OnBackPressedDispatcher
public final class OnBackPressedDispatcher
Dispatcher that can be used to register OnBackPressedCallback instances for handling the ComponentActivity.onBackPressed callback via composition.
class FormEntryFragment : Fragment() {
override fun onAttach(context: Context) {
super.onAttach(context)
val callback = object : OnBackPressedCallback(
true // default to enabled
) {
override fun handleOnBackPressed() {
showAreYouSureDialog()
}
}
requireActivity().onBackPressedDispatcher.addCallback(
this, // LifecycleOwner
callback
)
}
}
When constructing an instance of this class, the fallbackOnBackPressed can be set to receive a callback if onBackPressed is called when hasEnabledCallbacks returns false.
Summary
Public constructors |
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OnBackPressedDispatcher(Runnable fallbackOnBackPressed) |
OnBackPressedDispatcher( |
Public methods |
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final void |
@MainThreadAdd a new |
final void |
@MainThreadRegisters a new |
final void |
|
final void |
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final void |
|
final boolean |
Returns |
final void |
Trigger a call to the currently added |
final void |
@RequiresApi(value = 33)Sets the |
Extension functions |
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|---|---|
final @NonNull OnBackPressedCallback |
OnBackPressedDispatcherKt.addCallback(Creates and registers a new |
Public constructors
OnBackPressedDispatcher
public OnBackPressedDispatcher(Runnable fallbackOnBackPressed)
Public methods
addCallback
@MainThread
public final void addCallback(@NonNull OnBackPressedCallback onBackPressedCallback)
Add a new OnBackPressedCallback. Callbacks are invoked in the reverse order in which they are added, so this newly added OnBackPressedCallback will be the first callback to receive a callback if onBackPressed is called.
This method is not Lifecycle aware - if you'd like to ensure that you only get callbacks when at least started, use addCallback. It is expected that you call OnBackPressedCallback.remove to manually remove your callback.
| Parameters | |
|---|---|
@NonNull OnBackPressedCallback onBackPressedCallback |
The callback to add |
| See also | |
|---|---|
onBackPressed |
addCallback
@MainThread
public final void addCallback(
@NonNull LifecycleOwner owner,
@NonNull OnBackPressedCallback onBackPressedCallback
)
Registers a new OnBackPressedCallback that follows the lifecycle of the given LifecycleOwner.
The callback is registered once and stays attached for the lifetime of the LifecycleOwner. Its OnBackPressedCallback.isEnabled state automatically follows the lifecycle: it becomes enabled when the lifecycle is at least Lifecycle.State.STARTED and disabled otherwise.
When the LifecycleOwner is Lifecycle.State.DESTROYED, the callback is removed and lifecycle tracking stops. If the lifecycle is already destroyed when this method is called, the callback is not added.
Legacy Behavior
To restore the legacy add/remove behavior, set ActivityFlags.isOnBackPressedLifecycleOrderMaintained to false. In legacy mode, the handler is added on Lifecycle.Event.ON_START and removed on Lifecycle.Event.ON_STOP, which may change dispatch ordering across lifecycle transitions.
| Parameters | |
|---|---|
@NonNull LifecycleOwner owner |
The |
@NonNull OnBackPressedCallback onBackPressedCallback |
The callback to register. |
| See also | |
|---|---|
onBackPressed |
dispatchOnBackCancelled
@VisibleForTesting
@MainThread
public final void dispatchOnBackCancelled()
dispatchOnBackProgressed
@VisibleForTesting
@MainThread
public final void dispatchOnBackProgressed(@NonNull BackEventCompat backEvent)
dispatchOnBackStarted
@VisibleForTesting
@MainThread
public final void dispatchOnBackStarted(@NonNull BackEventCompat backEvent)
hasEnabledCallbacks
@MainThread
public final boolean hasEnabledCallbacks()
Returns true if there is at least one enabled callback registered with this dispatcher.
| Returns | |
|---|---|
boolean |
True if there is at least one enabled callback. |
onBackPressed
@MainThread
public final void onBackPressed()
Trigger a call to the currently added callbacks in reverse order in which they were added. Only if the most recently added callback is not enabled will any previously added callback be called.
If hasEnabledCallbacks is false when this method is called, the fallbackOnBackPressed set by the constructor will be triggered.
setOnBackInvokedDispatcher
@RequiresApi(value = 33)
public final void setOnBackInvokedDispatcher(@NonNull OnBackInvokedDispatcher invoker)
Sets the OnBackInvokedDispatcher for handling system back for Android SDK T+.
| Parameters | |
|---|---|
@NonNull OnBackInvokedDispatcher invoker |
the OnBackInvokedDispatcher to be set on this dispatcher |
Extension functions
OnBackPressedDispatcherKt.addCallback
public final @NonNull OnBackPressedCallback OnBackPressedDispatcherKt.addCallback(
@NonNull OnBackPressedDispatcher receiver,
LifecycleOwner owner,
boolean enabled,
@NonNull Function1<@NonNull OnBackPressedCallback, Unit> onBackPressed
)
Creates and registers a new OnBackPressedCallback that invokes onBackPressed.
If a LifecycleOwner is provided, the callback’s enabled state automatically follows the lifecycle: it is enabled while the lifecycle is at least State.STARTED and disabled otherwise. The callback stays registered until the LifecycleOwner is destroyed.
A default enabled state can be supplied.
Legacy Behavior
To restore the legacy add/remove behavior, set ActivityFlags.isOnBackPressedLifecycleOrderMaintained to false. In legacy mode, the handler is added on Lifecycle.Event.ON_START and removed on Lifecycle.Event.ON_STOP, which may change dispatch ordering across lifecycle transitions.