Programmatically trigger “detect displays.”

Question

I’m trying to trigger the same thing that the system does when you click on “Detect Displays” in the “Displays” System Preferences pane. Is there a way to do this programmatically? My goal is to create a LaunchAgent that does this at login to reset the display resolution in case a user messes with it.

Answer 1

You will need to use a private CoreGraphics routine to get the list of all displays including inactive ones, and then request a rescan of the bus. Try it like this:

#include <IOKit/IOKitLib.h>
#include <IOKit/IOTypes.h>

CGDisplayErr CGSGetDisplayList(CGDisplayCount maxDisplays,
                                    CGDirectDisplayID * onlineDspys,
                                    CGDisplayCount * dspyCnt);
static void DetectDisplays()
{
 CGDirectDisplayID    displays[8];
    CGDisplayCount  dspCount = 0;

 if (CGSGetDisplayList(8, displays, &dspCount) == noErr)
 {
  for(int i = 0; i < dspCount; i++)
  {
   io_service_t service = CGDisplayIOServicePort(displays[i]);
   if (service)
    IOServiceRequestProbe(service, kIOFBUserRequestProbe);
  }
 }
}

And link to ApplicationServices and IOKit.

Answer 2

Since the CGDisplayIOServicePort call has been deprecated in OS X 10.9 Mavericks, it may be better to acquire the IOFramebuffer service through an appropriate matching call like so:

void triggerDetectDisplays()
{
    // loop over all IOFramebuffer services
    CFMutableDictionaryRef matchingDict = IOServiceMatching("IOFramebuffer");

    mach_port_t masterPort;
    IOMasterPort(MACH_PORT_NULL, &masterPort);
    io_iterator_t serviceIterator;
    IOServiceGetMatchingServices(masterPort, matchingDict, &serviceIterator);

    io_service_t obj = IOIteratorNext(serviceIterator);
    while (obj)
    {
        kern_return_t kr = IOServiceRequestProbe(obj, 0);
        obj = IOIteratorNext(serviceIterator);
    }
}