Reverse UIBezierPath for single sided corners

Question

I want to create a mask path whose final output will be a corner with a concave face.

I have created a UIBezierPath using the -bezierPathWithRoundedRect:byRoundingCorners:cornerRadii: method and specified that only one corner need to be rounded (in this example, the UIRectCornerBottomLeft) and I thought that by reversing this path, I should get what would have been cut in the first place.
For this, I am trying -bezierPathByReversingPath but it doesn't seem to make any difference (with or without as I am getting the normal bezier path and not the reversed)

This is what I have tried so far:

UIView *vwTest = [[UIView alloc] init];
[vwTest setFrame:CGRectMake(100.0f, 100.0f, 100.0f, 100.0f)];
[vwTest setBackgroundColor:[UIColor redColor]];
[self.view addSubview:vwTest];

UIBezierPath *maskPath = [UIBezierPath bezierPathWithRoundedRect:vwTest.bounds
                                               byRoundingCorners:UIRectCornerBottomLeft
                                                     cornerRadii:CGSizeMake(50.0f, 50.0f)];

//get reverse path but doesn't seem to work as I think it should
maskPath = [maskPath bezierPathByReversingPath]; 

CAShapeLayer *maskLayer = [CAShapeLayer layer];
[maskLayer setFrame:vwTest.bounds];
[maskLayer setPath:maskPath.CGPath];

[vwTest.layer setMask:maskLayer];
[vwTest.layer setMasksToBounds:YES];
[vwTest setNeedsDisplay];

---

As per the image below, what I am trying to achieve is to show the red part of the UIView and eliminate the rest of the area.

---

For the time being, I am doing shit like subview on subview and syncing the color of one subview with the main view's background color (sucks but atleast I got the required output):

[self.view setBackgroundColor:[UIColor lightGrayColor]];

UIView *vwTest = [[UIView alloc] init];
[vwTest setFrame:CGRectMake(100.0f, 100.0f, 100.0f, 100.0f)];
[vwTest setBackgroundColor:[UIColor redColor]];
[self.view addSubview:vwTest];

UIView *vwPatch = [[UIView alloc] init];
[vwPatch setFrame:vwTest.bounds];
[vwPatch setBackgroundColor:vwTest.superview.backgroundColor];
[vwTest addSubview:vwPatch];

UIBezierPath *maskPath = [UIBezierPath bezierPathWithRoundedRect:vwPatch.bounds
                                               byRoundingCorners:UIRectCornerBottomLeft
                                                     cornerRadii:CGSizeMake(50.0f, 50.0f)];
CAShapeLayer *maskLayer = [CAShapeLayer layer];
[maskLayer setFrame:vwPatch.bounds];
[maskLayer setPath:maskPath.CGPath];

[vwPatch.layer setMask:maskLayer];
[vwPatch.layer setMasksToBounds:YES];
[vwPatch setNeedsDisplay];

---

Someone, kindly point me in the right direction.

Answer 1

From looking at the code in the question I've assumed you wanted the bottom left corner (as you are using UIRectCornerBottomLeft) be concave with a radius of 30. You can't use bezierPathByReversingPath as all is does it literally reverse the path, it also says in the docs:

Reversing a path does not necessarily change the appearance of the path when rendered.

EDIT:
Now that an image has been added to the question, the OP wants the inverse of the shape:

CAShapeLayer *mask = [CAShapeLayer layer];
mask.frame = view.layer.bounds;

UIBezierPath *path = [UIBezierPath bezierPath];
CGFloat radius = 50;
CGRect rect = mask.bounds;
[path moveToPoint:CGPointMake(CGRectGetMinX(rect), CGRectGetMaxY(rect))];
[path addLineToPoint:CGPointMake(CGRectGetMinX(rect), CGRectGetMaxY(rect) - radius)];
[path addArcWithCenter:CGPointMake(CGRectGetMinX(rect) + radius, CGRectGetMaxY(rect) - radius) radius:radius startAngle:M_PI endAngle:M_PI_2 clockwise:NO];
[path closePath];

mask.path = path.CGPath;
view.layer.mask = mask;

The shape of the path is then red bit from the OPs image in the question.

---

Answer before image was posted - might be used for others

The following code will produce this shape:

CGFloat cornerRadius = 30;

CGRect rect = CGRectMake(0, 0, 200, 100);

UIBezierPath *path = [UIBezierPath bezierPath];

// Draw the complete sides
[path moveToPoint:rect.origin];
[path addLineToPoint:CGPointMake(CGRectGetMaxX(rect), CGRectGetMinY(rect))];
[path addLineToPoint:CGPointMake(CGRectGetMaxX(rect), CGRectGetMaxY(rect))];

// Stop short for the start of the arc
[path addLineToPoint:CGPointMake(CGRectGetMinX(rect) + cornerRadius, CGRectGetMaxY(rect))];

// Concave arc
[path addArcWithCenter:CGPointMake(CGRectGetMinX(rect), CGRectGetMaxY(rect)) radius:cornerRadius startAngle:0 endAngle:M_PI_2 * 3 clockwise:NO];

// Complete the path
[path closePath];

Update: Useful category

Category on UIBezierPath so you can pick which corners (bit mask of UIRectCorners) and a radius.

UIBezierPath+RDHConcaveCorners.h

@interface UIBezierPath (RDHConcaveCorners)

+(instancetype)bezierPathWithRoundedRect:(CGRect)rect byConcaveRoundingCorners:(UIRectCorner)corners cornerRadius:(CGFloat)radius;

@end

UIBezierPath+RDHConcaveCorners.m

@implementation UIBezierPath (RDHConcaveCorners)

+(instancetype)bezierPathWithRoundedRect:(CGRect)rect byConcaveRoundingCorners:(UIRectCorner)corners cornerRadius:(CGFloat)cornerRadius
{
    CGFloat halfWidth = CGRectGetWidth(rect) / 2;
    CGFloat halfHeight = CGRectGetHeight(rect) / 2;
    if (cornerRadius > halfWidth || cornerRadius > halfHeight) {
        cornerRadius = MIN(halfWidth, halfHeight);
    }

    UIBezierPath *path = [self bezierPath];

    CGPoint topLeft = CGPointMake(CGRectGetMinX(rect), CGRectGetMinY(rect));
    if (corners & UIRectCornerTopLeft) {
        [path moveToPoint:CGPointMake(topLeft.x, topLeft.y + cornerRadius)];
        [path addArcWithCenter:topLeft radius:cornerRadius startAngle:M_PI_2 endAngle:0 clockwise:NO];
    } else {
        [path moveToPoint:topLeft];
    }

    CGPoint topRight = CGPointMake(CGRectGetMaxX(rect), CGRectGetMinY(rect));
    if (corners & UIRectCornerTopRight) {
        [path addLineToPoint:CGPointMake(topRight.x - cornerRadius, topRight.y)];
        [path addArcWithCenter:topRight radius:cornerRadius startAngle:M_PI endAngle:M_PI_2 clockwise:NO];
    } else {
        [path addLineToPoint:topRight];
    }

    CGPoint bottomRight = CGPointMake(CGRectGetMaxX(rect), CGRectGetMaxY(rect));
    if (corners & UIRectCornerBottomRight) {
        [path addLineToPoint:CGPointMake(bottomRight.x, bottomRight.y - cornerRadius)];
        [path addArcWithCenter:bottomRight radius:cornerRadius startAngle:M_PI_2 * 3 endAngle:M_PI clockwise:NO];

    } else {
        [path addLineToPoint:bottomRight];
    }

    CGPoint bottomLeft = CGPointMake(CGRectGetMinX(rect), CGRectGetMaxY(rect));
    if (corners & UIRectCornerBottomLeft) {
        [path addLineToPoint:CGPointMake(bottomLeft.x + cornerRadius, bottomLeft.y)];
        [path addArcWithCenter:bottomLeft radius:cornerRadius startAngle:0 endAngle:M_PI_2 * 3 clockwise:NO];
    } else {
        [path addLineToPoint:bottomLeft];
    }

    // Complete the path
    [path closePath];

    return path;
}

@end