu items with menu.add(). You can also override the super class's behavior for individual menu items.

Creating an Options Menu

Handling click events

更多相关文章

随机推荐