class_name AudioFader extends Node
## Controls an AudioStreamPlayer and allows for fading and crossfading.

## Audio fade-out target.
const AUDIO_FADE_MIN_VOLUME: float = -20.0
## Default audio transition time.
const DEFAULT_AUDIO_FADE: float = 1.0

## Audio bus.
@export var bus: StringName

## Current AudioStreamPlayer.
var player: AudioStreamPlayer = null
## Current TweenChannel.
var tween_channel: TweenChannel = null

## Creates an initial AudioStreamPlayer child and TweenChannel.
func _ready() -> void:
	_create_members()

## Creates an AudioStreamPlayer child and TweenChannel.
func _create_members() -> void:
	player = AudioStreamPlayer.new()
	add_child(player)
	tween_channel = TweenChannel.make_replacing()

## Fades in the current audio player.
func fade_in(stream: AudioStream, fade: float = DEFAULT_AUDIO_FADE) -> void:
	await fade_out()
	player.volume_db = AUDIO_FADE_MIN_VOLUME
	player.stream = stream
	player.play()
	var tween := await tween_channel.create_tween(player)
	tween.tween_property(player, ^'volume_db', 0.0, fade)
	await tween_channel.sync(tween)

## Fades out an arbitrary audio player on an arbitrary tween channel.
static func _fade_out_foreign(
	p_player: AudioStreamPlayer,
	p_tween_channel: TweenChannel,
	fade: float = DEFAULT_AUDIO_FADE
) -> void:
	if p_player.volume_db > AUDIO_FADE_MIN_VOLUME:
		var tween := await p_tween_channel.create_tween(p_player)
		tween.tween_property(
			p_player, ^'volume_db', AUDIO_FADE_MIN_VOLUME, fade
		)
		await p_tween_channel.sync(tween)
	p_player.stop()

## Fades out the current audio player.
func fade_out(fade: float = DEFAULT_AUDIO_FADE) -> void:
	await AudioFader._fade_out_foreign(player, tween_channel, fade)

## Plays audio without fading in.
func play(stream: AudioStream) -> void:
	await player.fade_out()
	player.volume_db = 0.0
	player.stream = stream
	player.play()

## Fades out old audio and fades in new audio at the same time.
func crossfade(stream: AudioStream, fade: float = DEFAULT_AUDIO_FADE) -> void:
	var old_player := player
	var old_tween_channel := tween_channel
	var thunk := func() -> void:
		await AudioFader._fade_out_foreign(old_player, old_tween_channel, fade)
		old_player.queue_free()
	_create_members()
	await Wait.until(player.is_node_ready)
	var task := Task.new(thunk)
	await fade_in(stream, fade)
	await task.sync()