[vtkusers] vtk Renderer don't Refresh when I add a new actor into it VTK6.1

[檐下听雪]

Dear:
I'm a new learner in vtk and I have a question in this days.


I want to make a mark line with vtk on the renderwindow.
The task I want to do is that when I press my Letfbutton, 
I begin to draw the line ,and when I release leftbutton, 
it is end.


I use class qvtkWidget to show it, a callback class which inherit
class vtkCommand. I got the mouse current postion by 
vtkRenderWindowInteractor, use it to creat vtkPoints and vtkLine,
and add them into vtkcellarray to create vtkpolydata. when a
new point add in , i create a new line add into the array, and I create 
a new actor to add into the renderer. But when i press left button, 
the mark line is not be drawn, and i release the button , the mark line 
can show finally.I use the same sentences in the callback function for the
even press and mousemove, the function in press works well , but another
was bad.


So I want to know why ,and how can i solve it. Thank you!
Yao
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