Re-order blocks when a block is placed in an occupied snap point

https://phabricator.endlessm.com/T35519

Author: dylanmccall

addons/block_code/drag_manager/drag_manager.gd

@@ -9,6 +9,7 @@ signal block_modified
 @export var block_canvas_path: NodePath
 
 const Constants = preload("res://addons/block_code/ui/constants.gd")
+const BLOCK_AUTO_PLACE_MARGIN: Vector2 = Vector2(16, 8)
 
 var drag_offset: Vector2
 var dragging: Block = null
@@ -33,10 +34,6 @@ func _process(_delta):
 
 		var dragging_global_pos: Vector2 = dragging.get_global_rect().position
 
-		# TODO: check if dropped snap point is occupied
-		# if so, replace with this node and attach the previous one
-		# to this node's bottom snap
-
 		# Find closest snap point not child of current node
 		var closest_snap_point: SnapPoint = null
 		var closest_dist: float = INF
@@ -105,7 +102,9 @@ func drag_block(block: Block, copied_from: Block = null):
 		new_pos += block.get_global_rect().position
 
 	var parent = block.get_parent()
-	if parent:
+	if parent is SnapPoint:
+		parent.remove_snapped_block(block)
+	elif parent:
 		parent.remove_child(block)
 
 	block.position = new_pos
@@ -142,7 +141,13 @@ func drag_ended():
 				# Can snap block
 				preview_block.free()
 				preview_block = null
-				previewing_snap_point.add_child(dragging)
+				var orphaned_block = previewing_snap_point.set_snapped_block(dragging)
+				if orphaned_block:
+					# Place the orphan block somewhere outside the snap point
+					orphaned_block.position = (
+						(previewing_snap_point.block.global_position - block_canvas_rect.position) + (previewing_snap_point.block.get_size() * Vector2.RIGHT) + BLOCK_AUTO_PLACE_MARGIN
+					)
+					_block_canvas.add_block(orphaned_block)
 			else:
 				# Block goes on screen somewhere
 				dragging.position = (get_global_mouse_position() - block_canvas_rect.position - drag_offset)

addons/block_code/ui/blocks/utilities/snap_point/snap_point.gd

@@ -9,6 +9,8 @@ extends MarginContainer
 ## When block_type is [enum Types.BlockType.VALUE], the type of the value that can be used at this snap point.
 @export var variant_type: Variant.Type
 
+signal snapped_block_changed(block: Block)
+
 var block: Block
 
 
@@ -18,7 +20,48 @@ func _ready():
 
 
 func get_snapped_block() -> Block:
-	if get_child_count() == 0:
-		return null
+	for node in get_children():
+		if node is Block:
+			return node
+	return null
+
+
+func has_snapped_block() -> bool:
+	return get_snapped_block() != null
+
+
+func set_snapped_block(snapped_block: Block) -> Block:
+	var orphaned_block: Block = _pop_snapped_block()
+
+	if snapped_block:
+		add_child(snapped_block)
+
+	if snapped_block and orphaned_block:
+		var last_snap = _get_last_snap(snapped_block)
+		if last_snap:
+			last_snap.set_snapped_block(orphaned_block)
+			orphaned_block = null
+
+	snapped_block_changed.emit(snapped_block)
+
+	return orphaned_block
+
+
+func remove_snapped_block(snapped_block: Block):
+	assert(snapped_block == get_snapped_block())
+	set_snapped_block(null)
+
+
+func _pop_snapped_block() -> Block:
+	var snapped_block = get_snapped_block()
+	if snapped_block:
+		remove_child(snapped_block)
+	return snapped_block
+
 
-	return get_child(0) as Block
+func _get_last_snap(block: Block) -> SnapPoint:
+	var last_snap: SnapPoint
+	while block:
+		last_snap = block.bottom_snap
+		block = last_snap.get_snapped_block() if last_snap else null
+	return last_snap