You want your UI components to have gravity, so that if they are dragged up to the top of the screen, they will descend on their own. Combining this with the collision behavior that you will learn later, you can create UI components that fall from their current location until they collide with a path that you’ll specify.
Initialize an object of type
UIGravityBehavior and add your UI components
that need gravity to this object. After you are done, create an instance
UIDynamicAnimator, add your
gravity behavior to the animator, and let the animator take care of the
rest of the work for you.
For the purpose of this recipe, we are going to create a simple colored square view in our single-view application and place that view at the center of the screen. We will then add gravity to that view and watch it fall from the center all the way down and eventually outside the bounds of the screen.
So let’s start by defining our animator and the view:
Next, we are going to create our little view, assign a color to
it, and place it at the center of our view controller’s view. Then we
will create an instance of the
UIGravityBehavior class using its
initWithItems: initializer. This ...