VTK QT Animation

import vtk
from vtk.qt.QVTKRenderWindowInteractor import QVTKRenderWindowInteractor
from PyQt5 import Qt

message = "tick"
def onTimerEvent(object, event):
    global message
    print(message)
    if message == "tick":
        message = "tock"
    else:
        message = "tick"

app        = Qt.QApplication([])
mainWindow = Qt.QMainWindow()
renderer = vtk.vtkRenderer()
vtkWidget = QVTKRenderWindowInteractor(mainWindow)
vtkWidget.GetRenderWindow().AddRenderer(renderer)
mainWindow.setCentralWidget(vtkWidget)

vtkWidget.GetRenderWindow().GetInteractor().Initialize()
timerId = vtkWidget.CreateRepeatingTimer(100)
vtkWidget.AddObserver("TimerEvent", onTimerEvent)

mainWindow.show()
app.exec_()