Perform Platform physics changes on scene changed again and use a custom operation for better op merging

PlatformPhysics.py

@@ -5,35 +5,27 @@ from UM.Math.Float import Float
 from UM.Math.Vector import Vector
 from UM.Application import Application
 
+from PlatformPhysicsOperation import PlatformPhysicsOperation
+
+import time
 import threading
 
 class PlatformPhysics:
     def __init__(self, controller):
         super().__init__()
         self._controller = controller
-        self._controller.activeToolChanged.connect(self._onActiveToolChanged)
-        self._active_tool = None
-        self._operation = False
-        self._onActiveToolChanged()
+        self._controller.getScene().sceneChanged.connect(self._onSceneChanged)
+        self._signal_source = None
 
-    def _onActiveToolChanged(self):
-        if self._active_tool:
-            self._active_tool.endOperation.disconnect(self._onEndOperation)
-
-        self._active_tool = self._controller.getActiveTool()
-
-        if self._active_tool:
-            self._active_tool.endOperation.connect(self._onEndOperation)
-
-    def _onEndOperation(self):
+    def _onSceneChanged(self, source):
         root = self._controller.getScene().getRoot()
         for node in BreadthFirstIterator(root):
-            if type(node) is not SceneNode:
+            if node is root or type(node) is not SceneNode:
                 continue
 
             bbox = node.getBoundingBox()
             if not Float.fuzzyCompare(bbox.bottom, 0.0):
                 self._signal_source = node
-                op = TranslateOperation(node, Vector(0, -bbox.bottom, 0))
+                op = PlatformPhysicsOperation(node, Vector(0, -bbox.bottom, 0))
                 Application.getInstance().getOperationStack().push(op)
                 self._signal_source = None

PlatformPhysicsOperation.py

@@ -0,0 +1,27 @@
+from UM.Operations.Operation import Operation
+from UM.Operations.AddSceneNodeOperation import AddSceneNodeOperation
+from UM.Operations.TranslateOperation import TranslateOperation
+
+##  A specialised operation designed specifically to modify the previous operation.
+class PlatformPhysicsOperation(Operation):
+    def __init__(self, node, translation):
+        super().__init__()
+        self._node = node
+        self._translation = translation
+
+    def undo(self):
+        pass
+
+    def redo(self):
+        pass
+
+    def mergeWith(self, other):
+        if type(other) is AddSceneNodeOperation:
+            other._node.translate(self._translation)
+            return other
+        elif type(other) is TranslateOperation:
+            other._translation += self._translation
+            return other
+        else:
+            return False
+