iOS UIKit Dynamics（物理碰撞，动画）

大部分人或许觉得动画用UIView 或是CoreAnimation的动画效果，那为何还需要UIKit 中的UIDynamic？
答：UIDynamic 为使用者提供更符合现实效果的互动行为（比如：自由落体能量消耗的过程）

一、需求：请实现下图所示的动画效果(物理碰撞)

JJ's Animation.gif

二、上面引入过UIDynamic 可以实现碰撞的物理效果动画，how to start ?

• 在工程中导入UIKit，从Headers 找一些UIDynamic，发现了UIDynamicAnimator.h,UIDynamicBehavior,UIDynamicItemBehavior
• 既然是要动画效果 那就应该是UIDynamicAnimator.h
• 到UIDynamicAnimator.h 头文件去查看注释

// 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;

• 大概意思是使用这个初始化方法的时候，需要为你的试图关联behaviors
• what's the behaviors ? jump into defination 然而还不是很懂，UIDynamicBehavior,那就查看API (注意文档中的如下描述)。
• This parent class, UIDynamicBehavior, is inherited by the primitive dynamic behavior classes UIAttachmentBehavior, UICollisionBehavior, UIGravityBehavior, UIDynamicItemBehavior, UIPushBehavior, and UISnapBehavior.
• 文档中提到UIDynamicBehavior的父类，那么UIDynamicBehavior父类(Dynamic 行为相关类)描述什么行为呢？
  • UIAttachmentBehavior：附着行为
  • UICollisionBehavior：碰撞行为
  • UIGravityBehavior：重力行为
  • UIDynamicItemBehavior：动态元素行为
  • UIPushBehavior：推行为
  • UISnapBehavior：吸附行为
• 上述的各种行为可单独使用，也可以组合使用更复杂的动画效果。

解决方案的思路就引导到这边，也就可以实现上述需求。Dynamic 行为相关类的属性可自行研究。

三、解决方案。

• 上述需求的解决方案（我这边用的是6sP的模拟器奥，因为在设置floor的y坐标是600）
• 既然是球落地效果，必然有两个模拟对象：floor，basketBall

  • 1.创建UIDynamicAnimator 对象
    self.animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];

  • 2.刚刚在API 看到了初始化UIDynamicAnimator需要关联一个或者多个行为

NSArray *animatorObjects = @[self.imageV1];
 // 刚刚在API 看到了初始化UIDynamicAnimator需要关联一个或者多个行为
 self.gravityBehav = [[UIGravityBehavior alloc] initWithItems:animatorObjects];
 self.colision = [[UICollisionBehavior alloc] initWithItems:animatorObjects];
     ```
   -  3.如果你想监听碰撞的状态，可以设置一下代理
   ```
 self.colision.collisionDelegate = self;
   ```
   - 4.设置碰撞边界
   ```
[self.colision addBoundaryWithIdentifier:@"boundaryLine" fromPoint:CGPointMake(0, 600) toPoint:CGPointMake(SCREENWITH, 600)];
   ```
   - 5.设置动态行为参数
     ```
// 设置动态行为参数
 UIDynamicItemBehavior *itemBehavior = [[UIDynamicItemBehavior alloc] initWithItems:animatorObjects];
 // 设置弹性
 [itemBehavior setElasticity:0.5];
     ```
   - 6.行为创建了，别忘了给animator添加上
     ```
[self.animator addBehavior:self.gravityBehav];
 [self.animator addBehavior:self.colision];
 [self.animator addBehavior:itemBehavior];
     ```

### 三、下面代码只能说功能实现
 - PS：以下代码不符合代码书写规范
 
```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 ()

@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 {
 // 创建碰撞需要的对象
 [self jjCreateBall];
 [self jjCreateFloor];
 // 创建UIDynamicAnimator 对象
 self.animator = [[UIDynamicAnimator alloc] initWithReferenceView:self.view];

 NSArray *animatorObjects = @[self.imageV1];
 // 刚刚在API 看到了初始化UIDynamicAnimator需要关联一个或者多个行为
 self.gravityBehav = [[UIGravityBehavior alloc] initWithItems:animatorObjects];
 self.colision = [[UICollisionBehavior alloc] initWithItems:animatorObjects];
 // 这里设置代理是为了监听碰撞状态的，可以去了解一下代理方法
 self.colision.collisionDelegate = self;

 // 设置碰撞边界
 [self.colision addBoundaryWithIdentifier:@"boundaryLine" fromPoint:CGPointMake(0, 600) toPoint:CGPointMake(SCREENWITH, 600)];
 // 设置动态行为参数
 UIDynamicItemBehavior *itemBehavior = [[UIDynamicItemBehavior alloc] initWithItems:animatorObjects];
 // 设置弹性
 [itemBehavior setElasticity:0.5];

 // 创建了行为需要animator添加
 [self.animator addBehavior:self.gravityBehav];
 [self.animator addBehavior:self.colision];
 [self.animator addBehavior:itemBehavior];

}

-(void)collisionBehavior:(UICollisionBehavior *)behavior endedContactForItem:(id)item withBoundaryIdentifier:(id)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点击下载

The last but not least：
1.如果想更好的玩转UIDynamics请查阅fancypixel有关Playing With UIDynamics in iOS 9
PS:1.录屏 Gif （for Mac）软件：GIPHY CAPTURE
2.那个boundary（边界）设置的适合下面的floor是一样高的，我选的图片是有白色背景的方形（Rect），没有把背景改成透明的，会感觉到球的底面没有和floor 重合的感觉。