UIView 的反彈動畫API
//usingSpringWithDamping的范圍為0.0f到1.0f饲宛，數(shù)值越小「彈簧」的振動效果越明顯忍啸。
//initialSpringVelocity則表示初始的速度魁索，數(shù)值越大一開始移動越快胆描。
[UIView animateWithDuration:2 delay:0 usingSpringWithDamping:0.8 initialSpringVelocity:3 options:UIViewAnimationOptionCurveEaseIn animations:^{
CGFloat y = [UIScreen mainScreen].bounds.size.height - frame.size.height;
self.sheetView.frame = CGRectMake(frame.origin.x, y, frame.size.width, frame.size.height);
} completion:^(BOOL finished) {
}];
