OnBackPressedDispatcher

Kotlin |Java

class OnBackPressedDispatcher

kotlin.Any
↳	androidx.activity.OnBackPressedDispatcher

Dispatcher that can be used to register OnBackPressedCallback instances for handling the ComponentActivity#onBackPressed() callback via composition.

public class FormEntryFragment extends Fragment {
      @Override
      public void onAttach(@NonNull Context context) {
          super.onAttach(context);
          OnBackPressedCallback callback = new OnBackPressedCallback(
              true // default to enabled
          ) {
              @Override
              public void handleOnBackPressed() {
                  showAreYouSureDialog();
              }
          };
          requireActivity().getOnBackPressedDispatcher().addCallback(
              this, // LifecycleOwner
              callback);
      }
  }

Summary

Public constructors
`<init>()` Create a new OnBackPressedDispatcher that dispatches System back button pressed events to one or more `OnBackPressedCallback` instances.
`<init>(@Nullable fallbackOnBackPressed: Runnable?)` Create a new OnBackPressedDispatcher that dispatches System back button pressed events to one or more `OnBackPressedCallback` instances.

Public methods
Unit	`addCallback(@NonNull onBackPressedCallback: OnBackPressedCallback)` Add a new `OnBackPressedCallback`.
Unit	`addCallback(@NonNull owner: LifecycleOwner, @NonNull onBackPressedCallback: OnBackPressedCallback)` Receive callbacks to a new `OnBackPressedCallback` when the given `LifecycleOwner` is at least `started`.
Boolean	`hasEnabledCallbacks()` Checks if there is at least one `enabled` callback registered with this dispatcher.
Unit	`onBackPressed()` Trigger a call to the currently added `callbacks` in reverse order in which they were added.

Extension functions

From androidx.activity

OnBackPressedCallback

OnBackPressedDispatcher.addCallback(owner: LifecycleOwner? = null, enabled: Boolean = true, onBackPressed: OnBackPressedCallback.() -> Unit)

Create and add a new OnBackPressedCallback that calls onBackPressed in OnBackPressedCallback.handleOnBackPressed.

Public constructors

<init>

OnBackPressedDispatcher()

Create a new OnBackPressedDispatcher that dispatches System back button pressed events to one or more OnBackPressedCallback instances.

<init>

OnBackPressedDispatcher(@Nullable fallbackOnBackPressed: Runnable?)

Create a new OnBackPressedDispatcher that dispatches System back button pressed events to one or more OnBackPressedCallback instances.

Parameters
`fallbackOnBackPressed`	Runnable?: The Runnable that should be triggered if `onBackPressed()` is called when `hasEnabledCallbacks()` returns false.

Public methods

addCallback

@MainThread fun addCallback(@NonNull onBackPressedCallback: OnBackPressedCallback): Unit

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(LifecycleOwner, OnBackPressedCallback). It is expected that you call OnBackPressedCallback#remove() to manually remove your callback.

Parameters
`onBackPressedCallback`	OnBackPressedCallback: The callback to add

See Also

#onBackPressed()

addCallback

@MainThread fun addCallback(
    @NonNull owner: LifecycleOwner, 
    @NonNull onBackPressedCallback: OnBackPressedCallback
): Unit

Receive callbacks to a new OnBackPressedCallback when the given LifecycleOwner is at least started.

This will automatically call addCallback(OnBackPressedCallback) and remove the callback as the lifecycle state changes. As a corollary, if your lifecycle is already at least started, calling this method will result in an immediate call to addCallback(OnBackPressedCallback).

When the LifecycleOwner is destroyed, it will automatically be removed from the list of callbacks. The only time you would need to manually call OnBackPressedCallback#remove() is if you'd like to remove the callback prior to destruction of the associated lifecycle.

If the Lifecycle is already destroyed when this method is called, the callback will not be added.

Parameters
`owner`	LifecycleOwner: The LifecycleOwner which controls when the callback should be invoked
`onBackPressedCallback`	OnBackPressedCallback: The callback to add

See Also

#onBackPressed()

hasEnabledCallbacks

@MainThread fun hasEnabledCallbacks(): Boolean

Checks if there is at least one enabled callback registered with this dispatcher.

Return
`Boolean`	True if there is at least one enabled callback.

onBackPressed

@MainThread fun onBackPressed(): Unit

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 fallback Runnable set by the constructor will be triggered.