@property (nonatomic, readonly, copy) NSArray* items;  

 @property (readwrite, nonatomic) CGFloat elasticity; // Usually between 0 (inelastic) and 1 (collide elastically) 弹性系数 在0~1之间

 @property (readwrite, nonatomic) CGFloat friction; // 0 being no friction between objects slide along each other 摩擦力系数

 @property (readwrite, nonatomic) CGFloat density; // 1 by default 跟size大小相关，计算物体块的质量。

 @property (readwrite, nonatomic) CGFloat resistance; // 0: no velocity damping 阻力系数

 @property (readwrite, nonatomic) CGFloat angularResistance; // 0: no angular velocity damping 旋转阻力

 @property (readwrite, nonatomic) BOOL allowsRotation; // force an item to never rotate  是否能旋转

 //

 //  YFDynamicItemViewController.m

 //  BigShow1949

 //

 //  Created by apple on 16/8/25.

 //  Copyright © 2016年 BigShowCompany. All rights reserved.

 //

 #import "YFDynamicItemViewController.h"

 @interface YFDynamicItemViewController ()

 @property(nonatomic,strong)UIDynamicAnimator *animator;

 @property (nonatomic, strong) UIView *redView;

 @end

 @implementation YFDynamicItemViewController

 - (void)viewDidLoad {

     [super viewDidLoad];

     self.view.backgroundColor = [UIColor whiteColor];

     UIView *redView = [[UIView alloc] init];

     redView.frame = CGRectMake(, , , );

     redView.backgroundColor = [UIColor redColor];

     self.redView = redView;

     [self.view addSubview:redView];

 }

 - (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {

     [self testGravityAndCollsion];

 }

 /**

  *  重力行为+碰撞检测

  */

 -(void)testGravityAndCollsion

 {

     //1.重力行为

     UIGravityBehavior *gravity=[[UIGravityBehavior alloc]init];

     [gravity addItem:self.redView];

     //2碰撞检测行为

     UICollisionBehavior *collision=[[UICollisionBehavior alloc]init];

     [collision addItem:self.redView];

     //让参照视图的边框成为碰撞检测的边界

     collision.translatesReferenceBoundsIntoBoundary=YES;

     // 动力元素行为

     UIDynamicItemBehavior * itemBehavior = [[UIDynamicItemBehavior alloc] initWithItems:@[self.redView]];

     itemBehavior.elasticity = 1.2;

     itemBehavior.friction = .;

     itemBehavior.density = ;

     itemBehavior.resistance = ; // 阻力系数

     itemBehavior.allowsRotation = NO;

     itemBehavior.angularResistance = ;

     //3.执行仿真

     [self.animator addBehavior:gravity];

     [self.animator addBehavior:collision];

     [self.animator addBehavior:itemBehavior];

 }

 -(UIDynamicAnimator *)animator

 {

     if (_animator==nil) {

         //创建物理仿真器（ReferenceView:参照视图，设置仿真范围）

         self.animator=[[UIDynamicAnimator alloc]initWithReferenceView:self.view];

     }

     return _animator;

 }

 @end