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有什么能让App界面变得更加生动呢，答案就是动画效果，其中带弹性的动画效果我尤其喜欢，iOS 给我们提供了一个api
+(void)animateWithDuration:(NSTimeInterval)duration delay:(NSTimeInterval)delay usingSpringWithDamping:(CGFloat)dampingRatio initialSpringVelocity:(CGFloat)velocity options:(UIViewAnimationOptions)options animations:(void (^)(void))animations completion:(void (^ __nullable)(BOOL finished))completion NS_AVAILABLE_IOS(7_0);
但是这个api有一个局限性，就是他只能控制一个view的整体变化，像上图那种只让矩形的底边做弹性运动，光靠这个api难以实现。

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等等，这里只是手指滑动的时候要做的事情，当手指松开后如何让控制点D动态地回到原处(即CE的中心位置)并且还要有弹性效果呢？这里就需要用到上面所说的弹性动画api了，其实我在D点这个位置放了一个view，就是动图上面的黑点，手指滑动的时候黑色view也跟着D点运动，当手指松开时对黑色view做一个弹性动画，并且运行一个CADisplayLink，并且在CADisplayLink触发的方法中不断获取黑色view.layer.presentationLayer.position的值，刷新D点的位置，这样D点也能跟着黑色view做弹性运动了，这样就实现了回弹和弹性效果。
是不是很麻烦！又是加黑色view，又是对黑色view做震荡动画，又是开CADisplayLink，又是监控黑色view的位置，其实就是为了获得一串带震荡效果的点，并连续的赋值给D点。所以这里我提供一个封装了的类，里面有一个方法就是产生连续的带震荡效果的值，并且不断的将值赋值给block，这样外界就能在block中直接获得这些值做动画了。我在这个类里面已经开了CADisplayLink，会不断的调用block，并且将新值付给block。
.h文件

#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>
typedef void(^CallBackBlockValue)(CGFloat value);
typedef void(^CallBackBlockPoint)(CGPoint point);
@interface animationTools : NSObject
//单个值的震荡
-(void)animationWithFormValue:(CGFloat)formValue
                      toValue:(CGFloat)toValue
                      damping:(CGFloat)damping
                     velocity:(CGFloat)velocity
                     duration:(CGFloat)duration
                     callback:(CallBackBlockValue)callback;

//点的震荡
-(void)animationWithFormPoint:(CGPoint)formPoint
                      toPoint:(CGPoint)toPoint
                      damping:(CGFloat)damping
                     velocity:(CGFloat)velocity
                     duration:(CGFloat)duration
                     callback:(CallBackBlockPoint)callback;
@end

.m文件

#import "animationTools.h"
#import <UIKit/UIKit.h>

@interface animationTools()
@property (nonatomic,strong) CADisplayLink *displaylink;
@property (nonatomic,assign) CGFloat formValue;
@property (nonatomic,assign) CGFloat toValue;
@property (nonatomic,assign) CGPoint formPoint;
@property (nonatomic,assign) CGPoint toPoint;
@property (nonatomic,assign) CGFloat damping;
@property (nonatomic,assign) CGFloat velocity;
@property (nonatomic,assign) NSInteger numberOfnum;
@property (nonatomic,assign) NSInteger beginNum;
@property (nonatomic,copy)   CallBackBlockValue callbackValue;
@property (nonatomic,copy)   CallBackBlockPoint callbackPoint;
@end
@implementation animationTools


/*
 * damping = 5;     //越小 幅度越大
 * velocity = 30;   //越大 震动次数越多
 */
-(void)animationWithFormValue:(CGFloat)formValue
                      toValue:(CGFloat)toValue
                      damping:(CGFloat)damping
                     velocity:(CGFloat)velocity
                     duration:(CGFloat)duration
                     callback:(CallBackBlockValue)callback
{
    [self.displaylink invalidate];
    self.displaylink = nil;
    self.numberOfnum = duration * 60;   //总帧数
    self.formValue = formValue;
    self.toValue = toValue;
    self.damping = damping;
    self.velocity = velocity;
    self.beginNum = 0;
    self.callbackValue = callback;
    self.displaylink = [CADisplayLink displayLinkWithTarget:self selector:@selector(SEL_Displaylink_value)];
    [self.displaylink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];
}

-(void)SEL_Displaylink_value
{
    if (self.beginNum >= self.numberOfnum) {
        [self.displaylink invalidate];
        self.displaylink = nil;
    }
    //计算
    CGFloat i = self.beginNum * 1.0/self.numberOfnum;
    CGFloat value = self.toValue - (self.toValue - self.formValue) * (pow(M_E, -self.damping * i) * cos(self.velocity * i)); //1 y = 1-e^{-5x} * cos(30x)
    if (self.callbackValue != nil) {
        self.callbackValue(value);
    }
    self.beginNum ++;
}

-(void)animationWithFormPoint:(CGPoint)formPoint
                      toPoint:(CGPoint)toPoint
                      damping:(CGFloat)damping
                     velocity:(CGFloat)velocity
                     duration:(CGFloat)duration
                     callback:(CallBackBlockPoint)callback
{
    [self.displaylink invalidate];
    self.displaylink = nil;
    self.numberOfnum = duration * 60;   //总帧数
    self.formPoint = formPoint;
    self.toPoint = toPoint;
    self.damping = damping;
    self.velocity = velocity;
    self.beginNum = 0;
    self.callbackPoint = callback;
    self.displaylink = [CADisplayLink displayLinkWithTarget:self selector:@selector(SEL_Displaylink_Point)];
    [self.displaylink addToRunLoop:[NSRunLoop currentRunLoop] forMode:NSRunLoopCommonModes];
}

-(void)SEL_Displaylink_Point
{
    if (self.beginNum >= self.numberOfnum) {
        [self.displaylink invalidate];
        self.displaylink = nil;
    }
    //计算
    CGFloat i = self.beginNum * 1.0/self.numberOfnum;
    CGFloat x = self.toPoint.x - (self.toPoint.x - self.formPoint.x) * (pow(M_E, -self.damping * i) * cos(self.velocity * i)); //1 y = 1-e^{-5x} * cos(30x)
    CGFloat y = self.toPoint.y - (self.toPoint.y - self.formPoint.y) * (pow(M_E, -self.damping * i) * cos(self.velocity * i));
    CGPoint point = CGPointMake(x, y);
    if (self.callbackPoint != nil) {
        self.callbackPoint(point);
    }
    self.beginNum ++;
}

@end

需要注意的是damping参数和duration参数，damping数值越小震动次数越多，可能duration设置的比较小，在duration时间结束后弹性效果还没结束，导致骤然停止。需要在使用的时候调试。

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