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The Core iOS Developer’s Cookbook, Fifth Edition by Rich Wardwell, Erica Sadun

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Recipe: Dragging from a Scroll View

iOS’s rich set of gesture recognizers doesn’t always accomplish exactly what you’re looking for. Here’s an example. Imagine a horizontal scrolling view filled with image views, one next to another, so you can scroll left and right to see the entire collection. Now, imagine that you want to be able to drag items out of that view and add them to a space directly below the scrolling area. To do this, you need to recognize downward touches on those child views (that is, orthogonal to the scrolling direction).

This was the puzzle developer Alex Hosgrove encountered while he was trying to build an application roughly equivalent to a set of refrigerator magnet letters. Users could drag those letters down into a workspace ...

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