Determine which display a window is on in macOS

Question

I have the CGWindowID of one window and all CGDirectDisplayIDs of my Mac. Then I want to know which display the window on. I try to get the CGWindowInfo of the Window，but can`t find useful informations.

CFArrayRef windowList =  CGWindowListCopyWindowInfo(kCGWindowListOptionIncludingWindow, windowID);
CFArrayApplyFunction(windowList, CFRangeMake(0, CFArrayGetCount(windowList)), &WindowListApplierFunction, this);
CFRelease(windowList);

Answer 1

You can use the NSScreen API for that. Use [NSScreen screens] to retrieve the screens your computer is connected to and then match the screen returned by [myWindow screen].

If you own the window that you want to know on which screen is at just do the following:

CGWindowID windowID = ... // init with your value
NSWindow *window = [NSApplication windowWithWindowNumber:(NSInteger)windowID];

if ([[NSScreen screens] count] > 1)
{
    // you have more than one screen attached
    NSScreen *currentScreen = [window screen];

    // you can then test if the window is on the main display
    if (currentScreen == [NSScreen mainScreen])
    {
        // your window is on the main screen
    }
    else
    {
       // your window is not on the main screen
    }
}

However, if you don't own the window, because it is owned by another app, then I suggest that you first understand the differences between the Quartz coordinate system used by NSScreen and the Core Graphics coordinate system used by the CGWindow API. There is a good article about this here (English):

http://www.thinkandbuild.it/deal-with-multiple-screens-programming/

and here (Japanese) (use Google translator if you don't know Japanese):

http://xcatsan.blogspot.com/2009/09/nsscreen-cgwindow.html

Second, you need to retrieve the window bounds as explained by Son of Grab sample code I recommended or as explained here:

Determine which screen a window is on given the window ID

Then you need to calculate the screen where it is at by the window bounds as suggested.