Problem
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.
Solution
Initialize an object of type UIGravityBehavior and add your UI components that need gravity to this object. After you are done, create an instance of UIDynamicAnimator, add your gravity behavior to the animator, and let the animator take care of the rest of the work for you.
Discussion
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:
#import "WSYViewController.h"
@interface WSYViewController ()
@property (nonatomic,strong)UIView *squareView;
@property (nonatomic,strong)UIDynamicAnimator * animator;
@end
@implementation WSYViewController
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view, typically from a nib.
//create our little square view add it to self.view
self.squareView = [[UIView alloc]initWithFrame:CGRectMake(0, 0, 100, 100)];
self.squareView.backgroundColor =[UIColor greenColor];
self.squareView.center = self.view.center;
[self.view addSubview:self.squareView];
self.animator = [[UIDynamicAnimator alloc]initWithReferenceView:self.view];
UIGravityBehavior * gravity = [[UIGravityBehavior alloc]initWithItems:@[self.squareView]];
[self.animator addBehavior:gravity];
}
@end
Now if you run your app, as soon as your view controller’s view appears on screen, you will see the colored view drop from the center of the screen all the way down and out of the screen.
Adding Gravity to your UI Components,布布扣,bubuko.com
Adding Gravity to your UI Components
原文：http://www.cnblogs.com/ios-mark/p/3762734.html