Android Dev Guide.Activities.Fragments文档章节翻译

译者署名: shadowl
版本:Android 4.0 r1

A Fragment represents a behavior or a portion of user interface in an Activity. You can combine multiple fragments in a single activity to build a multi-pane UI and reuse a fragment in multiple activities. You can think of a fragment as a modular section of an activity, which has its own lifecycle, receives its own input events, and which you can add or remove while the activity is running (sort of like a "sub activity" that you can reuse in different activities).


A fragment must always be embedded in an activity and the fragment's lifecycle is directly affected by the host activity's lifecycle. For example, when the activity is paused, so are all fragments in it, and when the activity is destroyed, so are all fragments. However, while an activity is running (it is in the resumed lifecycle state), you can manipulate each fragment independently, such as add or remove them. When you perform such a fragment transaction, you can also add it to a back stack that's managed by the activity—each back stack entry in the activity is a record of the fragment transaction that occurred. The back stack allows the user to reverse a fragment transaction (navigate backwards), by pressing the BACK button.


When you add a fragment as a part of your activity layout, it lives in a ViewGroup inside the activity's view hierarchy and the fragment defines its own view layout. You can insert a fragment into your activity layout by declaring the fragment in the activity's layout file, as a <fragment> element, or from your application code by adding it to an existing ViewGroup. However, a fragment is not required to be a part of the activity layout; you may also use a fragment without its own UI as an invisible worker for the activity.


This document describes how to build your application to use fragments, including how fragments can maintain their state when added to the activity's back stack, share events with the activity and other fragments in the activity, contribute to the activity's action bar, and more.

这份文档描述了如何使用fragment来创建你的应用程序,包括fragment在添加到后台栈时如何维护他们的状态,如何同activity和其他同属于该activity的fragment们共享事件,构建到activity的动作槽(action bar)中去,等等。

设计原理(Design Philosophy)
Android introduced fragments in Android 3.0 (API level 11), primarily to support more dynamic and flexible UI designs on large screens, such as tablets. Because a tablet's screen is much larger than that of a handset, there's more room to combine and interchange UI components. Fragments allow such designs without the need for you to manage complex changes to the view hierarchy. By dividing the layout of an activity into fragments, you become able to modify the activity's appearance at runtime and preserve those changes in a back stack that's managed by the activity.

Android在3.0这个版本中(API Level “Honeycomb”)引入了fragment的概念,主要是支持在大屏幕上更为动态和灵活的UI设计,比方说平板电脑。由于平板电脑的屏幕要比手持电话的大许多,这样就有更多的空间去组合和交换UI组件。有了fragment,你可以不必去管理视图体系的复杂变化。通过将activity的布局分割成若干个fragment,可以在运行时编辑activity的呈现,并且那些变化会被保存在由activity管理的后台栈里面。

For example, a news application can use one fragment to show a list of articles on the left and another fragment to display an article on the right—both fragments appear in one activity, side by side, and each fragment has its own set of lifecycle callback methods and handle their own user input events. Thus, instead of using one activity to select an article and another activity to read the article, the user can select an article and read it all within the same activity, as illustrated in the tablet layout in figure 1.


You should design each fragment as a modular and reusable activity component. That is, because each fragment defines its own layout and its own behavior with its own lifecycle callbacks, you can include one fragment in multiple activities, so you should design for reuse and avoid directly manipulating one fragment from another fragment. This is especially important because a modular fragment allows you to change your fragment combinations for different screen sizes. When designing your application to support both tablets and handsets, you can reuse your fragments in different layout configurations to optimize the user experience based on the available screen space. For example, on a handset, it might be necessary to separate fragments to provide a single-pane UI when more than one cannot fit within the same activity.


Figure 1. An example of how two UI modules defined by fragments can be combined into one activity for a tablet design, but separated for a handset design.  

For example—to continue with the news application example—the application can embed two fragments in Activity A, when running on a tablet-sized device. However, on a handset-sized screen, there's not be enough room for both fragments, so Activity A includes only the fragment for the list of articles, and when the user selects an article, it starts Activity B, which includes the second fragment to read the article. Thus, the application supports both tablets and handsets by reusing fragments in different combinations, as illustrated in figure 1.

例如——还是以那个新闻应用程序为例——当程序运行在平板尺寸屏幕设备上时,可以在Activity A中嵌入两个fragment。但是,当运行在手机尺寸屏幕上,就没有足够的空间容纳两个fragment了,因此Activity A只能引用包含文章列表的fragment,在当用户选择一篇文章时,可以启动Activity B,它包含了用来阅读文章的第二个fragment。这样,应用程序通过以不同组合的复用fragments支持了平板电脑和手机,如图1所示。

For more information about designing your application with different fragment combinations for different screen configurations, see the guide to Supporting Tablets and Handsets.

创建片元(Creating a Fragment)

To create a fragment, you must create a subclass of Fragment (or an existing subclass of it). The Fragment class has code that looks a lot like an Activity. It contains callback methods similar to an activity, such as onCreate(), onStart(), onPause(), and onStop(). In fact, if you're converting an existing Android application to use fragments, you might simply move code from your activity's callback methods into the respective callback methods of your fragment.


Usually, you should implement at least the following lifecycle methods:
The system calls this when creating the fragment. Within your implementation, you should initialize essential components of the fragment that you want to retain when the fragment is paused or stopped, then resumed. 

The system calls this when it's time for the fragment to draw its user interface for the first time. To draw a UI for your fragment, you must return a View from this method that is the root of your fragment's layout. You can return null if the fragment does not provide a UI. 

The system calls this method as the first indication that the user is leaving the fragment (though it does not always mean the fragment is being destroyed). This is usually where you should commit any changes that should be persisted beyond the current user session (because the user might not come back). 

Most applications should implement at least these three methods for every fragment, but there are several other callback methods you should also use to handle various stages of the fragment lifecycle. All the lifecycle callback methods are discussed more later, in the section about Handling the Fragment Lifecycle.

There are also a few subclasses that you might want to extend, instead of the base Fragment class:

Displays a floating dialog. Using this class to create a dialog is a good alternative to using the dialog helper methods in the Activity class, because you can incorporate a fragment dialog into the back stack of fragments managed by the activity, allowing the user to return to a dismissed fragment. 

Displays a list of items that are managed by an adapter (such as a SimpleCursorAdapter), similar to ListActivity. It provides several methods for managing a list view, such as the onListItemClick() callback to handle click events. 

Displays a hierarchy of Preference objects as a list, similar to PreferenceActivity. This is useful when creating a "settings" activity for your application. 

 添加用户界面(Adding a user interface)
A fragment is usually used as part of an activity's user interface and contributes its own layout to the activity.
To provide a layout for a fragment, you must implement the onCreateView() callback method, which the Android system calls when it's time for the fragment to draw its layout. Your implementation of this method must return a View that is the root of your fragment's layout.
Note: If your fragment is a subclass of ListFragment, the default implementation returns a ListView from onCreateView(), so you don't need to implement it.
To return a layout from onCreateView(), you can inflate it from a layout resource defined in XML. To help you do so, onCreateView() provides a LayoutInflater object.

为了从onCreateView()返回一个布局,你可以从layout resource定义的XML文件inflate它。为了便于你这样做,onCreateView()提供一个LayoutInflater对象。

For example, here's a subclass of Fragment that loads a layout from the example_fragment.xml file:

public static class ExampleFragment extends Fragment {
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
return inflater.inflate(R.layout.example_fragment, container, false);

The container parameter passed to onCreateView() is the parent ViewGroup (from the activity's layout) in which your fragment layout will be inserted. The savedInstanceState parameter is a Bundle that provides data about the previous instance of the fragment, if the fragment is being resumed (restoring state is discussed more in the section about Handling the Fragment Lifecycle).
The inflate() method takes three arguments:
The resource ID of the layout you want to inflate. 
The ViewGroup to be the parent of the inflated layout. Passing the container is important in order for the system to apply layout parameters to the root view of the inflated layout, specified by the parent view in which it's going. 
A boolean indicating whether the inflated layout should be attached to the ViewGroup (the second parameter) during inflation. (In this case, this is false because the system is already inserting the inflated layout into the container—passing true would create a redundant view group in the final layout.) 

传入onCreateView()的参数container 是你的frament布局将要被插入的父ViewGroup(来自activity的布局)。如果fragment处于resumed状态(恢复状态在操纵fragment生命周期一节中将作更多讨论),参数savedInstanceState是属于Bundle类,它提供了fragment之前实例的相关数据。

Now you've seen how to create a fragment that provides a layout. Next, you need to add the fragment to your activity.

将fragment添加到activity之中(Adding a fragment to an activity)
Usually, a fragment contributes a portion of UI to the host activity, which is embedded as a part of the activity's overall view hierarchy. There are two ways you can add a fragment to the activity layout:

In this case, you can specify layout properties for the fragment as if it were a view. For example, here's the layout file for an activity with two fragments:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android=""
<fragment android:name=""
<fragment android:name=""

The android:name attribute in the <fragment> specifies the Fragment class to instantiate in the layout.
When the system creates this activity layout, it instantiates each fragment specified in the layout and calls the onCreateView() method for each one, to retrieve each fragment's layout. The system inserts the View returned by the fragment directly in place of the <fragment> element.

<fragment>中的android:name 属性指定了布局中实例化的Fragment类。
Note: Each fragment requires a unique identifier that the system can use to restore the fragment if the activity is restarted (and which you can use to capture the fragment to perform transactions, such as remove it). There are three ways to provide an ID for a fragment:
Supply the android:id attribute with a unique ID. 
Supply the android:tag attribute with a unique string. 
If you provide neither of the previous two, the system uses the ID of the container view. 


Or, programmatically add the fragment to an existing ViewGroup.

At any time while your activity is running, you can add fragments to your activity layout. You simply need to specify a ViewGroup in which to place the fragment.
To make fragment transactions in your activity (such as add, remove, or replace a fragment), you must use APIs from FragmentTransaction. You can get an instance of FragmentTransaction from your Activity like this:


FragmentManager fragmentManager = getFragmentManager()
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

You can then add a fragment using the add() method, specifying the fragment to add and the view in which to insert it. For example:

ExampleFragment fragment = new ExampleFragment();
fragmentTransaction.add(, fragment);

The first argument passed to add() is the ViewGroup in which the fragment should be placed, specified by resource ID, and the second parameter is the fragment to add.
Once you've made your changes with FragmentTransaction, you must call commit() for the changes to take effect.
传入add()函数的第一个参数是fragment被放置的ViewGroup,它由资源ID(resource ID)指定,第二个参数就是要添加的fragment。
一旦通过FragmentTransaction 做了更改,都应当使用commit()使变化生效。

添加无界面的fragment(Adding a fragment without a UI)
The examples above show how to add a fragment to your activity in order to provide a UI. However, you can also use a fragment to provide a background behavior for the activity without presenting additional UI.
To add a fragment without a UI, add the fragment from the activity using add(Fragment, String) (supplying a unique string "tag" for the fragment, rather than a view ID). This adds the fragment, but, because it's not associated with a view in the activity layout, it does not receive a call to onCreateView(). So you don't need to implement that method.
Supplying a string tag for the fragment isn't strictly for non-UI fragments—you can also supply string tags to fragments that do have a UI—but if the fragment does not have a UI, then the string tag is the only way to identify it. If you want to get the fragment from the activity later, you need to use findFragmentByTag().
For an example activity that uses a fragment as a background worker, without a UI, see the sample.
想要添加没有界面的fragment ,可以使用add(Fragment, String)(为fragment提供一个唯一的字符串“tag”,而不是视图(view)ID)。这样添加了fragment,但是,因为还没有关联到activity布局中的视图(view) ,收不到onCreateView()的调用。所以不需要实现这个方法。

管理fragment(Managing Fragments)

To manage the fragments in your activity, you need to use FragmentManager. To get it, call getFragmentManager() from your activity.
Some things that you can do with FragmentManager include:
Get fragments that exist in the activity, with findFragmentById() (for fragments that provide a UI in the activity layout) or findFragmentByTag() (for fragments that do or don't provide a UI). 
Pop fragments off the back stack, with popBackStack() (simulating a BACK command by the user). 
Register a listener for changes to the back stack, with addOnBackStackChangedListener(). 
For more information about these methods and others, refer to the FragmentManager class documentation.
As demonstrated in the previous section, you can also use FragmentManager to open a FragmentTransaction, which allows you to perform transactions, such as add and remove fragments.

使用FragmentManager 可以做如下事情,包括:
关于这些函数和其它的更多信息,可参考FragmentManager 类文档。

处理fragment事务(Performing Fragment Transactions)
A great feature about using fragments in your activity is the ability to add, remove, replace, and perform other actions with them, in response to user interaction. Each set of changes that you commit to the activity is called a transaction and you can perform one using APIs in FragmentTransaction. You can also save each transaction to a back stack managed by the activity, allowing the user to navigate backward through the fragment changes (similar to navigating backward through activities).
You can acquire an instance of FragmentTransaction from the FragmentManager like this:
在activity中使用fragment的一大特点是具有添加、删除、替换,和执行其它动作的能力,以响应用户的互动。提交给activity的每一系列变化被称为事务,并且可以用FragmentTransaction 中的APIs处理。你也可以将每一个事务保存在由activity管理的后台栈中,并且允许用户导航回退fragment变更(类似于activity的导航回退)。
你可以从 FragmentManager 中获取FragmentTransaction 实例,像这样:

FragmentManager fragmentManager = getFragmentManager();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();

Each transaction is a set of changes that you want to perform at the same time. You can set up all the changes you want to perform for a given transaction using methods such as add(), remove(), and replace(). Then, to apply the transaction to the activity, you must call commit().
Before you call commit(), however, you might want to call addToBackStack(), in order to add the transaction to a back stack of fragment transactions. This back stack is managed by the activity and allows the user to return to the previous fragment state, by pressing the BACK key.
For example, here's how you can replace one fragment with another, and preserve the previous state in the back stack:

// Create new fragment and transaction
Fragment newFragment = new ExampleFragment();
FragmentTransaction transaction = getFragmentManager().beginTransaction();

// Replace whatever is in the fragment_container view with this fragment,
// and add the transaction to the back stack
transaction.replace(, newFragment);

// Commit the transaction

In this example, newFragment replaces whatever fragment (if any) is currently in the layout container identified by the ID. By calling addToBackStack(), the replace transaction is saved to the back stack so the user can reverse the transaction and bring back the previous fragment by pressing the BACK key.
If you add multiple changes to the transaction (such as another add() or remove()) and call addToBackStack(), then all changes applied before you call commit() are added to the back stack as a single transaction and the BACK key will reverse them all together.

The order in which you add changes to a FragmentTransaction doesn't matter, except:
You must call commit() last 
If you're adding multiple fragments to the same container, then the order in which you add them determines the order they appear in the view hierarchy 
If you do not call addToBackStack() when you perform a transaction that removes a fragment, then that fragment is destroyed when the transaction is committed and the user cannot navigate back to it. Whereas, if you do call addToBackStack() when removing a fragment, then the fragment is stopped and will be resumed if the user navigates back.
Tip: For each fragment transaction, you can apply a transition animation, by calling setTransition() before you commit.
Calling commit() does not perform the transaction immediately. Rather, it schedules it to run on the activity's UI thread (the "main" thread) as soon as the thread is able to do so. If necessary, however, you may call executePendingTransactions() from your UI thread to immediately execute transactions submitted by commit(). Doing so is usually not necessary unless the transaction is a dependency for jobs in other threads.
Caution: You can commit a transaction using commit() only prior to the activity saving its state (when the user leaves the activity). If you attempt to commit after that point, an exception will be thrown. This is because the state after the commit can be lost if the activity needs to be restored. For situations in which its okay that you lose the commit, use commitAllowingStateLoss().

如果你正将多个fragment添加到同一个容器中,那么添加顺序决定了它们在视图层次(view hierarchy)里显示的顺序。


与activity通讯(Communicating with the Activity)
Although a Fragment is implemented as an object that's independent from an Activity and can be used inside multiple activities, a given instance of a fragment is directly tied to the activity that contains it.
Specifically, the fragment can access the Activity instance with getActivity() and easily perform tasks such as find a view in the activity layout:

View listView = getActivity().findViewById(;

Likewise, your activity can call methods in the fragment by acquiring a reference to the Fragment from FragmentManager, using findFragmentById() or findFragmentByTag(). For example::

ExampleFragment fragment = (ExampleFragment) getFragmentManager().findFragmentById(;

创建activity事件回调函数(Creating event callbacks to the activity)
In some cases, you might need a fragment to share events with the activity. A good way to do that is to define a callback interface inside the fragment and require that the host activity implement it. When the activity receives a callback through the interface, it can share the information with other fragments in the layout as necessary.
For example, if a news application has two fragments in an activity—one to show a list of articles (fragment A) and another to display an article (fragment B)—then fragment A must tell the activity when a list item is selected so that it can tell fragment B to display the article. In this case, the OnArticleSelectedListener interface is declared inside fragment A:

举个例子,如果新闻应用的actvity中有两个fragment——一个显示文章列表(fragment A),另一个显示一篇文章(fragment B)——然后fragment A 必须要告诉activity列表项何时被选种,这样,activity可以通知fragment B显示这篇文章。这种情况下,在fragment A内部声明接口OnArticleSelectedListener:

public static class FragmentA extends ListFragment {
// Container Activity must implement this interface
public interface OnArticleSelectedListener {
public void onArticleSelected(Uri articleUri);

Then the activity that hosts the fragment implements the OnArticleSelectedListener interface and overrides onArticleSelected() to notify fragment B of the event from fragment A. To ensure that the host activity implements this interface, fragment A's onAttach() callback method (which the system calls when adding the fragment to the activity) instantiates an instance of OnArticleSelectedListener by casting the Activity that is passed into onAttach():
然后fragment的宿主activity实现了OnArticleSelectedListener接口,并且重写onArticleSelected()以通知fragment B来自于fragment A的事件。为了确保宿主activity实现了这个接口,fragment A的onAttach()回调函数(当添加fragment到activity中时系统会调用它)通过作为参数传入onAttach()的activity的类型转换来实例化一个OnArticleSelectedListener实例。

public static class FragmentA extends ListFragment {
OnArticleSelectedListener mListener;
public void onAttach(Activity activity) {
try {
mListener = (OnArticleSelectedListener) activity;
} catch (ClassCastException e) {
throw new ClassCastException(activity.toString() + " must implement OnArticleSelectedListener");

If the activity has not implemented the interface, then the fragment throws a ClassCastException. On success, the mListener member holds a reference to activity's implementation of OnArticleSelectedListener, so that fragment A can share events with the activity by calling methods defined by the OnArticleSelectedListener interface. For example, if fragment A is an extension of ListFragment, each time the user clicks a list item, the system calls onListItemClick() in the fragment, which then calls onArticleSelected() to share the event with the activity:
如果activity没有实现这个接口,那么fragment会抛出一个ClassCaseException异常。一旦成功,mListener成员会保留一个activity的OnArticleSelectedListener实现的引用,由此fragment A可以通过调用由OnArticleSelectedListener接口定义的方法与activity共享事件。例如,如果fragment A是ListFragment的子类,每次用户点击列表项时,系统都会调用fragment的onListItemClick()事件,然后fragment调用onArticleSelected()来与activity共享事件。

public static class FragmentA extends ListFragment {
OnArticleSelectedListener mListener;
public void onListItemClick(ListView l, View v, int position, long id) {
// Append the clicked item's row ID with the content provider Uri
Uri noteUri = ContentUris.withAppendedId(ArticleColumns.CONTENT_URI, id);
// Send the event and Uri to the host activity

The id parameter passed to onListItemClick() is the row ID of the clicked item, which the activity (or other fragment) uses to fetch the article from the application's ContentProvider.
More information about using a content provider is available in the Content Providers document.
关于使用content provider的更多资料可以在在Content Providers文档获取。

添加项目到Action Bar(Adding items to the Action Bar)
Your fragments can contribute menu items to the activity's Options Menu (and, consequently, the Action Bar) by implementing onCreateOptionsMenu(). In order for this method to receive calls, however, you must call setHasOptionsMenu() during onCreate(), to indicate that the fragment would like to add items to the Options Menu (otherwise, the fragment will not receive a call to onCreateOptionsMenu()).
你的fragments可以通过实现onCreateOptionsMenu()来构建菜单项到activity的Options Menu(因此Action Bar也一样)。为了使用这个方法接收到调用,不管怎样,你必须在onCreate()期间调用setHasOptionsMenu(),来指明想要添加项目到Options Menu的那个fragment(否则,fragment将接收不到onCreateOptionsMenu()的调用)。

Any items that you then add to the Options Menu from the fragment are appended to the existing menu items. The fragment also receives callbacks to onOptionsItemSelected() when a menu item is selected.
任何想要在fragment中的Options Menu添加的项目都追加到已有的菜单项后面。当菜单项被选中时,fragment也会接收到对onOptionsItemSelected()的回调。

You can also register a view in your fragment layout to provide a context menu by calling registerForContextMenu(). When the user opens the context menu, the fragment receives a call to onCreateContextMenu(). When the user selects an item, the fragment receives a call to onContextItemSelected().
你也可以通过调用registerForContextMenu()在fragment布局中注册一个view以提供一个context menu。当用户打开context menu时,fragment接收到对onCreateContextMenu()的回调。当用户选中一个项目时,fragment接收到对onContextItemSelected()的回调。

Note: Although your fragment receives an on-item-selected callback for each menu item it adds, the activity is first to receive the respective callback when the user selects a menu item. If the activity's implementation of the on-item-selected callback does not handle the selected item, then the event is passed to the fragment's callback. This is true for the Options Menu and context menus.
注意:尽管你的fragment会接收到为添加到每个菜单项被选择菜单项的回调,但当用户选择一个菜单项时,activity会首先接收到对应的回调。如果activity的选择菜单项回调的实现没有处理被选中的项目,那么该事件被传递给fragment的回调。这同样适用于Options Menu和context menu。

For more information about menus, see Creating Menus and Using the Action Bar.
关于menu的更多信息,参考Creating Menus 和Using the Action Bar。

操纵fragment生命周期(Handling the Fragment Lifecycle)  

Managing the lifecycle of a fragment is a lot like managing the lifecycle of an activity. Like an activity, a fragment can exist in three states:
The fragment is visible in the running activity. 
Another activity is in the foreground and has focus, but the activity in which this fragment lives is still visible (the foreground activity is partially transparent or doesn't cover the entire screen). 
The fragment is not visible. Either the host activity has been stopped or the fragment has been removed from the activity but added to the back stack. A stopped fragment is still alive (all state and member information is retained by the system). However, it is no longer visible to the user and will be killed if the activity is killed. 

Also like an activity, you can retain the state of a fragment using a Bundle, in case the activity's process is killed and you need to restore the fragment state when the activity is recreated. You can save the state during the fragment's onSaveInstanceState() callback and restore it during either onCreate(), onCreateView(), or onActivityCreated(). For more information about saving state, see the Activities document.

The most significant difference in lifecycle between an activity and a fragment is how one is stored in its respective back stack. An activity is placed into a back stack of activities that's managed by the system when it's stopped, by default (so that the user can navigate back to it with the BACK key, as discussed in Tasks and Back Stack). However, a fragment is placed into a back stack managed by the host activity only when you explicitly request that the instance be saved by calling addToBackStack() during a transaction that removes the fragment.

Otherwise, managing the fragment lifecycle is very similar to managing the activity lifecycle. So, the same practices for managing the activity lifecycle also apply to fragments. What you also need to understand, though, is how the life of the activity affects the life of the fragment.

与activity生命周期的协调合作(Coordinating with the activity lifecycle)

The lifecycle of the activity in which the fragment lives directly affects the lifecycle of the fragment, such that each lifecycle callback for the activity results in a similar callback for each fragment. For example, when the activity receives onPause(), each fragment in the activity receives onPause().

Fragments have a few extra lifecycle callbacks, however, that handle unique interaction with the activity in order to perform actions such as build and destroy the fragment's UI. These additional callback methods are:
Called when the fragment has been associated with the activity (the Activity is passed in here). 
Called to create the view hierarchy associated with the fragment. 
Called when the activity's onCreate() method has returned. 
Called when the view hierarchy associated with the fragment is being removed. 
Called when the fragment is being disassociated from the activity. 


The flow of a fragment's lifecycle, as it is affected by its host activity, is illustrated by figure 3. In this figure, you can see how each successive state of the activity determines which callback methods a fragment may receive. For example, when the activity has received its onCreate() callback, a fragment in the activity receives no more than the onActivityCreated() callback.

Once the activity reaches the resumed state, you can freely add and remove fragments to the activity. Thus, only while the activity is in the resumed state can the lifecycle of a fragment change independently.

However, when the activity leaves the resumed state, the fragment again is pushed through its lifecycle by the activity.


To bring everything discussed in this document together, here's an example of an activity using two fragments to create a two-pane layout. The activity below includes one fragment to show a list of Shakespeare play titles and another to show a summary of the play when selected from the list. It also demonstrates how to provide different configurations of the fragments, based on the screen configuration.
Note: The complete source code for this activity is available in
The main activity applies a layout in the usual way, during onCreate():

protected void onCreate(Bundle savedInstanceState) {

The layout applied is fragment_layout.xml:  

<LinearLayout xmlns:android=""
="match_parent" android:layout_height="match_parent">
<fragment class="$TitlesFragment"
="@+id/titles" android:layout_weight="1"
="0px" android:layout_height="match_parent"/>
<FrameLayout android:id="@+id/details" android:layout_weight="1"
="0px" android:layout_height="match_parent"

Using this layout, the system instantiates the TitlesFragment (which lists the play titles) as soon as the activity loads the layout, while the FrameLayout (where the fragment for showing the play summary will go) consumes space on the right side of the screen, but remains empty at first. As you'll see below, it's not until the user selects an item from the list that a fragment is placed into the FrameLayout.
However, not all screen configurations are wide enough to show both the list of plays and the summary, side by side. So, the layout above is used only for the landscape screen configuration, by saving it at res/layout-land/fragment_layout.xml.


Thus, when the screen is in portrait orientation, the system applies the following layout, which is saved atres/layout/fragment_layout.xml:

<FrameLayout xmlns:android=""
="match_parent" android:layout_height="match_parent">
<fragment class="$TitlesFragment"
="match_parent" android:layout_height="match_parent"/>

This layout includes only TitlesFragment. This means that, when the device is in portrait orientation, only the list of play titles is visible. So, when the user clicks a list item in this configuration, the application will start a new activity to show the summary, instead of loading a second fragment.

Next, you can see how this is accomplished in the fragment classes. First is TitlesFragment, which shows the list of Shakespeare play titles. This fragment extends ListFragment and relies on it to handle most of the list view work.
As you inspect this code, notice that there are two possible behaviors when the user clicks a list item: depending on which of the two layouts is active, it can either create and display a new fragment to show the details in the same activity (adding the fragment to theFrameLayout), or start a new activity (where the fragment can be shown).


public static class TitlesFragment extends ListFragment {
boolean mDualPane;
int mCurCheckPosition = 0;
public void onActivityCreated(Bundle savedInstanceState) {
// Populate list with our static array of titles.
setListAdapter(new ArrayAdapter<String>(getActivity(),
android.R.layout.simple_list_item_activated_1, Shakespeare.TITLES));
// Check to see if we have a frame in which to embed the details
// fragment directly in the containing UI.
View detailsFrame = getActivity().findViewById(;
mDualPane = detailsFrame != null && detailsFrame.getVisibility() == View.VISIBLE;
if (savedInstanceState != null) {
// Restore last state for checked position.
mCurCheckPosition = savedInstanceState.getInt("curChoice", 0);
if (mDualPane) {
// In dual-pane mode, the list view highlights the selected item.
// Make sure our UI is in the correct state.
public void onSaveInstanceState(Bundle outState) {
outState.putInt("curChoice", mCurCheckPosition);
public void onListItemClick(ListView l, View v, int position, long id) {
* Helper function to show the details of a selected item, either by
* displaying a fragment in-place in the current UI, or starting a
* whole new activity in which it is displayed.
void showDetails(int index) {
mCurCheckPosition = index;
if (mDualPane) {
// We can display everything in-place with fragments, so update
// the list to highlight the selected item and show the data.
getListView().setItemChecked(index, true);
// Check what fragment is currently shown, replace if needed.
DetailsFragment details = (DetailsFragment)
if (details == null || details.getShownIndex() != index) {
// Make new fragment to show this selection.
details = DetailsFragment.newInstance(index);
// Execute a transaction, replacing any existing fragment
// with this one inside the frame.
FragmentTransaction ft = getFragmentManager().beginTransaction();
ft.replace(, details);
} else {
// Otherwise we need to launch a new activity to display
// the dialog fragment with selected text.
Intent intent = new Intent();
intent.setClass(getActivity(), DetailsActivity.class);
intent.putExtra("index", index);

The second fragment, DetailsFragment shows the play summary for the item selected from the list from TitlesFragment:

public static class DetailsFragment extends Fragment {
* Create a new instance of DetailsFragment, initialized to
* show the text at 'index'.
public static DetailsFragment newInstance(int index) {
DetailsFragment f = new DetailsFragment();
// Supply index input as an argument.
Bundle args = new Bundle();
args.putInt("index", index);
return f;
public int getShownIndex() {
return getArguments().getInt("index", 0);
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
if (container == null) {
// We have different layouts, and in one of them this
// fragment's containing frame doesn't exist. The fragment
// may still be created from its saved state, but there is
// no reason to try to create its view hierarchy because it
// won't be displayed. Note this is not needed -- we could
// just run the code below, where we would create and return
// the view hierarchy; it would just never be used.
return null;
ScrollView scroller = new ScrollView(getActivity());
TextView text = new TextView(getActivity());
int padding = (int)TypedValue.applyDimension(TypedValue.COMPLEX_UNIT_DIP,
4, getActivity().getResources().getDisplayMetrics());
text.setPadding(padding, padding, padding, padding);
return scroller;

Recall from the TitlesFragment class, that, if the user clicks a list item and the current layout does not include the view (which is where the DetailsFragment belongs), then the application starts the DetailsActivity activity to display the content of the item.

Here is the DetailsActivity, which simply embeds theDetailsFragment to display the selected play summary when the screen is in portrait orientation:


public static class DetailsActivity extends Activity {
protected void onCreate(Bundle savedInstanceState) {
if (getResources().getConfiguration().orientation
// If the screen is now in landscape mode, we can show the
// dialog in-line with the list so we don't need this activity.
if (savedInstanceState == null) {
// During initial setup, plug in the details fragment.
DetailsFragment details = new DetailsFragment();
getFragmentManager().beginTransaction().add(, details).commit();

Notice that this activity finishes itself if the configuration is landscape, so that the main activity can take over and display the DetailsFragment alongside the TitlesFragment. This can happen if the user begins the DetailsActivity while in portrait orientation, but then rotates to landscape (which restarts the current activity).

For more samples using fragments (and complete source files for this example), see the sample code available in ApiDemos (available for download from the Samples SDK component).


更多的关于使用fragment的例子(还有这个例子的完整源码),请参阅ApiDemos中的示例代码(从 Samples SDK component中下载可用的)。

posted on 2012-08-13 16:04  lee0oo0  阅读(430)  评论(0编辑  收藏  举报