Clean up path and names

scripts/player/states/player_idle_state.gd

@@ -0,0 +1,13 @@
+class_name PlayerIdleState
+extends PlayerState
+
+func enter(_msg := {}):
+	player.velocity = Vector2.ZERO
+	player.animation_player.play("idle")
+
+func update(_delta):
+	if get_input_direction() != Vector2.ZERO:
+		state_machine.transition_to("Walk")
+
+func get_input_direction():
+	return Input.get_vector("move_left", "move_right", "move_up", "move_down")

scripts/player/states/player_state.gd

@@ -0,0 +1,9 @@
+class_name PlayerState
+extends State
+
+var player: Player
+
+func _ready():
+	await owner.ready
+	player = owner as Player
+	assert(player != null)

scripts/player/states/player_walk_state.gd

@@ -0,0 +1,24 @@
+class_name PlayerWalkState
+extends PlayerState
+
+func enter(_msg := {}):
+	player.animation_player.play("walk")
+
+func physics_update(_delta):
+	var direction = get_input_direction()
+	player.velocity = direction * player.speed
+	
+	player.move_and_slide() # This method calculate with delta, we don't need to do it.
+	
+	if player.velocity == Vector2.ZERO:
+		state_machine.transition_to("Idle")
+
+func get_input_direction():
+		var direction = Input.get_vector("move_left", "move_right", "move_up", "move_down")
+		
+		if Input.is_action_pressed("move_left"):
+			player.sprite.flip_h = true
+		elif Input.is_action_pressed("move_right"):
+			player.sprite.flip_h = false
+		
+		return direction