Automatically switching the dock position when connecting or disconnecting another monitor

dock_monitor.m

// compile command:
// xcrun clang -o dock_monitor dock_monitor.m -fobjc-arc -isysroot $(xcrun --show-sdk-path) -framework Foundation -framework AppKit -Wall -Wshadow -Wextra
#import <Foundation/Foundation.h>
#import <AppKit/AppKit.h>

void changeDockPosition(CGDirectDisplayID displayID, NSString *position)
{
    // Retrieve the defaults dictionary and change the orientation key to
    // the new position
    NSUserDefaults *defaults = [NSUserDefaults standardUserDefaults];
    NSDictionary<NSString *, id> *dockDefaults = [defaults persistentDomainForName:@"com.apple.dock"];
    NSMutableDictionary *newDict = [NSMutableDictionary dictionaryWithDictionary:dockDefaults];
    [newDict setObject:position forKey:@"orientation"];
    [defaults setPersistentDomain:newDict forName:@"com.apple.dock"];

    // kill the dock, which will cause it to restart and use the new setting.
    NSRunningApplication *dock = [NSRunningApplication runningApplicationsWithBundleIdentifier:@"com.apple.dock"][0];
    [dock terminate];

    // output the current position
    NSString *dockParameterYouWant = [[defaults persistentDomainForName:@"com.apple.dock"] valueForKey:@"orientation"];
    NSLog(@"%x added, dock is %@", displayID, dockParameterYouWant);
}

static void displayChanged(CGDirectDisplayID displayID, CGDisplayChangeSummaryFlags flags, void *userInfo) {
    (void)userInfo;
    @autoreleasepool
    {
        if (flags & kCGDisplayAddFlag) {
            // display has been added
            changeDockPosition(displayID, @"bottom");
        }
        else if (flags & kCGDisplayRemoveFlag) {
            // display has been removed
            changeDockPosition(displayID, @"left");
        }
    }
}

int main(int argc, const char * argv[])
{
    (void)argc;
    (void)argv;
    @autoreleasepool {

        CGDisplayRegisterReconfigurationCallback(displayChanged, NULL);

        NSApplicationLoad();  // establish a connection to the window server. In <Cocoa/Cocoa.h>
        CFRunLoopRun();       // run the event loop
    }
    return 0;
}