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Create a fallback button. Put it behind the main button. Set its background and text colors to
[UIColor clearColor] to ensure it won't show up. (You can't just set its alpha to 0 because that makes it ignore touches.) In Interface Builder, the fallback button should be above the main button in the list of subviews, like this:
Give it the same frame as the main button. If you're using autolayout, select both the main and fallback buttons and create constraints to keep all four edges equal.
When the main button is disabled, touches will pass through to the fallback button. When the main button is enabled, it will catch all the touches and the fallback button won't receive any.
Connect the fallback button to an action so you can detect when it's tapped.