UIDynamicAnimator

可以通過該類添加不同的行為，來實現一些動態效果。

初始化方法

// the behaviors (and their dynamic items) that you add to the animator employ the reference view’s coordinate system.
- (instancetype)initWithReferenceView:(UIView *)view

添加到animator上的行為使用reference view的坐標系統。

添加和移除行為

- (void)addBehavior:(UIDynamicBehavior *)behavior;//添加指定的行為動畫
- (void)removeBehavior:(UIDynamicBehavior *)behavior;//移除指定的行為動畫
- (void)removeAllBehaviors;//移除所有的行為

其他屬性

- (NSTimeInterval)elapsedTime;

行為執行的時間，需要注意的是如果我們對同一個行為進行添加和移除操作時，時間會累積。

@property (nonatomic, readonly, getter = isRunning) BOOL running;

判定animator中是否還有行為正在執行

@property (nullable, nonatomic, weak) id <UIDynamicAnimatorDelegate> delegate;

代理協議的實現

- (void)dynamicAnimatorWillResume:(UIDynamicAnimator *)animator;//add行為時調用
- (void)dynamicAnimatorDidPause:(UIDynamicAnimator *)animator;//remove行為時調用

UIDynamicItem

說明一下，在很多方法里注意到有很多UIDynamicItem參數的存在。如
- (void)updateItemUsingCurrentState:(id <UIDynamicItem>)item;
其實是說item是實現了UIDynamicItem協議的類，我們發現 UIView(所有視圖控件的父類)實現了該協議。一般情況下，在遇到UIDynamicItem時，直接傳我們的視圖就行了。

UISnapBehavior

將物體通過動畫吸附到一個點上

UITapGestureRecognizer *tap = [[UITapGestureRecognizer alloc]initWithTarget:self action:@selector(tapGesture:)];
[self.view addGestureRecognizer:tap];
- (void)tapGesture:(UITapGestureRecognizer *)gesture
{
    CGPoint tapPoint = [gesture locationInView:self.view];
    if (_snapBehavior) {
        [self.animator removeBehavior:_snapBehavior];
        _snapBehavior = nil;
    }
    _snapBehavior = [[UISnapBehavior alloc]initWithItem:self.imageView1 snapToPoint:tapPoint];
    _snapBehavior.action = ^(){
        NSLog(@"UISnapBehavior 在執行");
    };
    _snapBehavior.damping = 0.9;
    [self.animator addBehavior:_snapBehavior];
}

通過觀察action和協議方法的實現，發現物體到達指定點之后會停止行為。

UIGravityBehavior

給物體添加一個類似于自由落體的運動

_gravityBehavior = [[UIGravityBehavior alloc]initWithItems:@[_imageView1]];
_gravityBehavior.gravityDirection = CGVectorMake(0.0f, 1.0f);
[self.animator addBehavior:_gravityBehavior];

gravityDirection

運動的方向，默認是(0.0f, 1.0f)。dx為-1.0f時向左運動，dy為-1.0時向上運動，所以根據0.0~1.0可以定位所有的方向。

angle

也是設置運動的方向，是根據角度來計算的。坐標系為左側為x的正方向，下側為y的正方向。可使用0~M_PI*2之間的值來定位所有方向。

magnitude

可以理解為重力加速度，默認值為1.0。可取負值。

UICollisionBehavior

碰撞檢測，和上面的重力配合查看效果

if (!_gravityBehavior) {
    _gravityBehavior = [[UIGravityBehavior alloc]initWithItems:@[_imageView1]];
}
_gravityBehavior = [[UIGravityBehavior alloc]initWithItems:@[_imageView1]];
_gravityBehavior.angle = M_PI_2;
_gravityBehavior.magnitude = 2.0f;
//    _gravityBehavior.gravityDirection = CGVectorMake(1.0f, 0.0f);

NSLog(@"magni = %f",_gravityBehavior.magnitude);

if(!_collisionBehavior){
    _collisionBehavior = [[UICollisionBehavior alloc]initWithItems:@[_imageView1,_imageView2]];
}
//    _collisionBehavior.collisionMode = UICollisionBehaviorModeBoundaries;
_collisionBehavior.translatesReferenceBoundsIntoBoundary = YES;

[self.animator addBehavior:_gravityBehavior];
[self.animator addBehavior:_collisionBehavior];

translatesReferenceBoundsIntoBoundary默認是NO。

另外，UICollisionBehavior有它自己的代理

- (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;

UIPushBehavior

給物體一個力的作用

typedef NS_ENUM(NSInteger, UIPushBehaviorMode) {
UIPushBehaviorModeContinuous,//持續的力
UIPushBehaviorModeInstantaneous//一次性的力

}
使用代碼：

if (!_pushBehavior) {
    _pushBehavior = [[UIPushBehavior alloc]initWithItems:@[_imageView1] mode:UIPushBehaviorModeContinuous];
}

_pushBehavior.active = YES;
_pushBehavior.pushDirection = CGVectorMake(10.0f, 10.0f);
_pushBehavior.magnitude = 1.0f;
[self.animator addBehavior:_pushBehavior];

pushDirection

給了物體一個運動的方向和初速度

magnitude

給物體一個加速度，默認值為0.0f

active

默認值是YES，將其設置為NO，可以將行為停止。

UIAttachmentBehavior

實現一個效果讓物體跟隨手指移動
- (void)panGestureRecognizer:(UIPanGestureRecognizer *)gesture
{
CGPoint location = [gesture locationInView:self.view];
CGPoint imageLocation = [gesture locationInView:self.imageView1];
switch (gesture.state) {
case UIGestureRecognizerStateBegan:
{
NSLog(@"touch position %@",NSStringFromCGPoint(location));
NSLog(@"loction in image %@",NSStringFromCGPoint(imageLocation));
[self.animator removeAllBehaviors];

        UIOffset centerOffset = UIOffsetMake(imageLocation.x - CGRectGetMidX(self.imageView1.bounds), imageLocation.y - CGRectGetMidY(self.imageView1.bounds));
        _attachmentBehavior = [[UIAttachmentBehavior alloc]initWithItem:self.imageView1 offsetFromCenter:centerOffset attachedToAnchor:location];
        _attachmentBehavior.damping = 0.5;
        _attachmentBehavior.frequency = 0.8;
        [self.animator addBehavior:_attachmentBehavior];
        
    }
        break;
        case UIGestureRecognizerStateEnded:
    {
        [self.animator removeBehavior:_attachmentBehavior];
    }
        break;
    default:
    {
        [_attachmentBehavior setAnchorPoint:[gesture locationInView:self.view]];
    }
        break;
}
}