Android destroy all fragments

MichaЕ‚ ЕЃuszczuk

Personal thoughts about code, Android and more

Android Fragments restoration mechanism

In my last post, where I described a problem of incorrect usage of Fragments instantiation inside FragmentPageAdapter & ViewPager , I wrote:

After orientation change all fragments currently added to FragmentManager are automatically restored and instantiated so there is no need to create them once again.

Today, I want to focus more specifically on an issue how this automatic restoration works under the hood.

Beginning

The simplest way to use a fragment:

A custom fragment instance is created and added to FragmentManager with help of FragmentTransaction .
We could add that our Fragment will be added to container identified by android.R.id.content .

This is the easiest case. I want to point out that we are not using setRetainInstance(true) inside our custom fragment implementation, and our activity is not protected in manifest agains any type of configuration changes.

The Question

Now, what will happen with our fragment if suddenly our device configuration will change, i.e. orientation?

Android Source Code (especially FragmentActivity and FragmentManager / FragmentManagerImpl ) is a place where we should look for the answer.

1. Saving the state

Before Activity will be destroyed its onSaveInstanceState method will be called. Take a look what this method is doing internally.

It takes mFragments (reference to FragmentManager held in this Activity) and calls FragmentManager.saveAllState() method which will return a parcelable object ready to be saved inside the bundle which, you can already guess… later will be used to restore Fragments.

In reality result of saveAllState method call is an object of type FragmentManagerState which consists of information about all active fragments and back stack entries

Last quick look at FragmentState .

It consists of all data which describes a specific fragment object instance, like container id, tag, arguments but also savedFragmentState.

It looks sufficient to create fragments from scratch, and set them like they were before.

2. Destroying fragments and activities.

After state of fragments is saved via FragmentManager activity object is destroyed (removed from memory) with all its fragments (those which are not retained with setRetainInstance(true) ).

3. Creating activity and recreating fragments

Final point is a recreation of the activity and recreation of the fragments. It starts within the first Activity lifecycle callback method onCreate(Bundle savedInstanceState)

The previously saved fragment manager state bundle now is used inside FragmentManager.restoreAllState method. This metod declaration is quite long but I want to focus on the most important part.

Array with all FragmentState objects is iterated. And every FragmentState object is used to recreate (create new instance with state like before) specific Fragments.

A new instance of a fragment is created by platform with usage of reflection and default constructor – that’s why you must remember to always ensure the existence of public non-argument fragment constructor and initialize your fragment through arguments bundle (not with usage of fragment object setters from strange places).

This is short story how restoration of Fragments works.

Conclusion

Points to remember:

• Already created fragments are restored automatically after orientation change
• Magic behind this is just code written in Activity together with FragmentManager logic/implementation
• Avoid setter methods and parametrized constructors to modify fragment state, because platform uses only default (0 parameter) constructor, arguments bundle and saved state bundle to restore it later
• Fragments are not so bad

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Fragment lifecycle

Each Fragment instance has its own lifecycle. When a user navigates and interacts with your app, your fragments transition through various states in their lifecycle as they are added, removed, and enter or exit the screen.

To manage lifecycle, Fragment implements LifecycleOwner , exposing a Lifecycle object that you can access through the getLifecycle() method.

Each possible Lifecycle state is represented in the Lifecycle.State enum.

By building Fragment on top of Lifecycle , you can use the techniques and classes available for Handling Lifecycles with Lifecycle-Aware Components. For example, you might display the device’s location on the screen using a lifecycle-aware component. This component could automatically start listening when the fragment becomes active and stop when the fragment moves to an inactive state.

As an alternative to using a LifecycleObserver , the Fragment class includes callback methods that correspond to each of the changes in a fragment’s lifecycle. These include onCreate() , onStart() , onResume() , onPause() , onStop() , and onDestroy() .

A fragment’s view has a separate Lifecycle that is managed independently from that of the fragment’s Lifecycle . Fragments maintain a LifecycleOwner for their view, which can be accessed using getViewLifecycleOwner() or getViewLifecycleOwnerLiveData() . Having access to the view’s Lifecycle is useful for situations where a Lifecycle-aware component should only perform work while a fragment’s view exists, such as observing LiveData that is only meant to be displayed on the screen.

This topic discusses the Fragment lifecycle in detail, explaining some of the rules that determine a fragment’s lifecycle state and showing the relationship between the Lifecycle states and the fragment lifecycle callbacks.

Fragments and the fragment manager

When a fragment is instantiated, it begins in the INITIALIZED state. For a fragment to transition through the rest of its lifecycle, it must be added to a FragmentManager . The FragmentManager is responsible for determining what state its fragment should be in and then moving them into that state.

Beyond the fragment lifecycle, FragmentManager is also responsible for attaching fragments to their host activity and detaching them when the fragment is no longer in use. The Fragment class has two callback methods, onAttach() and onDetach() , that you can override to perform work when either of these events occur.

The onAttach() callback is invoked when the fragment has been added to a FragmentManager and is attached to its host activity. At this point, the fragment is active, and the FragmentManager is managing its lifecycle state. At this point, FragmentManager methods such as findFragmentById() return this fragment.

onAttach() is always called before any Lifecycle state changes .

The onDetach() callback is invoked when the fragment has been removed from a FragmentManager and is detached from its host activity. The fragment is no longer active and can no longer be retrieved using findFragmentById() .

onDetach() is always called after any Lifecycle state changes .

Note that these callbacks are unrelated to the FragmentTransaction methods attach() and detach() . For more information on these methods, see Fragment transactions.

Fragment lifecycle states and callbacks

When determining a fragment’s lifecycle state, FragmentManager considers the following:

• A fragment’s maximum state is determined by its FragmentManager . A fragment cannot progress beyond the state of its FragmentManager .
• As part of a FragmentTransaction , you can set a maximum lifecycle state on a fragment using setMaxLifecycle() .
• A fragment’s lifecycle state can never be greater than its parent. For example, a parent fragment or activity must be started before its child fragments. Likewise, child fragments must be stopped before their parent fragment or activity.

Caution: Avoid using the tag to add a fragment using XML, as the tag allows a fragment to move beyond the state of its FragmentManager . Instead, always use FragmentContainerView for adding a fragment using XML. Figure 1. Fragment Lifecycle states and their relation both the fragment’s lifecycle callbacks and the fragment’s view Lifecycle .

Figure 1 shows each of the fragment’s Lifecycle states and how they relate to both the fragment’s lifecycle callbacks and the fragment’s view Lifecycle .

As a fragment progresses through its lifecycle, it moves upward and downward through its states. For example, a fragment that is added to the top of the back stack moves upward from CREATED to STARTED to RESUMED . Conversely, when a fragment is popped off of the back stack, it moves downward through those states, going from RESUMED to STARTED to CREATED and finally DESTROYED .

Upward state transitions

When moving upward through its lifecycle states, a fragment first calls the associated lifecycle callback for its new state. Once this callback is finished, the relevant Lifecycle.Event is emitted to observers by the fragment’s Lifecycle , followed by the fragment’s view Lifecycle , if it has been instantiated.

Fragment CREATED

When your fragment reaches the CREATED state, it has been added to a FragmentManager and the onAttach() method has already been called.

This would be the appropriate place to restore any saved state associated with the fragment itself through the fragment’s SavedStateRegistry . Note that the fragment’s view has not been created at this time, and any state associated with the fragment’s view should be restored only after the view has been created.

This transition invokes the onCreate() callback. The callback also receives a savedInstanceState Bundle argument containing any state previously saved by onSaveInstanceState() . Note that savedInstanceState has a null value the first time the fragment is created, but it is always non-null for subsequent recreations, even if you do not override onSaveInstanceState() . See Saving state with fragments for more details.

Fragment CREATED and View INITIALIZED

The fragment’s view Lifecycle is created only when your Fragment provides a valid View instance. In most cases, you can use the fragment constructors that take a @LayoutId , which automatically inflates the view at the appropriate time. You can also override onCreateView() to programmatically inflate or create your fragment’s view.

If and only if your fragment’s view is instantiated with a non-null View , that View is set on the fragment and can be retrieved using getView() . The getViewLifecycleOwnerLiveData() is then updated with the newly INITIALIZED LifecycleOwner corresponding with the fragment’s view. The onViewCreated() lifecycle callback is also called at this time.

This is the appropriate place to set up the initial state of your view, to start observing LiveData instances whose callbacks update the fragment’s view, and to set up adapters on any RecyclerView or ViewPager2 instances in your fragment’s view.

Fragment and View CREATED

After the fragment’s view has been created, the previous view state, if any, is restored, and the view’s Lifecycle is then moved into the CREATED state. The view lifecycle owner also emits the ON_CREATE event to its observers. Here you should restore any additional state associated with the fragment’s view.

This transition also invokes the onViewStateRestored() callback.

Fragment and View STARTED

It is strongly recommended to tie Lifecycle-aware components to the STARTED state of a fragment, as this state guarantees that the fragment’s view is available, if one was created, and that it is safe to perform a FragmentTransaction on the child FragmentManager of the fragment. If the fragment’s view is non-null, the fragment’s view Lifecycle is moved to STARTED immediately after the fragment’s Lifecycle is moved to STARTED .

When the fragment becomes STARTED , the onStart() callback is invoked.

Note: Components such as ViewPager2 set the maximum Lifecycle of offscreen fragments to STARTED .

Fragment and View RESUMED

When the fragment is visible, all Animator and Transition effects have finished, and the fragment is ready for user interaction. The fragment’s Lifecycle moves to the RESUMED state, and the onResume() callback is invoked.

The transition to RESUMED is the appropriate signal to indicate that the user is now able to interact with your fragment. Fragments that are not RESUMED should not manually set focus on their views or attempt to handle input method visibility.

Downward state transitions

When a fragment moves downward to a lower lifecycle state, the relevant Lifecycle.Event is emitted to observers by the fragment’s view Lifecycle , if instantiated, followed by the fragment’s Lifecycle . After a fragment’s lifecycle event is emitted, the fragment calls the associated lifecycle callback.

Fragment and View STARTED

As the user begins to leave the fragment, and while the fragment is still visible, the Lifecycle s for the fragment and for its view are moved back to the STARTED state and emit the ON_PAUSE event to their observers. The fragment then invokes its onPause() callback.

Fragment and View CREATED

Once the fragment is no longer visible, the Lifecycle s for the fragment and for its view are moved into the CREATED state and emit the ON_STOP event to their observers. This state transition is triggered not only by the parent activity or fragment being stopped, but also by the saving of state by the parent activity or fragment. This behavior guarantees that the ON_STOP event is invoked before the fragment’s state is saved. This makes the ON_STOP event the last point where it is safe to perform a FragmentTransaction on the child FragmentManager .

As shown in figure 2, the ordering of the onStop() callback and the saving of the state with onSaveInstanceState() differs based on API level. For all API levels prior to API 28, onSaveInstanceState() is invoked before onStop() . For API levels 28 and higher, the calling order is reversed.

Figure 2. Calling order differences for onStop() and onSaveInstanceState() .

Fragment CREATED and View DESTROYED

After all of the exit animations and transitions have completed, and the fragment’s view has been detached from the window, the fragment’s view Lifecycle is moved into the DESTROYED state and emits the ON_DESTROY event to its observers. The fragment then invokes its onDestroyView() callback. At this point, the fragment’s view has reached the end of its lifecycle and getViewLifecycleOwnerLiveData() returns a null value.

At this point, all references to the fragment’s view should be removed, allowing the fragment’s view to be garbage collected.

Fragment DESTROYED

If the fragment is removed, or if the FragmentManager is destroyed, the fragment’s Lifecycle is moved into the DESTROYED state and sends the ON_DESTROY event to its observers. The fragment then invokes its onDestroy() callback. At this point, the fragment has reached the end of its lifecycle.

Additional resources

For more information related to the fragment lifecycle, see the following additional resources.

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