Why can't I draw a dashed line with different line caps/joins on OS X using Quartz?

Question

I am trying to build a test case for this question. To do so, I need to draw a dashed line with caps and joins that differ from the defaults; for instance, with round line caps. The Programming with Quartz book by Gelphman and Laden says I should be able to; page 125 talks a lot about drawing dashed lines with different caps and joins and even has a figure showing what a dashed line would look like with different caps.

However, when I run

CGFloat lengths[2] = { 5, 3 };
CGContextMoveToPoint(c, 50, 50);
CGContextAddLineToPoint(c, 100, 30);
CGContextAddLineToPoint(c, 150, 70);
CGContextAddLineToPoint(c, 200, 50);
CGContextSetLineWidth(c, 10);
CGContextSetLineJoin(c, kCGLineJoinBevel);
CGContextSetLineCap(c, kCGLineCapRound);
CGContextSetLineDash(c, 0, lengths, 2);
CGContextSetRGBStrokeColor(c, 0, 0, 0, 1);
CGContextStrokePath(c);

I see a solid line, not a dashed line. If I comment out both of the following lines:

CGContextSetLineJoin(c, kCGLineJoinBevel);
CGContextSetLineCap(c, kCGLineCapRound);

then I see my dashing. One of these lines by itself causes the drawing to revert back into a solid line.

I'm at a loss as to what's going on. A sample program has been provided. This is on OS X 10.10; I need to target as far back as 10.7.

This is for OS X, not iOS.

Thanks.

// 15 october 2015
#import <Cocoa/Cocoa.h>

@interface dashStrokeView : NSView
@end

void putstr(CGContextRef c, const char *str, double x, double y)
{
    NSFont *sysfont;
    CFStringRef string;
    CTFontRef font;
    CFStringRef keys[1];
    CFTypeRef values[1];
    CFDictionaryRef attrs;
    CFAttributedStringRef attrstr;
    CTLineRef line;

    sysfont = [NSFont systemFontOfSize:[NSFont systemFontSizeForControlSize:NSRegularControlSize]];
    font = (CTFontRef) sysfont;     // toll-free bridge

    string = CFStringCreateWithCString(kCFAllocatorDefault,
        str, kCFStringEncodingUTF8);
    keys[0] = kCTFontAttributeName;
    values[0] = font;
    attrs = CFDictionaryCreate(kCFAllocatorDefault,
        keys, values,
        1,
        &kCFTypeDictionaryKeyCallBacks,
        &kCFTypeDictionaryValueCallBacks);
    attrstr = CFAttributedStringCreate(kCFAllocatorDefault, string, attrs);

    line = CTLineCreateWithAttributedString(attrstr);
    CGContextSetTextPosition(c, x, y);
    CTLineDraw(line, c);

    CFRelease(line);
    CFRelease(attrstr);
    CFRelease(attrs);
    CFRelease(string);
}

@implementation dashStrokeView

- (void)drawRect:(NSRect)r
{
    CGContextRef c;
    CGFloat lengths[2] = { 5, 3 };
    CGMutablePathRef buildpath;
    CGPathRef copy, copy2;

    c = (CGContextRef) [[NSGraphicsContext currentContext] graphicsPort];

    CGContextSaveGState(c);

    putstr(c, "Dash + Stroke With CGContext Functions", 10, 10);
    CGContextMoveToPoint(c, 50, 50);
    CGContextAddLineToPoint(c, 100, 30);
    CGContextAddLineToPoint(c, 150, 70);
    CGContextAddLineToPoint(c, 200, 50);
    CGContextSetLineWidth(c, 10);
    CGContextSetLineJoin(c, kCGLineJoinBevel);
    CGContextSetLineCap(c, kCGLineCapRound);
    CGContextSetLineDash(c, 0, lengths, 2);
    CGContextSetRGBStrokeColor(c, 0, 0, 0, 1);
    CGContextStrokePath(c);

    CGContextTranslateCTM(c, 0, 100);
    putstr(c, "Dash With CGPath Functions", 10, 10);

    CGContextTranslateCTM(c, 0, 100);
    putstr(c, "Dash + Stroke With CGPath Functions", 10, 10);

    CGContextRestoreGState(c);
}

- (BOOL)isFlipped
{
    return YES;
}

@end

@interface appDelegate : NSObject<NSApplicationDelegate>
@end

@implementation appDelegate

- (void)applicationDidFinishLaunching:(NSNotification *)note
{
    NSWindow *mainwin;
    NSView *contentView;
    dashStrokeView *view;
    NSDictionary *views;
    NSArray *constraints;

    mainwin = [[NSWindow alloc] initWithContentRect: NSMakeRect(0, 0, 320, 240)
        styleMask:(NSTitledWindowMask | NSClosableWindowMask | NSMiniaturizableWindowMask | NSResizableWindowMask)
        backing:NSBackingStoreBuffered
        defer:YES];
    [mainwin setTitle:@"Dash/Stroke Example"];
    contentView = [mainwin contentView];

    view = [[dashStrokeView alloc] initWithFrame:NSZeroRect];
    [view setTranslatesAutoresizingMaskIntoConstraints:NO];
    [contentView addSubview:view];

    views = NSDictionaryOfVariableBindings(view);
    constraints = [NSLayoutConstraint constraintsWithVisualFormat:@"H:|-[view]-|"
        options:0
        metrics:nil
        views:views];
    [contentView addConstraints:constraints];
    constraints = [NSLayoutConstraint constraintsWithVisualFormat:@"V:|-[view]-|"
        options:0
        metrics:nil
        views:views];
    [contentView addConstraints:constraints];

    [mainwin cascadeTopLeftFromPoint:NSMakePoint(20, 20)];
    [mainwin makeKeyAndOrderFront:nil];
}

- (BOOL)applicationShouldTerminateAfterLastWindowClosed:(NSApplication *)app
{
    return YES;
}

@end

int main(void)
{
    NSApplication *app;

    app = [NSApplication sharedApplication];
    [app setActivationPolicy:NSApplicationActivationPolicyRegular];
    [app setDelegate:[appDelegate new]];
    [app run];
    return 0;
}

Answer 1

The problem is that CGFloat lengths is not a large enough value to be discernible from it's path. Increasing the float values will allow the bevel and caps to be spaced more sufficiently:

CGFloat lengths[2] = { 10, 13 };

The way you've got it now is actually drawing (not reverting), although it's so minuscule that it's overlapping itself which essentially creates a solid line.