How can I draw an arrow using Core Graphics?

UPDATE

I've posted a Swift version of this answer separately.

ORIGINAL

This is a fun little problem. First of all, there are lots of ways to draw arrows, with curved or straight sides. Let's pick a very simple way and label the measurements we'll need:

We want to write a function that takes the start point, the end point, the tail width, the head width, and the head length, and returns a path outlining the arrow shape. Let's create a category named dqd_arrowhead to add this method to UIBezierPath:

// UIBezierPath+dqd_arrowhead.h@interface UIBezierPath (dqd_arrowhead)+ (UIBezierPath *)dqd_bezierPathWithArrowFromPoint:(CGPoint)startPoint                                           toPoint:(CGPoint)endPoint                                         tailWidth:(CGFloat)tailWidth                                         headWidth:(CGFloat)headWidth                                        headLength:(CGFloat)headLength;@end

Since there are seven corners on the path of the arrow, let's start our implementation by naming that constant:

// UIBezierPath+dqd_arrowhead.m#import "UIBezierPath+dqd_arrowhead.h"#define kArrowPointCount 7@implementation UIBezierPath (dqd_arrowhead)+ (UIBezierPath *)dqd_bezierPathWithArrowFromPoint:(CGPoint)startPoint                                           toPoint:(CGPoint)endPoint                                         tailWidth:(CGFloat)tailWidth                                         headWidth:(CGFloat)headWidth                                        headLength:(CGFloat)headLength {

OK, the easy part is done. Now, how do we find the coordinates of those seven points on the path? It is much easier to find the points if the arrow is aligned along the X axis:

It's pretty easy to compute the point coordinates on an axis-aligned arrow, but we'll need the overall length of the arrow to do it. We'll use the hypotf function from the standard library:

    CGFloat length = hypotf(endPoint.x - startPoint.x, endPoint.y - startPoint.y);

We'll call on a helper method to actually compute the seven points:

    CGPoint points[kArrowPointCount];    [self dqd_getAxisAlignedArrowPoints:points                              forLength:length                              tailWidth:tailWidth                              headWidth:headWidth                             headLength:headLength];

But we need to transform those points, because in general we're not trying to create an axis-aligned arrow. Fortunately, Core Graphics supports a kind of transformation called an affine transformation, which lets us rotate and translate (slide) points. We'll call another helper method to create the transform that turns our axis-aligned arrow into the arrow we were asked for:

    CGAffineTransform transform = [self dqd_transformForStartPoint:startPoint                                                          endPoint:endPoint                                                            length:length];

Now we can create a Core Graphics path using the points of the axis-aligned arrow and the transform that turns it into the arrow we want:

    CGMutablePathRef cgPath = CGPathCreateMutable();    CGPathAddLines(cgPath, &transform, points, sizeof points / sizeof *points);    CGPathCloseSubpath(cgPath);

Finally, we can wrap a UIBezierPath around the CGPath and return it:

    UIBezierPath *uiPath = [UIBezierPath bezierPathWithCGPath:cgPath];    CGPathRelease(cgPath);    return uiPath;}

Here's the helper method that computes the point coordinates. It's quite simple. Refer back to the diagram of the axis-aligned arrow if you need to.

+ (void)dqd_getAxisAlignedArrowPoints:(CGPoint[kArrowPointCount])points                            forLength:(CGFloat)length                            tailWidth:(CGFloat)tailWidth                            headWidth:(CGFloat)headWidth                           headLength:(CGFloat)headLength {    CGFloat tailLength = length - headLength;    points[0] = CGPointMake(0, tailWidth / 2);    points[1] = CGPointMake(tailLength, tailWidth / 2);    points[2] = CGPointMake(tailLength, headWidth / 2);    points[3] = CGPointMake(length, 0);    points[4] = CGPointMake(tailLength, -headWidth / 2);    points[5] = CGPointMake(tailLength, -tailWidth / 2);    points[6] = CGPointMake(0, -tailWidth / 2);}

Computing the affine transform is more complicated. This is where the trigonometry comes in. You could use atan2 and the CGAffineTransformRotate and CGAffineTransformTranslate functions to create it, but if you remember enough trigonometry, you can create it directly. Consult “The Math Behind the Matrices” in the Quartz 2D Programming Guide for more information about what I'm doing here:

+ (CGAffineTransform)dqd_transformForStartPoint:(CGPoint)startPoint                                       endPoint:(CGPoint)endPoint                                         length:(CGFloat)length {    CGFloat cosine = (endPoint.x - startPoint.x) / length;    CGFloat sine = (endPoint.y - startPoint.y) / length;    return (CGAffineTransform){ cosine, sine, -sine, cosine, startPoint.x, startPoint.y };}@end

I have put all of the code in a gist for easy copy'n'paste.

With this category, you can easily draw arrows:

Since you're just generating a path, you can choose not to fill it, or not to stroke it as in this example:

You have to be careful, though. This code doesn't prevent you from getting funky results if you make the head width less than the tail width, or if you make the head length larger than the total arrow length:

Here's a Swift version of my old Objective-C code. It should work in Swift 3.2 and later versions.

extension UIBezierPath {    static func arrow(from start: CGPoint, to end: CGPoint, tailWidth: CGFloat, headWidth: CGFloat, headLength: CGFloat) -> UIBezierPath {        let length = hypot(end.x - start.x, end.y - start.y)        let tailLength = length - headLength        func p(_ x: CGFloat, _ y: CGFloat) -> CGPoint { return CGPoint(x: x, y: y) }        let points: [CGPoint] = [            p(0, tailWidth / 2),            p(tailLength, tailWidth / 2),            p(tailLength, headWidth / 2),            p(length, 0),            p(tailLength, -headWidth / 2),            p(tailLength, -tailWidth / 2),            p(0, -tailWidth / 2)        ]        let cosine = (end.x - start.x) / length        let sine = (end.y - start.y) / length        let transform = CGAffineTransform(a: cosine, b: sine, c: -sine, d: cosine, tx: start.x, ty: start.y)        let path = CGMutablePath()        path.addLines(between: points, transform: transform)        path.closeSubpath()        return self.init(cgPath: path)    }}

Here's an example of how you'd call it:

let arrow = UIBezierPath.arrow(from: CGPoint(x: 50, y: 100), to: CGPoint(x: 200, y: 50),        tailWidth: 10, headWidth: 25, headLength: 40)

//This is the integration into the view of the previous exemple//Attach the following class to your view in the xib file#import <UIKit/UIKit.h>@interface Arrow : UIView@end#import "Arrow.h"#import "UIBezierPath+dqd_arrowhead.h"@implementation Arrow{    CGPoint startPoint;    CGPoint endPoint;    CGFloat tailWidth;    CGFloat headWidth;    CGFloat headLength;    UIBezierPath *path;}- (id)initWithCoder:(NSCoder *)aDecoder{    if (self = [super initWithCoder:aDecoder])    {        [self setMultipleTouchEnabled:NO];        [self setBackgroundColor:[UIColor whiteColor]];    }    return self;}- (void)drawRect:(CGRect)rect {    [[UIColor redColor] setStroke];    tailWidth = 4;    headWidth = 8;    headLength = 8;    path = [UIBezierPath dqd_bezierPathWithArrowFromPoint:(CGPoint)startPoint                                                  toPoint:(CGPoint)endPoint                                                tailWidth:(CGFloat)tailWidth                                                headWidth:(CGFloat)headWidth                                               headLength:(CGFloat)headLength];    [path setLineWidth:2.0];    [path stroke];}- (void) touchesBegan:(NSSet*)touches withEvent:(UIEvent*)event{    UITouch* touchPoint = [touches anyObject];    startPoint = [touchPoint locationInView:self];    endPoint = [touchPoint locationInView:self];    [self setNeedsDisplay];}-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event{    UITouch* touch = [touches anyObject];    endPoint=[touch locationInView:self];    [self setNeedsDisplay];}-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{    UITouch* touch = [touches anyObject];    endPoint = [touch locationInView:self];    [self setNeedsDisplay];}@end