Android callback from fragment to fragment

Android Fragments Common Queries & Common Mistakes

Fragment class in Android is used to build dynamic User Interfaces. Fragment should be used within the Activity. A greatest advantage of fragments is that it simplifies the task of creating UI for multiple screen sizes. A activity can contain any number of fragments.

Now this meaning of fragment sounds good and easy, right? But there is lot more involved, this article covers main needs and common mistakes while using Fragments.

I am assuming you are having basic knowledge of Fragment and Fragment lifecycle callbacks also I am assuming you know how implement communication between two fragments this article goes beyond that

So here are a few obstacles related to fragments some of you must have faced already, some of you might face later.

• FragmentManager: getSupportFragmentManager and getChildFragmentManager. Which one to use when and avoid memory leaks while using them.
• Callback from DialogFragment, ChildFragment, BottomSheetFragment to parent fragment.
• Fragments when using ViewPager and when to use FragmentStateAdapter vs FragmentPagerAdapter.
• When to use FragmentTransaction add vs replace ?
• Fragment receivers, broadcasts and memory leaks.
• Fragment BottomBarNavigation and drawer. How to handle these?
• commit() and commitAllowingStateLoss()
• Fragment option menus.
• Fragment getActivity(), getView() and NullPointers Exceptions.
• onActivityResult with nested fragments.
• Fragment and Bundle.
• Back Navigation.

Whoa . see its a big list, reply in comment if anyone wish to add something more to the list.

getSupportFragmentManager and getChildFragmentManager

FragmentManager is class provided by the framework which is used to create transactions for adding, removing or replacing fragments.

• getSupportFragmentManager is associated with Activity consider it as a FragmentManager for your Activity .

So whenever you are using ViewPager, BottomSheetFragment and DialogFragment in an Activity you will use getSupportFragmentManager

• getChildFragmentManager is associated with fragment.

Whenever you are ViewPager inside Fragment you will use getChildFragmentManager

Here is official link for this for better understanding.

Now coming to common mistakes people do when they are using ViewPager inside a Fragment they pass getSupportFragmentManager which is fragment manager for Activity and it causes issues as such memory leaks, ViewPager not updated properly sometimes etc.

Most important issue caused by using getSupportFragmentManager in Fragment is memory leak, let me tell you how? Your Fragment has stack of fragments which is used by ViewPager or any other thing and all these fragments stack is in activity since you used getSupportFragmentManager , now if close your Parent fragment it will be closed but it will not be destroyed because all child Fragments are in activity and they are still in memory which does not allow to destroy Parent Fragment hence causing leak. It will not just leak parent fragment but also leak all child fragments since none of them can be cleared from heap memory. So never try to use getSupportFragmentManager in a Fragment

Callback from DialogFragment, ChildFragment, BottomSheetFragment to parent fragment

This is very common issue people face when they use BottomSheetFragment or DialogFragment or ChildFragment.

Add a child fragment

Another example bottomSheetFragment

Now suppose you want callback from these child fragments to parent fragments . Most of people create connection between two fragments using activity, few people pass interface listeners as a parameter to fragment which really a bad practice and one should avoid this. Best way calling getParentFragment() from your child fragment to create a call back this is very simple way consider example below.

then setting callback to parent fragment add following code in child fragment.

thats it you can give a callback to your parent fragment now easily.

Using same way you can create a callback from child fragment inside ViewPager to parent fragment who is holding ViewPager.

Fragments when using ViewPager and adapters FragmentStateAdapter and FragmentPagerAdapter which one one to use when

FragmentPagerAdapter stores the whole fragment in memory, and could increase a memory overhead if a large amount of fragments are used in ViewPager . FragmentStatePagerAdapter only stores the savedInstanceState of fragments, and destroys all the fragments when they lose focus.

So when your is going to have many Fragments use FragmentStateAdapter if ViewPager is going to have less than three fragments use FragmentPagerAdapter.

Commonly faced issues

Update ViewPager not working:

People always come across the issue remember ViewPager fragments are managed by FragmentManager either from Fragment or Activity and this FragmentManager holds instance of all ViewPager Fragments.

So when people say ViewPager is not refreshed it’s nothing but old instances of fragments are still being hold by FragmentManager. So you need to find out why FragmentManger is holding instance of Fragments is there any leak or not ideally to refresh ViewPager following code works if it is not you are doing something wrong

Access current Fragment from ViewPager:

This is also very common issue we come across. For this people either create array list of fragments inside adapter or try to access fragment using some tags according to me both methods are not reliable. FragmentStateAdapter and FragmentPagerAdapter both provides method setPrimaryItem this can be used to set current fragment as below.

I am leaving a Github link below to this simple ViewPager project so that everyone can understand better.

amodkanthe/ViewPagerTest

Contribute to amodkanthe/ViewPagerTest development by creating an account on GitHub.

FragmentTransaction add vs replace

In our Activity we have a container and inside this container we display our Fragments

add will simply add fragment to container suppose you add FragmentA and FragmentB to container. Container will have FragmentA and FragmentB both and suppose if container is FrameLayout fragments are added one above the other. replace will simply replace a fragment on top of container so if I call create FragmentC now and call replace FragmentB which was on top will removed from container unless you are not calling addToBackStack and now FragmentC will be on top.

So which one to use when. replace removes the existing fragment and adds a new fragment . This means when you press back button the fragment that got replaced will be created with its onCreateView being invoked. Whereas add retains the existing fragments and adds a new fragment that means existing fragment will be active and they wont be in ‘paused’ state hence when a back button is pressed onCreateView is not called for the existing fragment(the fragment which was there before new fragment was added). In terms of fragment’s life cycle events onPause, onResume, onCreateView and other life cycle events will be invoked in case of replace but they wont be invoked in case of add .

Use replace fragment if don’t need to revisit current fragment and current fragment is not require anymore. Also if your app has memory constraints use replace instead of add.

Fragment receivers, broadcasts and memory leaks

Mistakes people commonly do when using receivers inside a fragment forgot to unregister receiver in onPause or OnDestroy. If you are registering fragment to listen to receiver inside onCreate or OnResume you will have to unregister it inside onPause or onDestroy otherwise it will cause memory leak

Also if have multiple fragments listening to same broadcast receiver make sure you register in onResume and unregister in onPause. Because if you use onCreate and onDestroy for register and unregister other fragments will not receive the broadcast as this fragment is not destroyed

Fragment BottomBarNavigation and drawer how to handle these

When we are using BottomBarNavigation and NavigationDrawer people face issues like fragments recreated, same fragment is added multiple times etc.

So in such case you can use fragment transaction show and hide instead of add or replace.

There is also one beautiful library which take care of navigations and avoid recreation of fragments called FragNav below is link to it

Источник

Раскладываем на части FragmentLifecycleCallbacks

Привет! Сегодня я продолжаю рассказывать про инструменты, которые почему-то обделили вниманием. В своей предыдущей статье я написал про возможности ActivityLifecycleCallbacks и как их можно применять не только для логирования жизненного цикла. Но кроме Activity есть еще и Fragment, и нам хотелось получить для них подобное поведение.

Не долго думая, я открыл поиск по классам в AndroidStudio (Cmd/Ctrl + O) и ввел туда FragmentLifecycleCallbacks. И каково же было мое удивление, когда поиск показал мне FragmentManager.FragmentLifecycleCallbacks. Самые нетерпеливые читатели писали про это в комментариях, поэтому вот продолжение всей этой истории. Скорее под кат!

Что это такое

Интерфейс наподобие ActivityLifecycleCallbacks, только для Fragment.

В отличие от ActivityLifecycleCallbacks он управляется не самим Fragment, а FragmentManager, что дает ряд преимуществ. Например, у этого интерфейса методы с приставкой Pre-, которые вызываются до соответствующих методов Fragment. А методы без приставки вызываются после того, как сработают эти же методы Fragment.

К тому же FragmentLifecycleCallbacks — это абстрактный класс, а не интерфейс. Думаю, что это для того, чтобы у методов была реализация по умолчанию.

Но перейдем к интересному — как это запустить.

Как зарегистрировать

Чтобы заставить FragmentLifecycleCallbacks работать, его нужно зарегистрировать на FragmentManager. Для этого надо вызвать FragmentManager.registerFragmentLifecycleCallback(), передав в него два параметра: сам callback и флаг — recursive. Флаг показывает, нужно ли применить этот callback только к этому FragmentManager или его надо рекурсивно прокидывать во все childFragmentManager’ы, этого FragmentManager’а и дальше по иерархии.

FragmentLifecycleCallback стоит регистрировать до Activity.onCreate(), иначе мы можем получить не все события, например, при восстановлении состояния.

Выглядит не очень красиво, и в некоторых ситуациях потребует заводить что-то вроде базовой Activity. Но если ты уже прочитал мою статью про ActivityLifecycleCallbacks, то знаешь, на что базовые Activity отлично заменяются =).

И тут мы получаем потрясающую синергию callback’ов. Благодаря этому решению мы теперь можем дотянуться почти до любого объекта Activity и Fragment, создаваемых в системе. И теперь, когда мы видим все как на ладони, можно заставить всю эту систему работать на нас.

Примеры использования

Сразу про dependency injection: да, теперь можно распространять зависимости по всему приложению, даже если у вас Single Activity Application. Помнишь пример из предыдущей статьи, про RequireCoolTool? То же самое можно сделать для всех Activity и Fragment в приложении. И ты уже догадался как, да? Но я все-равно покажу пример.

И конечно же с Dagger’ом все тоже идеально работает.

Я думаю, что ты вполне справишься с другими DI-фреймворками, но если не получится, то давай обсудим это в комментариях.

Конечно, можно делать все то же самое, что и с Activity, например, отправлять аналитику.

Источник

Fragments

In this document

Key classes

See also

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 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.

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.

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 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.

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

Creating a Fragment

Figure 2. The lifecycle of a fragment (while its activity is running).

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:

onCreate() 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. onCreateView() 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. onPause() 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 in more detail 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:

DialogFragment 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. ListFragment 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. PreferenceFragment Displays a hierarchy of Preference objects as a list, similar to PreferenceActivity . This is useful when creating a «settings» activity for your application.

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.

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

Creating a layout

In the sample above, R.layout.example_fragment is a reference to a layout resource named example_fragment.xml saved in the application resources. For information about how to create a layout in XML, see the User Interface documentation.

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.)

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

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:

Declare the fragment inside the activity’s layout file.

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:

The android:name attribute in the 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 element.

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.

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:

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:

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.

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 FragmentRetainInstance.java sample, which is included in the SDK samples (available through the Android SDK Manager) and located on your system as /APIDemos/app/src/main/java/com/example/android/apis/app/FragmentRetainInstance.java .

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.

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:

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 button.

For example, here’s how you can replace one fragment with another, and preserve the previous state in the back stack:

In this example, newFragment replaces whatever fragment (if any) is currently in the layout container identified by the R.id.fragment_container 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 button.

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 button 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() .

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:

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

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:

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() :

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:

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.

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() ).

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.

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() .

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.

For more information about menus, see the Menus and Action Bar developer guides.

Handling the Fragment Lifecycle

Figure 3. The effect of the activity lifecycle on 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:

Resumed The fragment is visible in the running activity. Paused 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). Stopped 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 button, 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.

Caution: If you need a Context object within your Fragment , you can call getActivity() . However, be careful to call getActivity() only when the fragment is attached to an activity. When the fragment is not yet attached, or was detached during the end of its lifecycle, getActivity() will return null.

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:

onAttach() Called when the fragment has been associated with the activity (the Activity is passed in here). onCreateView() Called to create the view hierarchy associated with the fragment. onActivityCreated() Called when the activity’s onCreate() method has returned. onDestroyView() Called when the view hierarchy associated with the fragment is being removed. onDetach() 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.

Example

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 FragmentLayout.java .

The main activity applies a layout in the usual way, during onCreate() :

The layout applied is fragment_layout.xml :

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 at res/layout/fragment_layout.xml :

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 the FrameLayout ), or start a new activity (where the fragment can be shown).

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

Recall from the TitlesFragment class, that, if the user clicks a list item and the current layout does not include the R.id.details 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 the DetailsFragment to display the selected play summary when the screen is in portrait orientation:

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 API Demos sample app available in ApiDemos (available for download from the Samples SDK component).

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