Refactor grouping/ungrouping into an operation that is undoable

CURA-1543

cura/SetParentOperation.py

@@ -0,0 +1,50 @@
+# Copyright (c) 2016 Ultimaker B.V.
+# Uranium is released under the terms of the AGPLv3 or higher.
+
+from UM.Scene.SceneNode import SceneNode
+from UM.Operations import Operation
+
+from UM.Math.Vector import Vector
+
+##  An operation that parents a scene node to another scene node.
+
+class SetParentOperation(Operation.Operation):
+    ##  Initialises this SetParentOperation.
+    #
+    #   \param node The node which will be reparented.
+    #   \param parent_node The node which will be the parent.
+    def __init__(self, node, parent_node):
+        super().__init__()
+        self._node = node
+        self._parent = parent_node
+        self._old_parent = node.getParent() # To restore the previous parent in case of an undo.
+
+    ##  Undoes the set-parent operation, restoring the old parent.
+    def undo(self):
+        self._set_parent(self._old_parent)
+
+    ##  Re-applies the set-parent operation.
+    def redo(self):
+        self._set_parent(self._parent)
+
+    ##  Sets the parent of the node while applying transformations to the world-transform of the node stays the same.
+    #
+    #   \param new_parent The new parent. Note: this argument can be None, which would hide the node from the scene.
+    def _set_parent(self, new_parent):
+        if new_parent:
+            self._node.setPosition(self._node.getWorldPosition() - new_parent.getWorldPosition())
+            current_parent = self._node.getParent()
+            if current_parent:
+                self._node.scale(current_parent.getScale() / new_parent.getScale())
+                self._node.rotate(current_parent.getOrientation())
+            else:
+                self._node.scale(Vector(1, 1, 1) / new_parent.getScale())
+            self._node.rotate(new_parent.getOrientation().getInverse())
+
+        self._node.setParent(new_parent)
+
+    ##  Returns a programmer-readable representation of this operation.
+    #
+    #   \return A programmer-readable representation of this operation.
+    def __repr__(self):
+        return "SetParentOperation(node = {0}, parent_node={1})".format(self._node, self._parent)