Fragment manager

FragmentManager is the class responsible for performing actions on your app's fragments, such as adding, removing, or replacing them, and adding them to the back stack.

You might never interact with FragmentManager directly if you're using the Jetpack Navigation library, as it works with the FragmentManager on your behalf. That said, any app using fragments is using FragmentManager at some level, so it's important to understand what it is and how it works.

This topic covers how to access the FragmentManager, the role of the FragmentManager in relation to your activities and fragments, managing the back stack with FragmentManager, and providing data and dependencies to your fragments.

Access the FragmentManager

Accessing in an activity

Every FragmentActivity and subclasses thereof, such as AppCompatActivity, have access to the FragmentManager through the getSupportFragmentManager() method.

Accessing in a Fragment

Fragments are also capable of hosting one or more child fragments. Inside a fragment, you can get a reference to the FragmentManager that manages the fragment's children through getChildFragmentManager(). If you need to access its host FragmentManager, you can use getParentFragmentManager().

Let's look at a couple of examples to see the relationships between fragments, their hosts, and the FragmentManager instances associated with each.

two ui layout examples showing the relationships between
            fragments and their host activities
Figure 1. Two UI layout examples showing the relationships between fragments and their host activities.

Figure 1 shows two examples, each of which has a single activity host. The host activity in both of these examples displays top-level navigation to the user as a BottomNavigationView that is responsible for swapping out the host fragment with different screens in the app, with each screen implemented as a separate fragment.

The host fragment in Example 1, hosts two child fragments that make up a split-view screen. The host fragment in Example 2, hosts a single child fragment that makes up the display fragment of a swipe view.

Given this setup, you can think about each host as having a FragmentManager associated with it which manages its child fragments. This is illustrated in the figure 2, along with property mappings between supportFragmentManager, parentFragmentManager and childFragmentManager.

each host has its own FragmentManager associated with it
            that manages its child fragments
Figure 2. Each host has its own FragmentManager associated with it that manages its child fragments.

The appropriate FragmentManager property to reference depends on where the callsite is in the fragment hierarchy along with which fragment manager you are trying to access.

Once you have a reference to the FragmentManager, you can use it to manipulate the fragments being displayed to the user.

Child fragments

Generally speaking, your app should consist of a single or small number of activities in your application project, with each activity representing a group of related screens. The activity may provide a point to place top-level navigation and a place to scope ViewModels and other view-state between fragments. Each individual destination in your app should be represented by a fragment.

If you want to show multiple fragments at once, such as in a split-view or a dashboard, you should use child fragments that are managed by your destination fragment and its child fragment manager.

Other use cases for child fragments may include the following:

  • Screen slides, with a ViewPager2 in a parent fragment to manage a series of child fragment views.
  • Sub-navigation within a set of related screens.
  • Jetpack Navigation uses child fragments as individual destinations. An activity hosts a single parent NavHostFragment and fills its space with different child destination fragments as users navigate through your app.

Using the FragmentManager

The FragmentManager manages the fragment back stack. At runtime, the FragmentManager can perform back stack operations like adding or removing fragments in response to user interactions. Each set of changes are committed together as a single unit called a FragmentTransaction. For a more in-depth discussion about fragment transactions, see the fragment transactions guide.

When the user presses the Back button on their device, or when you call FragmentManager.popBackStack(), the top-most fragment transaction is popped off of the stack. In other words, the transaction is reversed. If there are no more fragment transactions on the stack, and if you aren't using child fragments, the back event bubbles up to the activity. If you are using child fragments, see special considerations for child and sibling fragments.

When you call addToBackStack() on a transaction, note that the transaction can include any number of operations, such as adding multiple fragments, replacing fragments in multiple containers, and so on. When the back stack is popped, all of these operations are reversed as a single atomic action. If you've committed additional transactions prior to the popBackStack() call, and if you did not use addToBackStack() for the transaction, these operations are not reversed. Therefore, within a single FragmentTransaction, avoid interleaving transactions that affect the back stack with those that do not.

Perform a transaction

To display a fragment within a layout container, use the FragmentManager to create a FragmentTransaction. Within the transaction, you can then perform an add() or replace() operation on the container.

For example, a simple FragmentTransaction might look like this:

Kotlin

supportFragmentManager.commit {
   replace<ExampleFragment>(R.id.fragment_container)
   setReorderingAllowed(true)
   addToBackStack("name") // name can be null
}

Java

FragmentManager fragmentManager = getSupportFragmentManager();
fragmentManager.beginTransaction()
    .replace(R.id.fragment_container, ExampleFragment.class, null)
    .setReorderingAllowed(true)
    .addToBackStack("name") // name can be null
    .commit();

In this example, ExampleFragment replaces the fragment, if any, that is currently in the layout container identified by the R.id.fragment_container ID. Providing the fragment's class to the replace() method allows the FragmentManager to handle instantiation using its FragmentFactory. For more information, see Providing dependencies.

setReorderingAllowed(true) optimizes the state changes of the fragments involved in the transaction so that animations and transitions work correctly. For more information on navigating with animations and transitions, see Fragment transactions and Navigate between fragments using animations.

Calling addToBackStack() commits the transaction to the back stack. The user can later reverse the transaction and bring back the previous fragment by pressing the Back button. If you added or removed multiple fragments within a single transaction, all of those operations are undone when the back stack is popped. The optional name provided in the addToBackStack() call gives you the ability to pop back to that specific transaction using popBackStack().

If you don't call addToBackStack() when you perform a transaction that removes a fragment, then the removed fragment is destroyed when the transaction is committed, and the user cannot navigate back to it. If you do call addToBackStack() when removing a fragment, then the fragment is only STOPPED and is later RESUMED when the user navigates back. Note that its view is destroyed in this case. For more information, see Fragment lifecycle.

Finding an existing fragment

You can get a reference to the current fragment within a layout container by using findFragmentById(). Use findFragmentById() to look up a fragment either by the given ID when inflated from XML or by the container ID when added in a FragmentTransaction. Here's an example:

Kotlin

supportFragmentManager.commit {
   replace<ExampleFragment>(R.id.fragment_container)
   setReorderingAllowed(true)
   addToBackStack(null)
}

...

val fragment: ExampleFragment =
        supportFragmentManager.findFragmentById(R.id.fragment_container) as ExampleFragment

Java

FragmentManager fragmentManager = getSupportFragmentManager();
fragmentManager.beginTransaction()
    .replace(R.id.fragment_container, ExampleFragment.class, null)
    .setReorderingAllowed(true)
    .addToBackStack(null)
    .commit();

...

ExampleFragment fragment =
        (ExampleFragment) fragmentManager.findFragmentById(R.id.fragment_container);

Alternatively, you can assign a unique tag to a fragment and get a reference using findFragmentByTag(). You can assign a tag using the android:tag XML attribute on fragments that are defined within your layout, or during an add() or replace() operation within a FragmentTransaction.

Kotlin

supportFragmentManager.commit {
   replace<ExampleFragment>(R.id.fragment_container, "tag")
   setReorderingAllowed(true)
   addToBackStack(null)
}

...

val fragment: ExampleFragment =
        supportFragmentManager.findFragmentByTag("tag") as ExampleFragment

Java

FragmentManager fragmentManager = getSupportFragmentManager();
fragmentManager.beginTransaction()
    .replace(R.id.fragment_container, ExampleFragment.class, null, "tag")
    .setReorderingAllowed(true)
    .addToBackStack(null)
    .commit();

...

ExampleFragment fragment = (ExampleFragment) fragmentManager.findFragmentByTag("tag");

Special considerations for child and sibling fragments

Only one FragmentManager is allowed to control the fragment back stack at any given time. If your app shows multiple sibling fragments on the screen at the same time, or if your app uses child fragments, then one FragmentManager must be designated to handle your app's primary navigation.

To define the primary navigation fragment inside of a fragment transaction, call the setPrimaryNavigationFragment() method on the transaction, passing in the instance of the fragment whose childFragmentManager should have primary control.

Consider the navigation structure as a series of layers, with the activity as the outermost layer, wrapping each layer of child fragments underneath. Each layer must have a single primary navigation fragment. When the Back event occurs, the innermost layer controls navigation behavior. Once the innermost layer has no more fragment transactions from which to pop back, control returns to the next layer out, and this process repeats until you reach the activity.

Note that when two or more fragments are displayed at the same time, only one of them can be the primary navigation fragment. Setting a fragment as the primary navigation fragment removes the designation from the previous fragment. Using the previous example, if you set the detail fragment as the primary navigation fragment, the main fragment's designation is removed.

Support multiple back stacks

In some cases, your app might need to support multiple back stacks. A common example is if your app uses a bottom navigation bar. FragmentManager allows you to support multiple back stacks with the saveBackStack() and restoreBackStack() methods. These methods allow you to swap between back stacks by saving one back stack and restoring a different one.

saveBackStack() works similarly to calling popBackStack() with the optional name parameter: the specified transaction and all transactions after it on the stack are popped. The difference is that saveBackStack() saves the state of all fragments in the popped transactions.

For example, suppose you previously added a fragment to the back stack by committing a FragmentTransaction using addToBackStack():

Kotlin

supportFragmentManager.commit {
  replace<ExampleFragment>(R.id.fragment_container)
  setReorderingAllowed(true)
  addToBackStack("replacement")
}

Java

supportFragmentManager.beginTransaction()
  .replace(R.id.fragment_container, ExampleFragment.class, null)
  // setReorderingAllowed(true) and the optional string argument for
  // addToBackStack() are both required if you want to use saveBackStack().
  .setReorderingAllowed(true)
  .addToBackStack("replacement")
  .commit();

In that case, you can save this fragment transaction and the state of ExampleFragment by calling saveState():

Kotlin

supportFragmentManager.saveBackStack("replacement")

Java

supportFragmentManager.saveBackStack("replacement");

You can call restoreBackStack() with the same name parameter to restore all of the popped transactions and all of the saved fragment states:

Kotlin

supportFragmentManager.restoreBackStack("replacement")

Java

supportFragmentManager.restoreBackStack("replacement");

Provide dependencies to your fragments

When adding a fragment, you can instantiate the fragment manually and add it to the FragmentTransaction.

Kotlin

fragmentManager.commit {
    // Instantiate a new instance before adding
    val myFragment = ExampleFragment()
    add(R.id.fragment_view_container, myFragment)
    setReorderingAllowed(true)
}

Java

// Instantiate a new instance before adding
ExampleFragment myFragment = new ExampleFragment();
fragmentManager.beginTransaction()
    .add(R.id.fragment_view_container, myFragment)
    .setReorderingAllowed(true)
    .commit();

When you commit the fragment transaction, the instance of the fragment you created is the instance used. However, during a configuration change, your activity and all of its fragments are destroyed and then recreated with the most applicable Android resources. The FragmentManager handles all of this for you. It recreates instances of your fragments, attaches them to the host, and recreates the back stack state.

By default, the FragmentManager uses a FragmentFactory that the framework provides to instantiate a new instance of your fragment. This default factory uses reflection to find and invoke a no-argument constructor for your fragment. This means that you can't use this default factory to provide dependencies to your fragment. It also means that any custom constructor you used to create your fragment the first time is not used during recreation by default.

To provide dependencies to your fragment, or to use any custom constructor, you must instead create a custom FragmentFactory subclass and then override FragmentFactory.instantiate. You can then override the FragmentManager's default factory with your custom factory, which is then used to instantiate your fragments.

Suppose you have a DessertsFragment that is responsible for displaying popular desserts in your hometown. Let's assume that DessertsFragment has a dependency on a DessertsRepository class that provides it with the information it needs to display the correct UI to your user.

You might define your DessertsFragment to require a DessertsRepository instance in its constructor.

Kotlin

class DessertsFragment(val dessertsRepository: DessertsRepository) : Fragment() {
    ...
}

Java

public class DessertsFragment extends Fragment {
    private DessertsRepository dessertsRepository;

    public DessertsFragment(DessertsRepository dessertsRepository) {
        super();
        this.dessertsRepository = dessertsRepository;
    }

    // Getter omitted.

    ...
}

A simple implementation of your FragmentFactory might look similar to the following.

Kotlin

class MyFragmentFactory(val repository: DessertsRepository) : FragmentFactory() {
    override fun instantiate(classLoader: ClassLoader, className: String): Fragment =
            when (loadFragmentClass(classLoader, className)) {
                DessertsFragment::class.java -> DessertsFragment(repository)
                else -> super.instantiate(classLoader, className)
            }
}

Java

public class MyFragmentFactory extends FragmentFactory {
    private DessertsRepository repository;

    public MyFragmentFactory(DessertsRepository repository) {
        super();
        this.repository = repository;
    }

    @NonNull
    @Override
    public Fragment instantiate(@NonNull ClassLoader classLoader, @NonNull String className) {
        Class<? extends Fragment> fragmentClass = loadFragmentClass(classLoader, className);
        if (fragmentClass == DessertsFragment.class) {
            return new DessertsFragment(repository);
        } else {
            return super.instantiate(classLoader, className);
        }
    }
}

This example subclasses FragmentFactory, overriding the instantiate() method to provide custom fragment creation logic for a DessertsFragment. Other fragment classes are handled by the default behavior of FragmentFactory through super.instantiate().

You can then designate MyFragmentFactory as the factory to use when constructing your app's fragments by setting a property on the FragmentManager. You must set this property prior to your activity's super.onCreate() to ensure that MyFragmentFactory is used when recreating your fragments.

Kotlin

class MealActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        supportFragmentManager.fragmentFactory = MyFragmentFactory(DessertsRepository.getInstance())
        super.onCreate(savedInstanceState)
    }
}

Java

public class MealActivity extends AppCompatActivity {
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        DessertsRepository repository = DessertsRepository.getInstance();
        getSupportFragmentManager().setFragmentFactory(new MyFragmentFactory(repository));
        super.onCreate(savedInstanceState);
    }
}

Note that setting the FragmentFactory in the activity overrides fragment creation throughout the activity's fragments hierarchy. In other words, the childFragmentManager of any child fragments you add uses the custom fragment factory set here unless overridden at a lower level.

Testing with FragmentFactory

In a single activity architecture, you should test your fragments in isolation using the FragmentScenario class. Since you cannot rely on the custom onCreate logic of your activity, you can instead pass the FragmentFactory in as an argument to your fragments test, as shown in the following example:

// Inside your test
val dessertRepository = mock(DessertsRepository::class.java)
launchFragment<DessertsFragment>(factory = MyFragmentFactory(dessertRepository)).onFragment {
    // Test Fragment logic
}

For detailed information about this testing process and for full examples, see Test your app's Fragments.