Most efficient way to check number of CGPoints in CGPathRef

Question

I want to find out if my CGMutablePathRef has more then 3 points! This checking will happen frequently so Im looking for an efficient solution.

This reason I need to do this is because in my project the user draws a shape. As the user drags his/her finger a CGPoint(current location of finger) is added to the path and a physical body is added when the touchEnded: is called.. now if the user just taps the screen the CGMutablePathRef only has one point in it(my reasoning in my head) and I get the following error when I use the my CGMutablePathRef for adding the physical body.

Assertion failed: (count >= 2), function CreateChain, file /SourceCache/PhysicsKit/PhysicsKit-6.5.4/PhysicsKit/Box2D/Collision/Shapes/b2ChainShape.cpp, line 45.

Im looking to make a function to call that takes a cgpathref as a parameter and counts the points until it reaches 3 (or the end if there isn't 3) and returns a bool

Thanks :)

What counts as a point? If the path contains a curve, do the control points of the curve count as points? — rob mayoff, Oct 10 '14 at 19:50
You don't add a CGPoint to a path. You add a line or a curve. I assume you are adding a line. You should probably go with @IanMacDonald's answer and keep a count alongside the path. — rob mayoff, Oct 10 '14 at 20:01
i would much prefer a solution like in my new edit @robmayoff — 4GetFullOf, Oct 10 '14 at 20:03

score 2 · Accepted Answer · edited May 23 '17 at 11:58

2

If you want to enumerate the elements of a CGPath, you have to use CGPathApply, and there is no support for early termination. You must enumerate all of the elements.

Grab my Rob_forEachElementOfCGPath function from this answer, and use it like this:

int numberOfSegmentsInCGPath(CGPathRef path) {
    __block int count = 0;
    Rob_forEachElementOfCGPath(path, ^(const CGPathElement *element) {
        if (element->type != kCGPathElementMoveToPoint) {
            ++count;
        }
    });
    return count;
}

edited May 23 '17 at 11:58

Community

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answered Oct 10 '14 at 20:27

rob mayoff

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Thanks, but if there is no support for early termination then keeping a point count is more efficient for my particular project. – 4GetFullOf Oct 11 '14 at 12:35

score 1 · Answer 2 · answered Oct 10 '14 at 19:57

1

There's no particularly efficient way to do this, but you could keep track of the number of points as you add them.

@interface MyPath : NSObject
@property (assign) int pointCount;
@property (assign) CGPathRef path;
@end

If you wanted to make sure you had complete control over it (and didn't want to worry about someone else incrementing your count or changing your path without incrementing), you could make the properties readonly and have an 'add point' function on the object ... Not great, but it would be more efficient than ApplyPath.

answered Oct 10 '14 at 19:57

Ian MacDonald

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Im looking more for a function to call that takes a cgpathref as a parameter and counts the points until it reaches 3 (or the end if there isn't 3) and returns a bool – 4GetFullOf Oct 10 '14 at 20:01
I understand, but there's only one way to do this and that's using `CGPathApply` and loop over the `points`, but I don't think it's particularly efficient for just checking a "3 or more" condition. – Ian MacDonald Oct 10 '14 at 20:16

Most efficient way to check number of CGPoints in CGPathRef

2 Answers2