Being notified of fold / unfold on iPhone Duo — what to observe, and when is the new size reliable?

When iPhone Duo is folded or unfolded while our app is running, what is the supported way to be notified, so the layout can re-evaluate?

Our app currently computes its board size once when the game screen appears and never recalculates, so a fold or unfold mid-game leaves it sized for the previous display. What should we observe, and at what point in the transition is the new size reliable to read?

Being notified of fold / unfold on iPhone Duo — what to observe, and when is the new size reliable?
 
 
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