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 itsFragmentManager. - 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.
Lifecycle states
and their relation to 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.
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 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.
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.