NavigationEventDispatcher

Artifact: androidx.navigationevent:navigationevent

Kotlin |Java

Cmn
```
class NavigationEventDispatcher
```

A dispatcher for navigation events that can be organized hierarchically.

This class acts as a localized entry point for registering NavigationEventCallback instances and dispatching navigation events within a specific UI scope, such as a composable or a fragment.

Dispatchers can be linked in a parent-child hierarchy. This structure allows for a sophisticated system where nested UI components can handle navigation events independently while still respecting the state of their parent. The core logic is delegated to a single, shared NavigationEventProcessor instance across the entire hierarchy, ensuring consistent event handling.

It is important to call dispose when the owner of this dispatcher is destroyed (e.g., in a DisposableEffect) to unregister callbacks and prevent memory leaks.

Summary

Public constructors
`NavigationEventDispatcher(parentDispatcher: NavigationEventDispatcher)` Creates a child `NavigationEventDispatcher` linked to a parent.	Cmn
`NavigationEventDispatcher( fallbackOnBackPressed: (() -> Unit)?, onHasEnabledCallbacksChanged: ((Boolean) -> Unit)? )` Creates a root `NavigationEventDispatcher`.	Cmn

Public functions
`Unit`	`@MainThread addCallback( callback: NavigationEventCallback, priority: NavigationEventPriority )` Adds a new `NavigationEventCallback` to receive navigation events.	Cmn
`Unit`	`addOnHasEnabledCallbacksChangedCallback(callback: (Boolean) -> Unit)` Adds a callback that will be notified when the overall enabled state of registered callbacks changes.	Cmn
`Unit`	`@MainThread dispose()` Removes this dispatcher and its entire chain of descendants from the hierarchy.	Cmn
`Boolean`	`hasEnabledCallbacks()` Returns `true` if there is at least one `NavigationEventCallback.isEnabled` callback registered with this dispatcher.	Cmn

Public properties
`Boolean`	`isEnabled` Controls whether this dispatcher is active and will process navigation events.	Cmn

Public constructors

NavigationEventDispatcher

Cmn

Added in 1.0.0-alpha05

NavigationEventDispatcher(parentDispatcher: NavigationEventDispatcher)

Creates a child NavigationEventDispatcher linked to a parent.

This constructor is used to create nested dispatchers within an existing hierarchy. The new dispatcher will share the same underlying NavigationEventProcessor as its parent, allowing it to participate in the same event stream.

Parameters
`parentDispatcher: NavigationEventDispatcher`	The parent `NavigationEventDispatcher` to which this new dispatcher will be attached.

NavigationEventDispatcher

Cmn

Added in 1.0.0-alpha05

NavigationEventDispatcher(
    fallbackOnBackPressed: (() -> Unit)? = null,
    onHasEnabledCallbacksChanged: ((Boolean) -> Unit)? = null
)

Creates a root NavigationEventDispatcher.

This constructor is used to establish the top-level dispatcher for a new navigation hierarchy, typically within a scope like an Activity or a top-level composable. It creates its own internal NavigationEventProcessor.

Parameters
`fallbackOnBackPressed: (() -> Unit)? = null`	An optional lambda to be invoked if a navigation event completes and no registered `NavigationEventCallback` handles it. This provides a default "back" action for the entire hierarchy.
`onHasEnabledCallbacksChanged: ((Boolean) -> Unit)? = null`	An optional lambda that will be called whenever the global state of whether there are any enabled callbacks changes.

Public functions

addCallback

Cmn

@MainThread
fun addCallback(
    callback: NavigationEventCallback,
    priority: NavigationEventPriority = Default
): Unit

Adds a new NavigationEventCallback to receive navigation events.

Callbacks are invoked based on priority, and then by recency. All Overlay callbacks are called before any Default callbacks. Within each priority group, callbacks are invoked in a Last-In, First-Out (LIFO) order—the most recently added callback is called first.

All callbacks are invoked on the main thread. To stop receiving events, a callback must be removed via NavigationEventCallback.remove.

Parameters
`callback: NavigationEventCallback`	The callback instance to be added.
`priority: NavigationEventPriority = Default`	The priority of the callback, determining its invocation order relative to others. See `NavigationEventPriority`.

Throws
`kotlin.IllegalArgumentException`	if the given callback is already registered with a different dispatcher.
`kotlin.IllegalStateException`	if the dispatcher has already been disposed.

addOnHasEnabledCallbacksChangedCallback

Cmn

Added in 1.0.0-alpha05

fun addOnHasEnabledCallbacksChangedCallback(callback: (Boolean) -> Unit): Unit

Adds a callback that will be notified when the overall enabled state of registered callbacks changes.

Parameters
`callback: (Boolean) -> Unit`	The callback to invoke when the enabled state changes.

dispose

Cmn

Added in 1.0.0-alpha05

@MainThread
fun dispose(): Unit

Removes this dispatcher and its entire chain of descendants from the hierarchy.

This is the primary cleanup method and should be called when the component owning this dispatcher is destroyed (e.g., in DisposableEffect in Compose).

This is a terminal operation; once a dispatcher is disposed, it cannot be reused.

Calling this method triggers a comprehensive, iterative cleanup:

It unregisters this dispatcher's onHasEnabledCallbacksChanged listener from the shared processor.
It iteratively processes and disposes of all child dispatchers and their descendants, ensuring a complete top-down cleanup of the entire sub-hierarchy without recursion.
It removes all NavigationEventCallback instances directly registered with this specific dispatcher from the shared NavigationEventProcessor, preventing memory leaks and ensuring callbacks are no longer active.
Finally, it removes itself from its parent's list of children, if a parent exists.

Throws
`kotlin.IllegalStateException`	if the dispatcher has already been disposed.

hasEnabledCallbacks

Cmn

Added in 1.0.0-alpha05

fun hasEnabledCallbacks(): Boolean

Returns true if there is at least one NavigationEventCallback.isEnabled callback registered with this dispatcher.

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

Public properties

isEnabled

Cmn

Added in 1.0.0-alpha05

var isEnabled: Boolean

Controls whether this dispatcher is active and will process navigation events.

A dispatcher's effective enabled state is hierarchical. It depends on both its own local isEnabled state and the state of its parent.

Getting the value:

This will return false if the parentDispatcher exists and its isEnabled state is false, regardless of this dispatcher's own setting. This provides a simple way to disable an entire branch of a navigation hierarchy by disabling its root.
If there is no parent or the parent is enabled, it will return the local value of this property (true by default).

Setting the value:

This only updates the local enabled state for this specific dispatcher. The getter will always re-evaluate the effective state based on the parent.

For this dispatcher to be truly active, its local isEnabled property must be true, and the isEnabled properties of all its ancestors must also be true.