Enable killing and restarting the engine when we get a change and are busy slicing

CuraEngineBackend.py

@@ -4,6 +4,7 @@ from UM.Scene.SceneNode import SceneNode
 from UM.Scene.Iterator.DepthFirstIterator import DepthFirstIterator
 from UM.Preferences import Preferences
 from UM.Math.Vector import Vector
+from UM.Signal import Signal
 
 from . import Cura_pb2
 from . import ProcessSlicedObjectListJob
@@ -37,9 +38,17 @@ class CuraEngineBackend(Backend):
 
         self._slice_interval = 0.5
 
+        self._slicing = False
+        self._restart = False
+
+        self.changeTimerFinished.connect(self._onChangeTimerFinished)
+        self.backendConnected.connect(self._onBackendConnected)
+
     def getEngineCommand(self):
         return [Preferences.getInstance().getValue("backend/location"), '--connect', "127.0.0.1:{0}".format(self._port)]
 
+    changeTimerFinished = Signal()
+
     def _onSceneChanged(self, source):
         if (type(source) is not SceneNode) or (source is self._scene.getRoot()):
             return
@@ -63,6 +72,8 @@ class CuraEngineBackend(Backend):
         job.start()
 
     def _onProgressMessage(self, message):
+        if message.amount >= 0.99:
+            self._slicing = False
         self.processingProgress.emit(message.amount)
 
     def _onGCodeMessage(self, message):
@@ -89,10 +100,16 @@ class CuraEngineBackend(Backend):
         if self._changeTimer:
             self._changeTimer.cancel()
 
-        self._changeTimer = threading.Timer(self._slice_interval, self._onChangeTimerFinished)
+        self._changeTimer = threading.Timer(self._slice_interval, lambda: self.changeTimerFinished.emit())
         self._changeTimer.start()
 
     def _onChangeTimerFinished(self):
+        if self._slicing:
+            self._slicing = False
+            self._restart = True
+            self._process.terminate()
+            return
+
         objects = []
         for node in DepthFirstIterator(self._scene.getRoot()):
             if type(node) is SceneNode and node.getMeshData():
@@ -102,6 +119,8 @@ class CuraEngineBackend(Backend):
             self._changeTimer = None
             return #No point in slicing an empty build plate
 
+        self._slicing = True
+
         self._sendSettings()
 
         self._scene.acquireLock()
@@ -302,3 +321,8 @@ class CuraEngineBackend(Backend):
             s.value = str(value).encode('utf-8')
 
         self._socket.sendMessage(msg)
+
+    def _onBackendConnected(self):
+        if self._restart:
+            self._onChanged()
+            self._restart = False