How to get a list of points from a UIBezierPath?

Question

I have a UIBezierPath that I need to take a list of points from.

In Qt there is a function called pointAtPercent that would fit my needs but I can't find anything equivalent in Objective-C.

Does anyone know how to do this?

Answer 1

You might try this:

UIBezierPath *yourPath; // Assume this has some points in it
CGPath yourCGPath = yourPath.CGPath;
NSMutableArray *bezierPoints = [NSMutableArray array];
CGPathApply(yourCGPath, bezierPoints, MyCGPathApplierFunc);

The path applier function will be handed each of the path's path elements in turn.
Check out CGPathApplierFunction and CGPathApply.

The path applier function might look something like this

void MyCGPathApplierFunc (void *info, const CGPathElement *element) {
    NSMutableArray *bezierPoints = (NSMutableArray *)info;

    CGPoint *points = element->points;
    CGPathElementType type = element->type;

    switch(type) {
        case kCGPathElementMoveToPoint: // contains 1 point
            [bezierPoints addObject:[NSValue valueWithCGPoint:points[0]]];
            break;
        
        case kCGPathElementAddLineToPoint: // contains 1 point
            [bezierPoints addObject:[NSValue valueWithCGPoint:points[0]]];            
            break;
        
        case kCGPathElementAddQuadCurveToPoint: // contains 2 points
            [bezierPoints addObject:[NSValue valueWithCGPoint:points[0]]];
            [bezierPoints addObject:[NSValue valueWithCGPoint:points[1]]];            
            break;
        
        case kCGPathElementAddCurveToPoint: // contains 3 points
            [bezierPoints addObject:[NSValue valueWithCGPoint:points[0]]];
            [bezierPoints addObject:[NSValue valueWithCGPoint:points[1]]];
            [bezierPoints addObject:[NSValue valueWithCGPoint:points[2]]];
            break;
        
        case kCGPathElementCloseSubpath: // contains no point
            break;
    }
}

Answer 2

@Moritz Great answer! To find a Swift like solution I strumbled across an approach in this post and implemented a CGPath extension like this to get the points from the path:

extension CGPath {
    func points() -> [CGPoint]
    {
        var bezierPoints = [CGPoint]()
        forEach(body: { (element: CGPathElement) in
            let numberOfPoints: Int = {
                switch element.type {
                case .moveToPoint, .addLineToPoint: // contains 1 point
                    return 1
                case .addQuadCurveToPoint: // contains 2 points
                    return 2
                case .addCurveToPoint: // contains 3 points
                    return 3
                case .closeSubpath:
                    return 0
                }
            }()
            for index in 0..<numberOfPoints {
                let point = element.points[index]
                bezierPoints.append(point)
            }
        })
        return bezierPoints
    }
    
    func forEach(body: @escaping @convention(block) (CGPathElement) -> Void) {
        typealias Body = @convention(block) (CGPathElement) -> Void
        
        func callback(info: UnsafeMutableRawPointer?, element: UnsafePointer<CGPathElement>) {
            let body = unsafeBitCast(info, to: Body.self)
            body(element.pointee)
        }
        
        let unsafeBody = unsafeBitCast(body, to: UnsafeMutableRawPointer.self)
        apply(info: unsafeBody, function: callback)
    }
}