CGDisplayRegisterReconfigurationCallback: callback not called - objective-c

Question

Why doesn't the function displayChanged get fired in the following code?
#import <Cocoa/Cocoa.h>
static void displayChanged(CGDirectDisplayID displayID, CGDisplayChangeSummaryFlags flags, void *userInfo) {
NSLog(#"%#, %#", displayID, flags);
}
int main(int argc, const char * argv[])
{
#autoreleasepool {
CGDisplayRegisterReconfigurationCallback(displayChanged, NULL);
CFRunLoopRun();
}
return 0;
}
I'm physically removing (and plugging in) my external display, but the function is never run.
Why?

Answer 1

Just found the solution on this other question
Before calling CFRunLoopRun you have to call NSApplicationLoad to establish a connection with the window server. This is the fixed code for the original question:
#import <Cocoa/Cocoa.h>
static void displayChanged(CGDirectDisplayID displayID, CGDisplayChangeSummaryFlags flags, void *userInfo) {
NSLog(#"%u, %u", displayID, flags);
}
int main(int argc, const char * argv[])
{
#autoreleasepool {
CGDisplayRegisterReconfigurationCallback(displayChanged, NULL);
NSApplicationLoad(); // establish a connection to the window server. In <Cocoa/Cocoa.h>
CFRunLoopRun(); // run the event loop
}
return 0;
}

Answer 2

I couldn't get CGDisplayRegisterReconfigurationCallback to work, so I used distributed notifications instead:
int main(int argc, const char * argv[]) {
#autoreleasepool {
[[NSDistributedNotificationCenter defaultCenter] addObserverForName:#"com.apple.BezelServices.BMDisplayHWReconfiguredEvent" object:nil queue:nil usingBlock:^(NSNotification *notification) {
NSLog(#"Displays changed!");
}];
CFRunLoopRun();
}
return 0;
}

Answer 3

If you're using AppKit (and have a running NSApplication event loop), you can listen for the NSApplicationDidChangeScreenParametersNotification notification. Or, you can implement the -applicationDidChangeScreenParameters: method in your application delegate, which amounts to the same thing.