example_NSViewGeometryInWindowDidChangeNotification.m

#import "ViewController.h"
#import <objc/message.h>
#import <objc/runtime.h>

NSNotificationCenter * __viewGeometryNotificationCenter;
id (*nc_originalInit)(id, SEL);
id nc_customInit(id self, SEL _cmd) {
    __viewGeometryNotificationCenter = self;
    return nc_originalInit(self, _cmd);
}

@interface ViewController ()
@property (weak) IBOutlet NSButton *upButton;
@end

@implementation ViewController

+ (void)load {
    Method method = class_getInstanceMethod([NSNotificationCenter class], @selector(init));
    nc_originalInit = (typeof(nc_originalInit))method_getImplementation(method);
    method_setImplementation(method, (IMP)nc_customInit);
    
    [[NSTrackingSeparatorToolbarItem alloc] initWithItemIdentifier:@""];
    
    method_setImplementation(method, (IMP)nc_originalInit);
}

- (void)viewDidLoad {
    [super viewDidLoad];

    NSButton *upButton = self.upButton;
    
    ((void (*)(id, SEL))objc_msgSend)(upButton, sel_registerName("enableGeometryInWindowDidChangeNotification"));
    [__viewGeometryNotificationCenter addObserver:self selector:@selector(foo:) name:@"NSViewGeometryInWindowDidChangeNotification" object:upButton];
}

- (IBAction)up:(NSButton *)sender {
    NSRect oldRect = sender.frame;
    sender.frame = NSMakeRect(oldRect.origin.x, oldRect.origin.y + 1., oldRect.size.width, oldRect.size.height);
}

- (void)foo:(NSNotification *)notification {
    NSLog(@"%@", notification);
}

@end