Bezier curve animation. See https://dribbble.com/shots/1950331-Interactive-shapes-iOS-demo?list=searches&offset=11

gistfile1.txt

- (void)drawRect:(CGRect)rect {
    [[UIColor lightGrayColor] setFill];
    [[UIColor darkGrayColor] setStroke];
    UIBezierPath *path = [[UIBezierPath alloc] init];
    CGPoint touchPoint = [self.lastTouch locationInView:self];
    CGFloat y0 = self.bounds.origin.y;
    CGFloat weight = 0.75; // Vary from 0 - 1. Values closed to 1 are more pointy.
    CGFloat y1 = y0 + (touchPoint.y - y0) * weight; //  between top of screen and touch
    CGFloat y2 = touchPoint.y;
    CGFloat y4 = CGRectGetMaxY(self.bounds);
    CGFloat y3 = y4 - (y4 - y2) * weight; // between touch point and bottom of screen
    CGFloat x0 = self.bounds.origin.y;
    CGFloat x1 = touchPoint.x;
    [path moveToPoint:CGPointZero];
    [path addCurveToPoint:touchPoint controlPoint1:CGPointMake(x0, y1) controlPoint2:CGPointMake(x1, y1)];
    [path addCurveToPoint:CGPointMake(x0, y4) controlPoint1:CGPointMake(x1, y3) controlPoint2:CGPointMake(x0, y3)];
    [path closePath];
    [path fill];
    [path stroke];
}

Bezier curve animation. See https://dribbble.com/shots/1950331-Interactive-shapes-iOS-demo?list=searches&offset=11