大部分人或許覺得動畫用UIView 或是CoreAnimation的動畫效果，那為何還需要UIKit 中的UIDynamic？
答：UIDynamic 為使用者提供更符合現實效果的互動行為（比如：自由落體能量消耗的過程）

一、需求：請實現下圖所示的動畫效果(物理碰撞)

JJ's Animation.gif

二、上面引入過UIDynamic 可以實現碰撞的物理效果動畫，how to start ?

• 在工程中導入UIKit，從Headers 找一些UIDynamic，發現了UIDynamicAnimator.h,UIDynamicBehavior,UIDynamicItemBehavior
• 既然是要動畫效果 那就應該是UIDynamicAnimator.h
• 到UIDynamicAnimator.h 頭文件去查看注釋

// When you initialize a dynamic animator with this method, you should only associates views with your behaviors.
// the behaviors (and their dynamic items) that you add to the animator employ the reference view’s coordinate system.
- (instancetype)initWithReferenceView:(UIView *)view NS_DESIGNATED_INITIALIZER;

• 大概意思是使用這個初始化方法的時候，需要為你的試圖關聯behaviors
• what's the behaviors ? jump into defination 然而還不是很懂，UIDynamicBehavior,那就查看API (注意文檔中的如下描述)。
• This parent class, UIDynamicBehavior, is inherited by the primitive dynamic behavior classes UIAttachmentBehavior, UICollisionBehavior, UIGravityBehavior, UIDynamicItemBehavior, UIPushBehavior, and UISnapBehavior.
• 文檔中提到UIDynamicBehavior的父類，那么UIDynamicBehavior父類(Dynamic 行為相關類)描述什么行為呢？
  • UIAttachmentBehavior：附著行為
  • UICollisionBehavior：碰撞行為
  • UIGravityBehavior：重力行為
  • UIDynamicItemBehavior：動態元素行為
  • UIPushBehavior：推行為
  • UISnapBehavior：吸附行為
• 上述的各種行為可單獨使用，也可以組合使用更復雜的動畫效果。

解決方案的思路就引導到這邊，也就可以實現上述需求。Dynamic 行為相關類的屬性可自行研究。

三、解決方案。

• 上述需求的解決方案（我這邊用的是6sP的模擬器奧，因為在設置floor的y坐標是600）
• 既然是球落地效果，必然有兩個模擬對象：floor，basketBall

  • 1.創建UIDynamicAnimator 對象
    self.animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];

  • 2.剛剛在API 看到了初始化UIDynamicAnimator需要關聯一個或者多個行為

NSArray *animatorObjects = @[self.imageV1];
 // 剛剛在API 看到了初始化UIDynamicAnimator需要關聯一個或者多個行為
 self.gravityBehav = [[UIGravityBehavior alloc] initWithItems:animatorObjects];
 self.colision = [[UICollisionBehavior alloc] initWithItems:animatorObjects];
     ```
   -  3.如果你想監聽碰撞的狀態，可以設置一下代理
   ```
 self.colision.collisionDelegate = self;
   ```
   - 4.設置碰撞邊界
   ```
[self.colision addBoundaryWithIdentifier:@"boundaryLine" fromPoint:CGPointMake(0, 600) toPoint:CGPointMake(SCREENWITH, 600)];
   ```
   - 5.設置動態行為參數
     ```
// 設置動態行為參數
 UIDynamicItemBehavior *itemBehavior = [[UIDynamicItemBehavior alloc] initWithItems:animatorObjects];
 // 設置彈性
 [itemBehavior setElasticity:0.5];
     ```
   - 6.行為創建了，別忘了給animator添加上
     ```
[self.animator addBehavior:self.gravityBehav];
 [self.animator addBehavior:self.colision];
 [self.animator addBehavior:itemBehavior];
     ```

### 三、下面代碼只能說功能實現
 - PS：以下代碼不符合代碼書寫規范
 
```objc
//
// ViewController.m
// DynamicAnimation
//
// Created by JeversonJee on 16/5/27.
// Copyright ? 2016年 JeversonJee. All rights reserved.
//

#import "ViewController.h"

#define SCREENWITH [UIScreen mainScreen].bounds.size.width
#define SCREENHEIGHT [UIScreen mainScreen].bounds.size.height

@interface ViewController ()<UICollisionBehaviorDelegate>

@property (nonatomic, strong) UIDynamicAnimator *animator;
@property (nonatomic, strong) UIGravityBehavior *gravityBehav;
@property (nonatomic, strong) UICollisionBehavior *colision;

@property (nonatomic, strong) UIImageView *imageV1;
@property (nonatomic, strong) UIImageView *imageV2;

@end

@implementation ViewController

- (void)viewDidLoad {
 [super viewDidLoad];
 // Do any additional setup after loading the view, typically from a nib.
 [self jjCollisionBehav];
}

- (void)jjCollisionBehav {
 // 創建碰撞需要的對象
 [self jjCreateBall];
 [self jjCreateFloor];
 // 創建UIDynamicAnimator 對象
 self.animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];

 NSArray *animatorObjects = @[self.imageV1];
 // 剛剛在API 看到了初始化UIDynamicAnimator需要關聯一個或者多個行為
 self.gravityBehav = [[UIGravityBehavior alloc] initWithItems:animatorObjects];
 self.colision = [[UICollisionBehavior alloc] initWithItems:animatorObjects];
 // 這里設置代理是為了監聽碰撞狀態的，可以去了解一下代理方法
 self.colision.collisionDelegate = self;

 // 設置碰撞邊界
 [self.colision addBoundaryWithIdentifier:@"boundaryLine" fromPoint:CGPointMake(0, 600) toPoint:CGPointMake(SCREENWITH, 600)];
 // 設置動態行為參數
 UIDynamicItemBehavior *itemBehavior = [[UIDynamicItemBehavior alloc] initWithItems:animatorObjects];
 // 設置彈性
 [itemBehavior setElasticity:0.5];

 // 創建了行為需要animator添加
 [self.animator addBehavior:self.gravityBehav];
 [self.animator addBehavior:self.colision];
 [self.animator addBehavior:itemBehavior];

}

-(void)collisionBehavior:(UICollisionBehavior *)behavior endedContactForItem:(id<UIDynamicItem>)item withBoundaryIdentifier:(id<NSCopying>)identifier {

 NSLog(@"collisionBehavior=%@", item);
 NSString *identifierStr = [NSString stringWithFormat:@"%@", identifier];
 NSLog(@"s=%@", identifier);
 if ( [identifierStr isEqualToString:@"boundaryLine"] ) {
   [self.imageV2 setBackgroundColor:[UIColor grayColor]];
   [UIView animateWithDuration:2 animations:^(void){
     [self.imageV2 setBackgroundColor:[UIColor grayColor]];
   }];
 }

}

-(void)jjCreateBall {

 _imageV1 = [[UIImageView alloc] initWithImage:[UIImage imageNamed:@"basketBall"]];
 _imageV1.frame = CGRectMake(100, 100, 60, 60);
 [self.view addSubview:_imageV1];
}

- (void)jjCreateFloor {

 _imageV2 = [[UIImageView alloc] init];

 [_imageV2 setBackgroundColor:[UIColor grayColor]];
 _imageV2.frame = CGRectMake(0, 600, SCREENWITH, SCREENHEIGHT - 400);
 [self.view addSubview:_imageV2];
}

-(void)didReceiveMemoryWarning {
 [super didReceiveMemoryWarning];
 // Dispose of any resources that can be recreated.
}

@end

四、UIDynamicDemo：GitHub/jeversonjee點擊下載

The last but not least：
1.如果想更好的玩轉UIDynamics請查閱fancypixel有關Playing With UIDynamics in iOS 9
PS:1.錄屏 Gif （for Mac）軟件：GIPHY CAPTURE
2.那個boundary（邊界）設置的適合下面的floor是一樣高的，我選的圖片是有白色背景的方形（Rect），沒有把背景改成透明的，會感覺到球的底面沒有和floor 重合的感覺。