Use Quarz Window Services API in XPC service - macos

Question

I have an App which does screen scrape and window scrape, both are working pretty well in the application, later I decided to move both screen scrape and window scrape into the XPC service which belongs to this App, the display scrape works ok, but window scrape malfunction:
My first try is that the host app pass the CGWindowID to the XPC, in the XPC, I use
CGWindowListCreateImage and CGWindowListCreateImageFromArray, both them always return null.
Then I try to pass the pid of the window to be scrape from the host, and In the XPC
I use CGWindowListCopyWindowInfo to enumerate Windows to match the pid to find the related CGWindowID inside the XPC service process, anyway, CGWindowListCopyWindowInfo always return null
My next try is to use
auto findWindowIds = [](uint32_t pId) -> CFArrayRef
{
auto appRef = AXUIElementCreateApplication(pId);
CFMutableArrayRef idArray = CFArrayCreateMutable(0, 0, nullptr);
CFIndex count = 0;
CFArrayRef windowArray = NULL;
auto err = AXUIElementGetAttributeValueCount(appRef,
CFSTR("AXWindows"), &count);
if (err == kAXErrorSuccess && count)
{
AXUIElementCopyAttributeValues(appRef, CFSTR("AXWindows"), 0, count, &windowArray);
for (int idx = 0; idx < count; idx ++)
{
AXUIElementRef element = (AXUIElementRef)
CFArrayGetValueAtIndex(windowArray, idx);
CGWindowID temp = 0;
_AXUIElementGetWindow(element, &temp);
LOGEX("windowId: %u", temp);
CFArrayAppendValue(idArray, reinterpret_cast<void*>(temp));
}
SAFE_CFRELEASE(windowArray);
}
SAFE_CFRELEASE(appRef);
return idArray;
};
to reverse the Pid to WindowID, the above code works as expected (the CGWindowID is exactly the same as the host app) according what I debugged into the code, anyway,
CGWindowListCreateImage and CGWindowListCreateImageFromArray are still return null.
Since host app is using Quartz Window Services APIs to enumerate windows, the window server is running, and the XPC inherit the host App's GUI security session, beside that,
Quartz Display Streaming are working pretty well inside XPC service, don't why this is happening.
Could we use Quartz Window Service API inside XPC service?

Answer 1

It turns out I need to add joinExistingSession in XPC settings so that it could access Windows Server.