macOS: Provide helper property for resolving the display ID of a screen

Change-Id: I144bd33a2c122d53ea1435a53483a3d8b46fd093
Reviewed-by: Gabriel de Dietrich <gabriel.dedietrich@qt.io>

src/plugins/platforms/cocoa/qcocoaintegration.mm

@@ -244,7 +244,7 @@ void QCocoaIntegration::updateScreens()
     uint screenCount = [screens count];
     for (uint i = 0; i < screenCount; i++) {
         NSScreen* scr = [screens objectAtIndex:i];
-        CGDirectDisplayID dpy = [[[scr deviceDescription] objectForKey:@"NSScreenNumber"] unsignedIntValue];
+        CGDirectDisplayID dpy = scr.qt_displayId;
         // If this screen is a mirror and is not the primary one of the mirror set, ignore it.
         // Exception: The NSScreen API has been observed to a return a screen list with one
         // mirrored, non-primary screen when Qt is running as a startup item. Always use the

src/plugins/platforms/cocoa/qcocoascreen.h

@@ -104,5 +104,9 @@ QDebug operator<<(QDebug debug, const QCocoaScreen *screen);
 
 QT_END_NAMESPACE
 
+@interface NSScreen (QtExtras)
+@property(readonly) CGDirectDisplayID qt_displayId;
+@end
+
 #endif
 

src/plugins/platforms/cocoa/qcocoascreen.mm

@@ -121,8 +121,7 @@ void QCocoaScreen::updateGeometry()
     m_format = QImage::Format_RGB32;
     m_depth = NSBitsPerPixelFromDepth([nsScreen depth]);
 
-    NSDictionary *devDesc = [nsScreen deviceDescription];
-    CGDirectDisplayID dpy = [[devDesc objectForKey:@"NSScreenNumber"] unsignedIntValue];
+    CGDirectDisplayID dpy = nsScreen.qt_displayId;
     CGSize size = CGDisplayScreenSize(dpy);
     m_physicalSize = QSizeF(size.width, size.height);
     m_logicalDpi.first = 72;
@@ -310,3 +309,12 @@ QDebug operator<<(QDebug debug, const QCocoaScreen *screen)
 #endif // !QT_NO_DEBUG_STREAM
 
 QT_END_NAMESPACE
+
+@implementation NSScreen (QtExtras)
+
+- (CGDirectDisplayID)qt_displayId
+{
+    return [self.deviceDescription[@"NSScreenNumber"] unsignedIntValue];
+}
+
+@end