How do I redraw the Cycles viewport after a material update?

Question

I use a modal operator that assigns materials to meshes via a shortcut.

Everything works fine except that there is a lag (close to a second) between when a material is assigned with the shortcut and the time that's visible on screen.

Is there a way to overcome this? I have used

bpy.context.view_layer.update()

but it seems to do nothing.