Loading and saving of ItemStack based inventory works.

inventory.gd

@@ -4,19 +4,18 @@ var _content:Array[ItemStack] = []
 const INVENTORY_SIZE:int = 36
 
 func _init() -> void:
-	_content.resize(INVENTORY_SIZE)
+	clear()
 
 func get_free_item_stack_index() -> int:
 	for i:int in range(_content.size()):
-		if _content[i] == null:
+		if _content[i].count == 0:
 			return i
 	
 	return -1
 
-
 func add_item(item:Item) -> bool:
 	for item_stack:ItemStack in _content:
-		if item_stack == null:
+		if item_stack.count == 0:
 			continue
 		
 		if item_stack.item.resource_path != item.resource_path:
@@ -28,10 +27,8 @@ func add_item(item:Item) -> bool:
 
 	var item_stack_index:int = get_free_item_stack_index()
 	if item_stack_index >= 0:
-		var item_stack:ItemStack = ItemStack.new()
-		item_stack.item = item
-		item_stack.count = 1
-		_content[item_stack_index] = item_stack
+		_content[item_stack_index].item = item
+		_content[item_stack_index].count = 1
 		return true
 
 	return false
@@ -39,14 +36,12 @@ func add_item(item:Item) -> bool:
 func remove_item(item:Item):
 	for i in range(_content.size() - 1, -1, -1):
 		var item_stack:ItemStack = _content[i]
-		if item_stack == null:
+		if item_stack.count == 0:
 			continue
 	
 		if item_stack.item.resource_path == item.resource_path:
 			if item_stack.count > 1:
 				item_stack.count = item_stack.count - 1
-			else:
-				_content[i] = null
 			
 			return
 
@@ -57,13 +52,17 @@ func get_item_stacks() -> Array[ItemStack]:
 
 func clear() -> void:
 	_content.clear()
+	_content.resize(INVENTORY_SIZE)
+	for i:int in range(_content.size()):
+		if _content[i] == null:
+			_content[i] = ItemStack.new()
 
 
 func has_all(items:Array[Item]) -> bool:
 	var needed:Array[Item] = items.duplicate()
 	
 	for item_stack:ItemStack in _content:
-		if item_stack == null:
+		if item_stack.count == 0:
 			continue
 		
 		for i in range (item_stack.count):

model/save_game.gd

@@ -5,5 +5,5 @@ extends Resource
 @export var player_name:String
 @export var quest_states:Dictionary
 @export var player_transform:Transform3D
-@export var inventory:Array[Item]
+@export var inventory:Array
 @export var level_pickup_items:Array[NodePath]

scenes/main.gd

@@ -28,8 +28,8 @@ func save_game():
 			if (property.usage & PROPERTY_USAGE_STORAGE != 0) and (property.usage & PROPERTY_USAGE_SCRIPT_VARIABLE) != 0:
 				save_game.quest_states[node.name][property.name] = node.get(property.name)
 
-	for item in player.inventory.get_items():
-		save_game.inventory.append(item)
+	for item_stack in player.inventory.get_item_stacks():
+		save_game.inventory.append({"item": item_stack.item, "count": item_stack.count})
 
 	ResourceSaver.save(save_game, "user://savegame.tres")
 	root_ui.activate_ui_panel(%GameUI)
@@ -57,8 +57,13 @@ func load_game():
 	
 	# Inventory
 	player.inventory.clear()
-	for item in save_game.inventory:
-		player.inventory.add_item(item)
+	var inventory_item_stacks:Array[ItemStack] = player.inventory.get_item_stacks()
+	
+	assert(inventory_item_stacks.size() >= save_game.inventory.size())
+	
+	for i in range(save_game.inventory.size()):
+		inventory_item_stacks[i].item = save_game.inventory[i]["item"]
+		inventory_item_stacks[i].count = save_game.inventory[i]["count"]
 	
 	# Picked up items
 	for item_path in save_game.level_pickup_items:

ui/item_drag.gd

@@ -0,0 +1,16 @@
+class_name ItemDrag
+
+signal drag_completed(data:ItemDrag)
+
+var source:Control = null
+var destination:Control = null
+
+var preview:Control
+
+func _init(source_control: Control, preview_control:Control):
+	self.source = source_control
+	self.preview = preview_control
+	self.preview.tree_exiting.connect(self._on_tree_exiting)
+	
+func _on_tree_exiting()->void:
+	drag_completed.emit(self)

ui/item_grid.gd

@@ -11,10 +11,11 @@ func displayStacks(item_stacks:Array[ItemStack]):
 
 	for item_stack in item_stacks:
 		var slot:ItemSlot = slot_scene.instantiate()
+		slot.set_drag_drop_flags(ItemSlot.ALLOW_DRAG | ItemSlot.ALLOW_DROP)
 		add_child(slot)
 		
 		if item_stack != null:
-			slot.display(item_stack.item, item_stack.count)
+			slot.display(item_stack)
 	
 	var item_count = rows * columns
 		
@@ -29,7 +30,10 @@ func display(item_stacks:Array[Item]):
 	for item in item_stacks:
 		var slot:ItemSlot = slot_scene.instantiate()
 		add_child(slot)
-		slot.display(item, 1)
+		var item_stack:ItemStack = ItemStack.new()
+		item_stack.item = item
+		item_stack.count = 1
+		slot.display(item_stack)
 	
 	var item_count = rows * columns
 		

ui/item_slot.gd

@@ -1,13 +1,33 @@
 class_name ItemSlot
 extends PanelContainer
 
+enum {
+	ALLOW_DRAG = 1,
+	ALLOW_DROP = 2
+}
+
 @onready var texture_rect:TextureRect = %TextureRect
 @onready var count_label = %CountLabel
 
-func display(item:Item, count:int):
-	if item != null:
-		texture_rect.texture = item.icon
-		update_quantity_text(count)
+var drag_drop_flags:int = 0
+var _item_stack:ItemStack = null
+
+func set_drag_drop_flags(flags:int) -> void:
+	drag_drop_flags = flags
+
+func display(item_stack):
+	_item_stack = item_stack
+
+func _process(_delta)->void:
+	if _item_stack == null:
+		return
+	
+	if _item_stack.count == 0:
+		texture_rect.texture = null
+		update_quantity_text(0)
+	else:
+		texture_rect.texture = _item_stack.item.icon
+		update_quantity_text(_item_stack.count)
 		
 func update_quantity_text(stack_size) -> void:
 	if stack_size <= 1:
@@ -15,3 +35,50 @@ func update_quantity_text(stack_size) -> void:
 	else:
 		count_label.text = str(stack_size)
 		count_label.show()
+
+func _get_drag_data(_at_position:Vector2)->Variant:
+	if drag_drop_flags & ALLOW_DRAG == 0:
+		return null
+	
+	if _item_stack == null:
+		return null
+	
+	var drag_data:ItemDrag = ItemDrag.new(self, self)
+	
+	var drag_preview = load("res://ui/item_slot.tscn").instantiate()
+	var drag_stack = ItemStack.new()
+	drag_stack.item = _item_stack.item
+	drag_stack.count = _item_stack.count
+	drag_preview.display(drag_stack)
+	set_drag_preview(drag_preview)
+	
+	return drag_data
+
+func _can_drop_data(_at_position:Vector2, data:Variant)->bool:
+	if drag_drop_flags & ALLOW_DROP == 0:
+		return false
+		
+	if !data is ItemDrag:
+		return false
+		
+	return _item_stack.count == 0
+
+func _drop_data(_at_position:Vector2, data:Variant)->void:
+	if drag_drop_flags & ALLOW_DROP == 0:
+		return
+		
+	if !data is ItemDrag:
+		return
+	
+	var drag_data := data as ItemDrag
+	
+	drag_data.destination = self
+	if drag_data.source is ItemSlot:
+		var source_slot := drag_data.source as ItemSlot
+		
+		# add stuff to the destination
+		self._item_stack.item = source_slot._item_stack.item
+		self._item_stack.count = source_slot._item_stack.count
+		
+		# remove it from the source
+		source_slot._item_stack.count = 0

ui/item_stack.gd

@@ -2,8 +2,4 @@ class_name ItemStack
 extends Node
 
 var item:Item = null
-@export var count:int = 1
-
-# Called when the node enters the scene tree for the first time.
-func _ready():
-	pass # Replace with function body.
+var count:int = 0

ui/tool_container.gd

@@ -6,8 +6,3 @@ extends HBoxContainer
 # Called when the node enters the scene tree for the first time.
 func _ready():
 	pass # Replace with function body.
-
-
-# Called every frame. 'delta' is the elapsed time since the previous frame.
-func _process(delta):
-	pass