[Doc Review]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.
- INITIALIZED
- CREATED
- STARTED
- RESUMED
- DESTROYED
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 following:
- onCreate()
- onStart()
- onResume()
- onPause()
- onStop()
- 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.
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.
Once Fragment is instantiated, you should use FragmentManager to handle the fragment. It manages the state of the fragment and is responsible for attaching and detaching fragment to their host activity.
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().
Caution: Avoid reusing Fragment instances after they are removed from the FragmentManager. While the fragment handles its own internal state cleanup, you might inadvertently carry over your own state into the reused instance.
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 usingsetMaxLifecycle(). - 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.
Don't use tag. Use
Figure 1. Fragment Lifecycle states and their relation 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
CREATED -> STARTED -> RESUMED
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.
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
This is the appropriate place to set up the initial state of your view, to observe LiveData instances that update the fragment's view, and to set up adapters on any RecyclerView or ViewPager2 instances in your 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.
Note: Components such as ViewPager2 set the maximum Lifecycle of offscreen fragments to STARTED.
Maximum Lifecycle of ViewPager2's offscreen is set to STARTED, meaning that the fragment is partially active but not fully activated overall.
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.
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.
Downward state transitions
RESUMED -> STARTED -> CREATED -> DESTROYED
When heading in the upward and downward directions, the state names are the same, but the functions called within each state are different.
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 Lifecycles 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 Lifecycles 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.
The creation of a fragment's view precedes the creation of the fragment. However, the order of destruction is reversed.
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.
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