Question

I am using a pan gesture to drag an item in my app. I know I could use a UIScrollView to get that “bounce” effect when dragging to an edge. However, that seems like a hack, so I’d like to find a way to get that effect when manually dragging an item (setting its frame or center point). I’m curious if there’s a standard way, or a physics library in the Accelerate framework, something like that which wouldn’t require me implementing my own physics engine.

Answer 1

The standard UIView animation curve is one that uses the accelerate / decelerate curve, which should gives a nice “bounce back” action.

It’s really easy to use. Once you decide you want to “snap back” to a location, do this:

[UIView beginAnimations:nil context:nil];
CGRect newFrame = viewToAnimate.frame;
newFrame.origin.x = snapBackXcoordinate;
viewToAnimate.frame = newFrame;
[UIView commitAnimations];

to entirely emulate the UIScrollView you’ll also need to set a “maximum stretch” distance before you stop stretching more.

But why re-invent the wheel? The UIKit components you’re given are there to use. If putting your view into a UIScrollView does the trick, go ahead and use it. Saves you develop and debug time.