Paint View

- (void) touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    NSLog(@"%s", __func__);
    UITouch *touch = [touches anyObject];
    point0 = CGPointMake(-1, -1);
    point1 = CGPointMake(-1, -1); // previous previous point
    point2 = CGPointMake(-1, -1); // previous touch point
    point3 = [touch locationInView:self]; // current touch point
}

- (void) touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    point0 = point1;
    point1 = point2;
    point2 = point3;
    point3 = [touch locationInView:self];
    [self drawToCache];
}

- (void) drawToCache {
    if(point1.x > -1){
        hue += 0.005;
        if(hue > 1.0) hue = 0.0;
        UIColor *color = [UIColor colorWithHue:hue saturation:0.7 brightness:1.0 alpha:1.0];

        CGContextSetStrokeColorWithColor(cacheContext, [color CGColor]);
        CGContextSetLineCap(cacheContext, kCGLineCapRound);
        CGContextSetLineWidth(cacheContext, 8);

        double x0 = (point0.x > -1) ? point0.x : point1.x; //after 4 touches we should have a back anchor point, if not, use the current anchor point
        double y0 = (point0.y > -1) ? point0.y : point1.y; //after 4 touches we should have a back anchor point, if not, use the current anchor point
        double x1 = point1.x;
        double y1 = point1.y;
        double x2 = point2.x;
        double y2 = point2.y;
        double x3 = point3.x;
        double y3 = point3.y;
        // Assume we need to calculate the control
        // points between (x1,y1) and (x2,y2).
        // Then x0,y0 - the previous vertex,
        //      x3,y3 - the next one.

        double xc1 = (x0 + x1) / 2.0;
        double yc1 = (y0 + y1) / 2.0;
        double xc2 = (x1 + x2) / 2.0;
        double yc2 = (y1 + y2) / 2.0;
        double xc3 = (x2 + x3) / 2.0;
        double yc3 = (y2 + y3) / 2.0;

        double len1 = sqrt((x1-x0) * (x1-x0) + (y1-y0) * (y1-y0));
        double len2 = sqrt((x2-x1) * (x2-x1) + (y2-y1) * (y2-y1));
        double len3 = sqrt((x3-x2) * (x3-x2) + (y3-y2) * (y3-y2));

        double k1 = len1 / (len1 + len2);
        double k2 = len2 / (len2 + len3);

        double xm1 = xc1 + (xc2 - xc1) * k1;
        double ym1 = yc1 + (yc2 - yc1) * k1;

        double xm2 = xc2 + (xc3 - xc2) * k2;
        double ym2 = yc2 + (yc3 - yc2) * k2;
        double smooth_value = 0.8;
        // Resulting control points. Here smooth_value is mentioned
        // above coefficient K whose value should be in range [0...1].
        float ctrl1_x = xm1 + (xc2 - xm1) * smooth_value + x1 - xm1;
        float ctrl1_y = ym1 + (yc2 - ym1) * smooth_value + y1 - ym1;

        float ctrl2_x = xm2 + (xc2 - xm2) * smooth_value + x2 - xm2;
        float ctrl2_y = ym2 + (yc2 - ym2) * smooth_value + y2 - ym2;

        CGContextMoveToPoint(cacheContext, point1.x, point1.y);
        CGContextAddCurveToPoint(cacheContext, ctrl1_x, ctrl1_y, ctrl2_x, ctrl2_y, point2.x, point2.y);
        CGContextStrokePath(cacheContext);

        CGRect dirtyPoint1 = CGRectMake(point1.x-10, point1.y-10, 20, 20);
        CGRect dirtyPoint2 = CGRectMake(point2.x-10, point2.y-10, 20, 20);
        [self setNeedsDisplayInRect:CGRectUnion(dirtyPoint1, dirtyPoint2)];
    }
}