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Programming iOS 4 by Matt Neuburg

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Scrolling

For the most part, the purpose of a scroll view will be to let the user scroll. A number of properties affect the user experience with regard to scrolling:

scrollEnabled
If NO, the user can’t scroll, but you can still scroll in code (as explained later in this section). You could put a UIScrollView to various creative purposes other than letting the user scroll; for example, scrolling in code to a different region of the content view might be a way of replacing one piece of interface by another.
scrollsToTop
If YES (the default), and assuming scrolling is enabled, the user can tap on the status bar as a way of making the scroll view scroll its content view to the top.
bounces
If YES (the default), then when the user scrolls to a limit of the content, it is possible to scroll somewhat further (possibly revealing the scroll view’s backgroundColor behind the content view, if a subview was covering it); the content then snaps back into place when the user releases it. Otherwise, the user experiences the limit as a sudden inability to scroll further in that direction.
alwaysBounceVertical, alwaysBounceHorizontal
If YES, and assuming that bounces is YES, then even if the contentSize in the given dimension isn’t larger than the scroll view (so that no scrolling is actually possible in that dimension), the user can nevertheless scroll somewhat and the content then snaps back into place when the user releases it; otherwise, the user experiences a simple inability to scroll ...

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