Thanks. I assumed iOS was like Windows. Although Windows also has scroller controls that you create on the UI thread but delegate the actual event handling to the compositor.
A compositor can only do scrolling if the app is written with a retained mode API. The low-level APIs in Windows are immediate mode, whether GDI or Direct2D.
For most of time, ScrollWindow / ScrollWindowEx was how you scrolled. It doesn't know what to draw in the invalidated region because it's an immediate mode API.
In Direct2D you can use e.g. a translation to get the renderer to draw a bigger bitmap in a different section of the window, but you need to render the bigger bitmap (i.e. assemble your own retained mode). There's no free lunch.
For most of time, ScrollWindow / ScrollWindowEx was how you scrolled. It doesn't know what to draw in the invalidated region because it's an immediate mode API.
In Direct2D you can use e.g. a translation to get the renderer to draw a bigger bitmap in a different section of the window, but you need to render the bigger bitmap (i.e. assemble your own retained mode). There's no free lunch.