iOS 7 Programming Cookbook

2.5. Adding a Dynamic Snap Effect to Your UI Components

Problem

Using an animation, you want to snap a view in your UI to a specific point on the screen, with the elasticity of a real-world snap effect. This means that when your UI component snaps to the given point, you will feel that it has elasticity built into it.

Solution

Instantiate an object of type UISnapBehavior and add it to an animator of type UIDynamicAnimator.

Discussion

To really understand how the snap dynamic behavior works, think about a small amount of jelly covered in oil with a string attached to it, sitting on a very smooth table. I know that sentence sounds really odd. But bear with me. Now imagine from another point on the table, pulling on that string to get the jelly to move from its initial point to the point you ordered it to move to. With the oil all around it, the jelly will move smoothly from that point to where you want it to go, and because it is jelly, it will wiggle when it snaps to position. This behavior is exactly what you can achieve with the UISnapBehavior class.

One of the use cases for this is when you have an app and some views on the screen, such as images, and you want the user to be able to dictate where those views have to be moved to create a customized UI for the user. One way of handling this is using the technique that we learned in Recipe 2.3, but that solution is quite rigid and has its own use cases. Here in this recipe, we have a view on our screen, and we want to allow the user to ...