谈谈iOS中的动力学！！！！

UIDynamic物理仿真 下面介绍一下各种属性

一、简单介绍

1.什么是UIDynamic

UIDynamic是从iOS 7开始引入的一种新技术，隶属于UIKit框架,可以认为是一种物理引擎，能模拟和仿真现实生活中的物理现象
如：重力、弹性碰撞等现象

2.物理引擎的价值

广泛用于游戏开发，经典成功案例是“愤怒的小鸟”
让开发人员可以在远离物理学公式的情况下，实现炫酷的物理仿真效果
提高了游戏开发效率，产生更多优秀好玩的物理仿真游戏

3.知名的2D物理引擎

Box2d
Chipmunk

二、使用步骤

要想使用UIDynamic来实现物理仿真效果，大致的步骤如下

（1）创建一个物理仿真器（顺便设置仿真范围）
（2）创建相应的物理仿真行为（顺便添加物理仿真元素）
（3）将物理仿真行为添加到物理仿真器中  开始仿真

三、相关说明

1.三个概念

（1）谁要进行物理仿真？
　　物理仿真元素（Dynamic Item）
 
（2）执行怎样的物理仿真效果？怎样的动画效果？
　　物理仿真行为（Dynamic Behavior） 
 
（3）让物理仿真元素执行具体的物理仿真行为
　　物理仿真器（Dynamic Animator）

2.物理仿真元素

注意：

不是任何对象都能做物理仿真元素
不是任何对象都能进行物理仿真

物理仿真元素要素：

任何遵守了UIDynamicItem协议的对象
UIView默认已经遵守了UIDynamicItem协议，因此任何UI控件都能做物理仿真
UICollectionViewLayoutAttributes类默认也遵守UIDynamicItem协议

四、物理仿真器 ------------UIDynamicAnimator

须知：它可以让物理仿真元素执行物理仿真行为，它是UIDynamicAnimator类型的对象

    物理仿真器属性方法说明

（1）UIDynamicAnimator的常见方法
- (instancetype)initWithReferenceView:(UIView *)view; //初始化一个仿真器。view参数：是一个参照视图，表示物理仿真的范围
　　- (void)addBehavior:(UIDynamicBehavior *)behavior; 　　//添加1个物理仿真行为
　　- (void)removeBehavior:(UIDynamicBehavior *)behavior;　　//移除1个物理仿真行为
　　- (void)removeAllBehaviors; 　　//移除之前添加过的所有物理仿真行为

   - (NSArray<id<UIDynamicItem>> *)itemsInRect:(CGRect)rect; 
   - (void)updateItemUsingCurrentState:(id <UIDynamicItem>)item;//(更新dynamic item在UIDynamicAnimator内部的代表的状态)，dynamic item被加入到UIDynamicAnimator后，你更改了dynamic item的状态，你应该使用这个方法更新dynamic item的状态。
   - (NSTimeInterval)elapsedTime;   //返回动画开始到现在的时间间隔

（2）UIDynamicAnimator的常见属性

　　@property (nonatomic, readonly) UIView* referenceView;  //参照视图 
　　@property (nonatomic, readonly, copy) NSArray* behaviors;//添加到物理仿真器中的所有物理仿真行为
　　@property (nonatomic, readonly, getter = isRunning) BOOL running;//是否正在进行物理仿真
　　@property (nonatomic, assign) id <UIDynamicAnimatorDelegate> delegate;//代理对象（能监听物理仿真器的仿真过程，比如开始和结束）{ 
        @optional
       - (void)dynamicAnimatorWillResume:(UIDynamicAnimator *)animator;
       - (void)dynamicAnimatorDidPause:(UIDynamicAnimator *)animator;
   }

（3）UIDynamicAnimator中的Collection View Additions：     
    •    - (UICollectionViewLayoutAttributes *)layoutAttributesForCellAtIndexPath:(NSIndexPath *)indexPath；//返回指定位置collection view cell的布局属性
    •    - (UICollectionViewLayoutAttributes *)layoutAttributesForDecorationViewOfKind:(NSString *)decorationViewKind atIndexPath:(NSIndexPath *)indexPath； //返回indexPath位置上collection view cell的指定(用decorationViewKind指定)decorationView的布局属性。
    •    - (UICollectionViewLayoutAttributes *)layoutAttributesForSupplementaryViewOfKind:(NSString *)kind atIndexPath:(NSIndexPath *)indexPath；//返回indexPath位置上collection view cell的指定(用kind指定)SupplementaryView的布局属性。

五、物理仿真行为 ------------UIDynamicBehavior

（1）UIDynamic提供了以下几种物理仿真行为
UIGravityBehavior：重力行为
UICollisionBehavior：碰撞行为
UISnapBehavior：捕捉行为
UIPushBehavior：推动行为
UIAttachmentBehavior：附着行为
UIDynamicItemBehavior：动力元素行为

（2）物理仿真行为须知

上述所有物理仿真行为都继承自UIDynamicBehavior，所有的UIDynamicBehavior都可以独立进行，组合使用多种行为时，可以实现一些比较复杂的效果。

（3）UIDynamicBehavior属性方法介绍

- (void)addChildBehavior:(UIDynamicBehavior *)behavior;
- (void)removeChildBehavior:(UIDynamicBehavior *)behavior;

@property (nonatomic, readonly, copy) NSArray<__kindof UIDynamicBehavior *> *childBehaviors;

@property (nullable, nonatomic,copy) void (^action)(void);

- (void)willMoveToAnimator:(nullable UIDynamicAnimator *)dynamicAnimator; 

@property (nullable, nonatomic, readonly) UIDynamicAnimator *dynamicAnimator;

UIGravityBehavior（重力）

         - (instancetype)initWithItems:(NSArray<id <UIDynamicItem>> *)items;//初始化

        - (void)addItem:(id <UIDynamicItem>)item;//添加
        - (void)removeItem:(id <UIDynamicItem>)item;//移除
        @property (nonatomic, readonly, copy) NSArray<id <UIDynamicItem>> *items;//行为数组

        @property (readwrite, nonatomic) CGVector gravityDirection;//运动方向，默认是(0.0f, 1.0f)。dx为-1.0f时向左运动，dy为-1.0时向上运动，所以根据0.0~1.0可以定位所有的方向。

        @property (readwrite, nonatomic) CGFloat angle;//重力方向（是一个角度，以x轴正方向为0°，顺时针正数，逆时针负数）
        @property (readwrite, nonatomic) CGFloat magnitude;/量级（用来控制加速度，1.0代表加速度是1000 points /second²）可以去负数
       - (void)setAngle:(CGFloat)angle magnitude:(CGFloat)magnitude;//

UICollisionBehavior（碰撞）

- (instancetype)initWithItems:(NSArray<id <UIDynamicItem>> *)items;//初始化

- (void)addItem:(id <UIDynamicItem>)item;//添加
- (void)removeItem:(id <UIDynamicItem>)item;//移除

@property (nonatomic, readonly, copy) NSArray<id <UIDynamicItem>> *items;//碰撞行为数组

@property (nonatomic, readwrite) UICollisionBehaviorMode collisionMode;//碰撞模式（分三种，   
        UICollisionBehaviorModeItems        = 1 << 0,元素碰撞
        UICollisionBehaviorModeBoundaries   = 1 << 1, 边界碰撞
        UICollisionBehaviorModeEverything   = NSUIntegerMax 全体碰撞）

@property (nonatomic, readwrite) BOOL translatesReferenceBoundsIntoBoundary;//是否以参照视图的bounds为边界

@property (nullable, nonatomic, weak, readwrite) id <UICollisionBehaviorDelegate> collisionDelegate;//代理
{-------------------------------------代理方法
- (void)collisionBehavior:(UICollisionBehavior *)behavior beganContactForItem:(id <UIDynamicItem>)item1 withItem:(id <UIDynamicItem>)item2 atPoint:(CGPoint)p;
- (void)collisionBehavior:(UICollisionBehavior *)behavior endedContactForItem:(id <UIDynamicItem>)item1 withItem:(id <UIDynamicItem>)item2;

// The identifier of a boundary created with translatesReferenceBoundsIntoBoundary or setTranslatesReferenceBoundsIntoBoundaryWithInsets is nil
- (void)collisionBehavior:(UICollisionBehavior*)behavior beganContactForItem:(id <UIDynamicItem>)item withBoundaryIdentifier:(nullable id <NSCopying>)identifier atPoint:(CGPoint)p;
- (void)collisionBehavior:(UICollisionBehavior*)behavior endedContactForItem:(id <UIDynamicItem>)item withBoundaryIdentifier:(nullable id <NSCopying>)identifier;
}
 - (void)setTranslatesReferenceBoundsIntoBoundaryWithInsets:(UIEdgeInsets)insets;//

- (void)addBoundaryWithIdentifier:(id <NSCopying>)identifier forPath:(UIBezierPath *)bezierPath;//添加一个path作为碰撞边界
- (void)addBoundaryWithIdentifier:(id <NSCopying>)identifier fromPoint:(CGPoint)p1 toPoint:(CGPoint)p2;//添加一条线作为碰撞边界
- (nullable UIBezierPath *)boundaryWithIdentifier:(id <NSCopying>)identifier;//
- (void)removeBoundaryWithIdentifier:(id <NSCopying>)identifier; //移除某个的
@property (nullable, nonatomic, readonly, copy) NSArray<id <NSCopying>> *boundaryIdentifiers;//
- (void)removeAllBoundaries;//移除所有

UISnapBehavior（捕捉）

- (instancetype)initWithItem:(id <UIDynamicItem>)item snapToPoint:(CGPoint)point;//初始化

@property (nonatomic, assign) CGPoint snapPoint;
@property (nonatomic, assign) CGFloat damping;//用于减幅、减震（取值范围是0.0 ~ 1.0，值越大，震动幅度越小）
注意：
//2.执行捕捉行为
43     //注意：这个控件只能用在一个仿真行为上，如果要拥有持续的仿真行为，那么需要把之前的所有仿真行为删除
44     //删除之前的所有仿真行为
45     [self.animator removeAllBehaviors];
46     [self.animator addBehavior:snap];

UIPushBehavior（推动）

- (instancetype)initWithItems:(NSArray<id <UIDynamicItem>> *)items mode:(UIPushBehaviorMode)mode;
- (void)addItem:(id <UIDynamicItem>)item;
- (void)removeItem:(id <UIDynamicItem>)item;
@property (nonatomic, readonly, copy) NSArray<id <UIDynamicItem>> *items;

- (UIOffset)targetOffsetFromCenterForItem:(id <UIDynamicItem>)item;//
- (void)setTargetOffsetFromCenter:(UIOffset)o forItem:(id <UIDynamicItem>)item;//

@property (nonatomic, readonly) UIPushBehaviorMode mode;//{
    UIPushBehaviorModeContinuous,//持续的力
    UIPushBehaviorModeInstantaneous //一次性的力
}
@property (nonatomic, readwrite) BOOL active;//默认值是YES，将其设置为NO，可以将行为停止。

@property (readwrite, nonatomic) CGFloat angle;//角度
@property (readwrite, nonatomic) CGFloat magnitude;//加速度
@property (readwrite, nonatomic) CGVector pushDirection;//运动方向

- (void)setAngle:(CGFloat)angle magnitude:(CGFloat)magnitude;//

UIAttachmentBehavior(附着)

- (instancetype)initWithItem:(id <UIDynamicItem>)item attachedToAnchor:(CGPoint)point;
- (instancetype)initWithItem:(id <UIDynamicItem>)item offsetFromCenter:(UIOffset)offset attachedToAnchor:(CGPoint)point;

- (instancetype)initWithItem:(id <UIDynamicItem>)item1 attachedToItem:(id <UIDynamicItem>)item2;
- (instancetype)initWithItem:(id <UIDynamicItem>)item1 offsetFromCenter:(UIOffset)offset1 attachedToItem:(id <UIDynamicItem>)item2 offsetFromCenter:(UIOffset)offset2;


+ (instancetype)slidingAttachmentWithItem:(id <UIDynamicItem>)item1 attachedToItem:(id <UIDynamicItem>)item2 attachmentAnchor:(CGPoint)point axisOfTranslation:(CGVector)axis;

+ (instancetype)slidingAttachmentWithItem:(id <UIDynamicItem>)item attachmentAnchor:(CGPoint)point axisOfTranslation:(CGVector)axis;

+ (instancetype)limitAttachmentWithItem:(id <UIDynamicItem>)item1 offsetFromCenter:(UIOffset)offset1 attachedToItem:(id <UIDynamicItem>)item2 offsetFromCenter:(UIOffset)offset2;

+ (instancetype)fixedAttachmentWithItem:(id <UIDynamicItem>)item1 attachedToItem:(id <UIDynamicItem>)item2 attachmentAnchor:(CGPoint)point;

+ (instancetype)pinAttachmentWithItem:(id <UIDynamicItem>)item1 attachedToItem:(id <UIDynamicItem>)item2 attachmentAnchor:(CGPoint)point;

@property (nonatomic, readonly, copy) NSArray<id <UIDynamicItem>> *items;

@property (readonly, nonatomic) UIAttachmentBehaviorType attachedBehaviorType;//{
    UIAttachmentBehaviorTypeItems,
    UIAttachmentBehaviorTypeAnchor 
}
@property (readwrite, nonatomic) CGPoint anchorPoint;//

@property (readwrite, nonatomic) CGFloat length; //
@property (readwrite, nonatomic) CGFloat damping; //振幅
@property (readwrite, nonatomic) CGFloat frequency; //频率
@property (readwrite, nonatomic) CGFloat frictionTorque;//
@property (readwrite, nonatomic) UIFloatRange attachmentRange; //

UIDynamicItemBehavior：动力元素行为

- (instancetype)initWithItems:(NSArray<id <UIDynamicItem>> *)items;//

- (void)addItem:(id <UIDynamicItem>)item;//
- (void)removeItem:(id <UIDynamicItem>)item;//
@property (nonatomic, readonly, copy) NSArray<id <UIDynamicItem>> *items;//

@property (readwrite, nonatomic) CGFloat elasticity; //（弹性系数）决定了碰撞的弹性程度，比如碰撞时物体的弹性
@property (readwrite, nonatomic) CGFloat friction;//（摩擦系数）决定了沿接触面滑动时的摩擦力大小
@property (readwrite, nonatomic) CGFloat density;//（密度） 跟size结合使用，计算物体的总质量。质量越大，物体加速或减速就越困难
@property (readwrite, nonatomic) CGFloat resistance;//（阻力）：决定线性移动的阻力大小，与摩擦系数不同，摩擦系数只作用于滑动运动
@property (readwrite, nonatomic) CGFloat angularResistance;//（角阻力） ：决定旋转运动时的阻力大小

@property (readwrite, nonatomic) CGFloat charge;//

@property (nonatomic, getter = isAnchored) BOOL anchored;//

@property (readwrite, nonatomic) BOOL allowsRotation; //（允许旋转）：这个属性很有意思，它在真实的物理世界没有对应的模型。设置这个属性为 NO 物体就完全不会转动，而无论施加多大的转动力

- (void)addLinearVelocity:(CGPoint)velocity forItem:(id <UIDynamicItem>)item;//
- (CGPoint)linearVelocityForItem:(id <UIDynamicItem>)item;//

- (void)addAngularVelocity:(CGFloat)velocity forItem:(id <UIDynamicItem>)item;//
- (CGFloat)angularVelocityForItem:(id <UIDynamicItem>)item;/