I have a loop in the OnUpdate() method and inside the loop, before it finishes, I need to refresh the screen. My question is, there a way to call the OnRender() method manually to force a screen refresh?
There's no way you can call or force the OnRender method from OnUpdate method. The only possible way is to use a list to store your own custom draw instructions that you can deal with in the OnRender method. You also have to remember that SetUpdateRate affects how many times OnRender will be called,