An NSSegmentedControl subclass with support for conditional segment selection via a delegate.

MTSegmentedControl.h

//
//  MTSegmentedControl.h
//
//  Created by Michal Tomlein on 14/08/15.
//  Copyright (c) 2015 Michal Tomlein. All rights reserved.
//

#import <Cocoa/Cocoa.h>

@class MTSegmentedControl;

@protocol MTSegmentedControlDelegate <NSObject>

- (BOOL)segmentedControl:(MTSegmentedControl *)control shouldSelectSegment:(NSInteger)segment;

@end

@interface MTSegmentedControl : NSSegmentedControl

@property (nonatomic, weak) id <MTSegmentedControlDelegate> delegate;

@end

MTSegmentedControl.m

//
//  MTSegmentedControl.m
//
//  Created by Michal Tomlein on 14/08/15.
//  Copyright (c) 2015 Michal Tomlein. All rights reserved.
//

#import "MTSegmentedControl.h"

@interface MTSegmentedCell : NSSegmentedCell {
    CGFloat _segmentContentInset;
    NSMutableDictionary *_segmentOffsets;
}

@end

@implementation MTSegmentedCell

- (void)drawSegment:(NSInteger)segment inFrame:(NSRect)frame withView:(NSView *)controlView
{
    if (segment) {
        if (!_segmentOffsets)
            _segmentOffsets = [[NSMutableDictionary alloc] initWithCapacity:self.segmentCount];
        _segmentOffsets[@(segment)] = @(frame.origin.x - _segmentContentInset + 1.0);
    } else {
        _segmentContentInset = frame.origin.x;
    }

    [super drawSegment:segment inFrame:frame withView:controlView];
}

- (void)stopTracking:(NSPoint)lastPoint at:(NSPoint)stopPoint inView:(id)controlView mouseIsUp:(BOOL)flag
{
    if ([controlView isKindOfClass:[MTSegmentedControl class]]) {
        id <MTSegmentedControlDelegate> delegate = [controlView delegate];

        if (delegate) {
            NSInteger segment = self.segmentCount - 1;
            while (segment >= 0) {
                if (stopPoint.x < [_segmentOffsets[@(segment)] doubleValue]) {
                    segment--;
                } else {
                    break;
                }
            }

            if (segment >= 0 && ![delegate segmentedControl:controlView shouldSelectSegment:segment]) {
                return;
            }
        }
    }

    [super stopTracking:lastPoint at:stopPoint inView:controlView mouseIsUp:flag];
}

@end

@implementation MTSegmentedControl

+ (Class)cellClass
{
    return [MTSegmentedCell class];
}

@end