UIView动画之阻尼动画

iOS 7 之后新出来一个阻尼动画，类似于弹簧效果的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);

dampingRatio（阻尼系数）
范围 0~1 当它设置为1时，动画是平滑的没有振动的达到静止状态，越接近0 振动越大

velocity （弹性速率）
就是形变的速度，从视觉上看可以理解弹簧的形变速度，到动画结束，该速度减为0，所以，velocity速度越大，那么形变会越快，当然在同等时间内，速度的变化（就是速率）也会越快，因为速度最后都要到0。

效果：

动画.gif

- (void)viewDidLoad {
    [super viewDidLoad];
   
    
    
    UIButton *button =[UIButton buttonWithType:UIButtonTypeCustom];
    button.backgroundColor =[UIColor greenColor];
    [button setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
    [button setTitleColor:[UIColor redColor] forState:UIControlStateNormal];

    [button setTitle:@"展开" forState:UIControlStateNormal];
    [button setTitle:@"收起" forState:UIControlStateSelected];
    button.frame = CGRectMake(Device_Width - 100, 200, 80, 40);
    
    [button addTarget:self action:@selector(btnClick:) forControlEvents:UIControlEventTouchUpInside];
    [self.view addSubview:button];
    
    _label =[[UILabel alloc]initWithFrame:CGRectMake(Device_Width - 100, 200, 0, 40)];
    _label.backgroundColor =[UIColor cyanColor];
    _label.text =  @"哈哈哈";
    [self.view addSubview:_label];
    
}
-(void)btnClick:(UIButton *)button
{
    button.selected = !button.selected;
    
    if (button.selected) {

        [UIView animateWithDuration:1 delay:0 usingSpringWithDamping:0.5 initialSpringVelocity:0.6 options:UIViewAnimationOptionCurveLinear animations:^{
            _label.frame = CGRectMake(Device_Width - 300, 200, 100, 40 );

        } completion:^(BOOL finished) {
        }];
    }
    if (!button.selected) {
        
        [UIView animateWithDuration:0.3 delay:0 usingSpringWithDamping:0.5 initialSpringVelocity:0.6 options:UIViewAnimationOptionCurveLinear animations:^{
            _label.frame = CGRectMake(Device_Width - 100, 200, 0, 40 );
        } completion:^(BOOL finished) {
            
        }];
    }
    
    
}