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

Public functions
`Unit`	`@MainThread addCallback( callback: NavigationEventCallback<*>, priority: NavigationEventPriority )` Adds a new `NavigationEventCallback` to receive navigation events.	Cmn
`Unit`	`@MainThread addInput(input: NavigationEventInput)` Adds an input, registering it with the shared processor and binding it to this dispatcher's lifecycle.	Cmn
`Unit`	`@MainThread dispose()` Removes this dispatcher and its entire chain of descendants from the hierarchy.	Cmn
`inline StateFlow<NavigationEventState<T>>`	`<T : NavigationEventInfo> getState(scope: CoroutineScope, initialInfo: T)` Creates a `StateFlow` that only emits states for a specific `NavigationEventInfo` type.	Cmn
`Boolean`	`hasEnabledCallbacks()` Returns `true` if there is at least one `NavigationEventCallback.isEnabled` callback registered with this dispatcher.	Cmn
`Unit`	`@MainThread removeInput(input: NavigationEventInput)` Removes and detaches an input from this dispatcher and the shared processor.	Cmn

Public properties
`Boolean`	`isEnabled` Controls whether this dispatcher is active and will process navigation events.	Cmn
`StateFlow<NavigationEventState<NavigationEventInfo>>`	`state` The `StateFlow` from the highest-priority, enabled navigation callback.	Cmn

Public constructors

NavigationEventDispatcher

Cmn

Added in 1.0.0-alpha07

NavigationEventDispatcher(fallbackOnBackPressed: (() -> 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.

NavigationEventDispatcher

Cmn

Added in 1.0.0-alpha07

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.

Public functions

addCallback

Cmn

Added in 1.0.0-alpha07

@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.

addInput

Cmn

Added in 1.0.0-alpha07

@MainThread
fun addInput(input: NavigationEventInput): Unit

Adds an input, registering it with the shared processor and binding it to this dispatcher's lifecycle.

The input is registered globally with the sharedProcessor to receive system-wide state updates (e.g., whether any callbacks are enabled). It is also tracked locally by this dispatcher for lifecycle management.

The input's NavigationEventInput.onAdded method is invoked immediately upon addition. It will be automatically detached when this dispatcher dispose is called.

Parameters
`input: NavigationEventInput`	The input to add.

Throws
`kotlin.IllegalStateException`	if the dispatcher has already been disposed or if `input` is already added to a dispatcher.

See also
`removeInput`
`onRemoved`

dispose

Cmn

Added in 1.0.0-alpha07

@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 iteratively processes and disposes of all child dispatchers and their descendants, ensuring a complete top-down cleanup of the entire sub-hierarchy without recursion.
For each dispatcher, it first detaches all registered NavigationEventInput instances by calling NavigationEventInput.onRemoved. This severs their lifecycle link to the dispatcher and allows them to release any tied resources.
It then removes all NavigationEventCallback instances registered with that dispatcher from the shared processor, preventing memory leaks.
Finally, it removes the dispatcher from its parent's list of children, fully dismantling the hierarchy.

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

getState

Cmn

inline fun <T : NavigationEventInfo> getState(scope: CoroutineScope, initialInfo: T): StateFlow<NavigationEventState<T>>

Creates a StateFlow that only emits states for a specific NavigationEventInfo type.

Parameters
`<T : NavigationEventInfo>`	The `NavigationEventInfo` type to filter for.
`scope: CoroutineScope`	The `CoroutineScope` in which the new `StateFlow` is created.
`initialInfo: T`	The initial `NavigationEventInfo` of type `T` to be used when the `StateFlow` starts.

Returns
`StateFlow<NavigationEventState<T>>`	A `StateFlow` that emits values only when the state's destination is of type `T`.

hasEnabledCallbacks

Cmn

Added in 1.0.0-alpha07

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.

removeInput

Cmn

Added in 1.0.0-alpha07

@MainThread
fun removeInput(input: NavigationEventInput): Unit

Removes and detaches an input from this dispatcher and the shared processor.

This severs the input's lifecycle link to the dispatcher. Its NavigationEventInput.onRemoved method is invoked, and it will no longer receive lifecycle calls or global state updates from the processor.

Parameters
`input: NavigationEventInput`	The input to remove.

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

See also
`addInput`
`onAdded`

Public properties

isEnabled

Cmn

Added in 1.0.0-alpha07

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.

state

Cmn

Added in 1.0.0-alpha07

val state: StateFlow<NavigationEventState<NavigationEventInfo>>

The StateFlow from the highest-priority, enabled navigation callback.

This represents the navigation state of the currently active component.