How to check input manually?

This problem originate from a multi-stage drawing. The drawing is splited into several stages so that each stage costs less time to let the user interrupt it. After each stage, I call DrawDeferred to make it goes to the next stage. However, HandleCommandL is never called until the last stage finished.
And I tried to add User::After(1) before DrawDeferred. This time HandleCommandL can be called in the middle of the stages, but the performance of the Drawing is dropped dramatically.
Is there any method that I can check whether there's an input? Or any better method to achieve a multi-stage draw?