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I have an app with drag-and-drop functionality, and I'd like to make it conform to some sort of rule so as to make it as intuitive for my audience as possible. The iOS Human Interface Guidelines provide no such rules, as far as I could tell. However, the OS X HIG has an entire section dedicated to drag-and-drop functionality.

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Drag-and-drop has major accessibility implications, so one of the cardinal rules is that it shouldn't be the only way to accomplish something. Always think of it as a shortcut that people who are dexterous enough might want to use once they get really familiar with the UI. For mobile the common idiom is to tap the source, then tap the destination.

The other big rule is to make it so that you can tell whether or not the drop is allowed before attempting it. I'm sure you don't need examples (try dragging files around in your OS if you do), it's just really easy to forget to implement so it bears repeating.

As a side note, I think that d-n-d works extremely poorly on touch screens, especially for smart phones. Your finger ends up covering what you need to see. You can get away with it on a tablet if your targets are big enough but I have yet to use a d-n-d interface on my iPhone that didn't cause me to start swearing within seconds. And that includes Apple's built-in stuff like the text selection, which I honestly feel like is doing the best it can.

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Think affordances: The draggable things must look draggable. In Gmail, when you mouse over the message list, a little grippy texture appears on the messages and the cursor turns into an open hand.

Gmail's grippy texture

While you're dragging, the allowable targets should look drop-onto-able. Highlight them in some way to show that they can receive the object you're dragging.

In addition to @MattPutnam's comment about dragging on small screens, keep in mind that it's hard to maintain a drag across page boundaries. That is, the farther you drag a thing, the more likely you are to drop it midway. Best case scenario is you have to start over. Worst case is you drop it into something else and you have to go find it. (The very worst case is, I suppose, you don't realize you've dropped it in the wrong place. But you get the idea.)

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