Accessing and Pretty-Printing the Elements of a CGPath

Peter Steinberger asked this question on Twitter yesterday:

Is there an easy way to pretty-print CGPath’s?



December 6th, 2012

The short answer is: there may not be an easy way to log the contents of a CGPath, but it is certainly possible.

CGPath and UIBezierPath are Opaque

CGPath is an opaque data type that does not expose a straightforward way to list or access the individual elements of the path. Similarly, the UIKit wrapper for CGPath, UIBezierPath, does not provide a nice log output or a method to access the path elements.

Things are different with the AppKit counterpart, NSBezierPath. This class does have an APIs for individual element access (the elementAtIndex: and related methods) and produces a very nice log output in its description method. Here’s an example:

NSLog(@"%@", bezierPath);
> Path <0x101411090>
  Bounds: {{10, 10}, {100, 122.5}}
  Control point bounds: {{10, 10}, {100, 130}}
    10.000000 10.000000 moveto
    10.000000 110.000000 lineto
    30.000000 140.000000 80.000000 140.000000 110.000000 110.000000 curveto
    110.000000 10.000000 lineto
    10.000000 10.000000 lineto

Replicating NSBezierPath Behavior

Let’s try to replicate this log output for a UIBezierPath. Rather than working with CGPath directly, I decided to implement a category on UIBezierPath because I find it is a convenient place to put this code. If you have a plain CGPath, simply instantiate a UIBezierPath with it. Alternatively, the code should be easy to adapt for use with regular CGPath objects.

Accessing Individual Path Elements

While we cannot access the individual path elements directly, the CGPathApply() function lets us specify a callback function that will be called for each element. The prototype of this callback function looks like this:

void MyCGPathApplierFunc (void *info, const CGPathElement *element);

The info parameter is a pointer to an arbitrary piece of data we can pass to the callback from CGPathApply(). The second parameter, element, points to a struct that describes an individual path element.


A CGPathElement can have one of five types, describing the drawing command this particular element represents. Depending on the element type, the points member of CGPathElement points to an array of zero (kCGPathElementCloseSubpath), one (kCGPathElementMoveToPoint, kCGPathElementAddLineToPoint), two (kCGPathElementAddQuadCurveToPoint) or three (kCGPathElementAddCurveToPoint) points that define the exact form and endpoint of the drawing command.


Rather than just writing a better description method for UIBezierPath, I opted for a slightly more general solution. The new method ob_enumerateElementsUsingBlock: allows you to iterate over the path elements much like CGPathApply(). Instead of a callback function pointer, however, it takes a handler block in which you can specify what you want to do with the path elements. The implementation is simple: we use the info parameter of CGPathApply() to pass the block to the callback function.

#import <UIKit/UIKit.h>

typedef void(^OBUIBezierPathEnumerationHandler)(const CGPathElement *element);

@interface UIBezierPath (OBAdditions)

- (void)ob_enumerateElementsUsingBlock:(OBUIBezierPathEnumerationHandler)handler;
- (NSString *)ob_description;


Implementing the New description method

With this in place, writing the new ob_description method is a matter of enumerating over the path elements and creating an appropriate log string for each element in the block. Since the creation of the log string involves a rather large switch statement, I extracted this part into a separate method. The entire implementation looks like this:

#import "UIBezierPath+OBAdditions.h"

@implementation UIBezierPath (OBAdditions)

- (void)ob_enumerateElementsUsingBlock:(OBUIBezierPathEnumerationHandler)handler
    CGPathRef cgPath = self.CGPath;
    void CGPathEnumerationCallback(void *info, const CGPathElement *element);
    CGPathApply(cgPath, handler, CGPathEnumerationCallback);

- (NSString *)ob_description
    CGPathRef cgPath = self.CGPath;
    CGRect bounds = CGPathGetPathBoundingBox(cgPath);
    CGRect controlPointBounds = CGPathGetBoundingBox(cgPath);

    NSMutableString *mutableDescription = [NSMutableString string];
    [mutableDescription appendFormat:@"%@ <%p>\n", [self class], self];
    [mutableDescription appendFormat:@"  Bounds: %@\n", NSStringFromCGRect(bounds)];
    [mutableDescription appendFormat:@"  Control Point Bounds: %@\n", NSStringFromCGRect(controlPointBounds)];

    [self ob_enumerateElementsUsingBlock:^(const CGPathElement *element) {
        [mutableDescription appendFormat:@"    %@\n", [self ob_descriptionForPathElement:element]];

    return [mutableDescription copy];

- (NSString *)ob_descriptionForPathElement:(const CGPathElement *)element
    NSString *description = nil;
    switch (element->type) {
        case kCGPathElementMoveToPoint: {
            CGPoint point = element ->points[0];
            description = [NSString stringWithFormat:@"%f %f %@", point.x, point.y, @"moveto"];
        case kCGPathElementAddLineToPoint: {
            CGPoint point = element ->points[0];
            description = [NSString stringWithFormat:@"%f %f %@", point.x, point.y, @"lineto"];
        case kCGPathElementAddQuadCurveToPoint: {
            CGPoint point1 = element->points[0];
            CGPoint point2 = element->points[1];
            description = [NSString stringWithFormat:@"%f %f %f %f %@", point1.x, point1.y, point2.x, point2.y, @"quadcurveto"];
        case kCGPathElementAddCurveToPoint: {
            CGPoint point1 = element->points[0];
            CGPoint point2 = element->points[1];
            CGPoint point3 = element->points[2];
            description = [NSString stringWithFormat:@"%f %f %f %f %f %f %@", point1.x, point1.y, point2.x, point2.y, point3.x, point3.y, @"curveto"];
        case kCGPathElementCloseSubpath: {
            description = @"closepath";
    return description;


void CGPathEnumerationCallback(void *info, const CGPathElement *element)
    OBUIBezierPathEnumerationHandler handler = info;
    if (handler) {