Change NSSlider bar color

This is correct, you have to subclass the NSSliderCell class to redraw the bar or the knob.

NSRect is just a rectangular container, you have to draw inside this container. I made an example based on an custom NSLevelIndicator that I have in one of my program.

First you need to calculate the position of the knob. You must pay attention to the control minimum and maximum value. Next you draw a NSBezierPath for the background and another for the left part.

#import "MyCustomSlider.h"

@implementation MyCustomSlider

- (void)drawBarInside:(NSRect)rect flipped:(BOOL)flipped {

//  [super drawBarInside:rect flipped:flipped];

    rect.size.height = 5.0;

    // Bar radius
    CGFloat barRadius = 2.5;

    // Knob position depending on control min/max value and current control value.
    CGFloat value = ([self doubleValue]  - [self minValue]) / ([self maxValue] - [self minValue]);

    // Final Left Part Width
    CGFloat finalWidth = value * ([[self controlView] frame].size.width - 8);

    // Left Part Rect
    NSRect leftRect = rect;
    leftRect.size.width = finalWidth;

    NSLog(@"- Current Rect:%@ \n- Value:%f \n- Final Width:%f", NSStringFromRect(rect), value, finalWidth);

    // Draw Left Part
    NSBezierPath* bg = [NSBezierPath bezierPathWithRoundedRect: rect xRadius: barRadius yRadius: barRadius];
    [NSColor.orangeColor setFill];
    [bg fill];

    // Draw Right Part
    NSBezierPath* active = [NSBezierPath bezierPathWithRoundedRect: leftRect xRadius: barRadius yRadius: barRadius];
    [NSColor.purpleColor setFill];
    [active fill];


}

@end

First, I created an image of a slider bar and copied it in my project.

Then I used this image in the drawBarInside method to draw in the bar rect before the normal one, so we'll see only the remainder part (I wanted to keep the blue part intact).

This has to be done in a subclass of NSSliderCell:

class CustomSliderCell: NSSliderCell {

    let bar: NSImage

    required init(coder aDecoder: NSCoder) {
        self.bar = NSImage(named: "bar")!
        super.init(coder: aDecoder)
    }

    override func drawBarInside(aRect: NSRect, flipped: Bool) {
        var rect = aRect
        rect.size = NSSize(width: rect.width, height: 3)
        self.bar.drawInRect(rect)
        super.drawBarInside(rect, flipped: flipped)
    }

}

Pro: it works. :)

Con: it removes the rounded edges of the bar and I haven't found a way to redraw this yet.

UPDATE:

I made a Swift version of the accepted answer, it works very well:

class CustomSliderCell: NSSliderCell {

    required init(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
    }

    override func drawBarInside(aRect: NSRect, flipped: Bool) {
        var rect = aRect
        rect.size.height = CGFloat(5)
        let barRadius = CGFloat(2.5)
        let value = CGFloat((self.doubleValue - self.minValue) / (self.maxValue - self.minValue))
        let finalWidth = CGFloat(value * (self.controlView!.frame.size.width - 8))
        var leftRect = rect
        leftRect.size.width = finalWidth
        let bg = NSBezierPath(roundedRect: rect, xRadius: barRadius, yRadius: barRadius)
        NSColor.orangeColor().setFill()
        bg.fill()
        let active = NSBezierPath(roundedRect: leftRect, xRadius: barRadius, yRadius: barRadius)
        NSColor.purpleColor().setFill()
        active.fill()
    }

}

I have achieved this without redraw or override cell at all. Using "False color" filter seems work very well and it is only a few codes!

class RLSlider: NSSlider {
init() {
    super.init(frame: NSZeroRect)
    addFilter()
}

required init?(coder: NSCoder) {
    super.init(coder: coder)
    addFilter()
}

func addFilter() {
    let colorFilter = CIFilter(name: "CIFalseColor")!
    colorFilter.setDefaults()
    colorFilter.setValue(CIColor(cgColor: NSColor.white.cgColor), forKey: "inputColor0")
    colorFilter.setValue(CIColor(cgColor: NSColor.yellow.cgColor), forKey: "inputColor1")

//        colorFilter.setValue(CIColor(cgColor: NSColor.yellow.cgColor), forKey: "inputColor0")
//        colorFilter.setValue(CIColor(cgColor: NSColor.yellow.cgColor), forKey: "inputColor1")

    self.contentFilters = [colorFilter]
}
}