How to implement Doevents function in WPF like winform?

How to implement Doevents function in WPF like winform?
and if checkforIllegalCrossThreadCalls exists in WPF?

by the way, my program like below

step 1 (long time)
Doevents
step 2 (long time)
Doevents
step 3 (long time)
Doevents

thanks

Joey