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Programming iOS 4 by Matt Neuburg

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Scroll View Touches

Improvements in the scroll view implementation have eliminated most of the worry once associated with scroll view touches. A scroll view will interpret a drag or a pinch as a command to scroll or zoom, and any other gesture will fall through to the subviews; thus buttons and similar interface objects inside a scroll view work just fine.

You can even put a scroll view inside a scroll view, and this can be quite a useful thing to do, in contexts where you might not think of it at first. A WWDC 2010 presentation uses as an example Apple’s Photos app, where a single photo fills the screen: you can page-scroll from one photo to the next, and you can zoom the current photo with a pinch-out gesture. This, the presentation suggests, can be implemented with a scroll view inside a scroll view: the outer scroll view is for paging between images, and the inner scroll view contains the current image and is for zooming.

Gesture recognizers (Chapter 18) have also greatly simplified the task of adding custom gestures to a scroll view. For instance, some older code in Apple’s documentation, showing how to implement a double-tap to zoom in and a two-finger tap to zoom out, uses old-fashioned touch handling, but this is no longer necessary. Simply attach to your scroll view’s scalable subview any gesture recognizers for these sorts of gesture, and they will mediate automatically among the possibilities.

In the past, making something inside a scroll view draggable required setting ...

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