Implement MonitorId::get_hidpi_factor for macOS (#410)

* Implement MonitorId::get_hidpi_factor for macOS

* Added changelog entry

src/platform/macos/monitor.rs

@@ -1,6 +1,10 @@
+use cocoa::appkit::NSScreen;
+use cocoa::base::{id, nil};
+use cocoa::foundation::{NSString, NSUInteger};
 use core_graphics::display::{CGDirectDisplayID, CGDisplay};
 use std::collections::VecDeque;
 use super::EventsLoop;
+use super::window::IdRef;
 
 #[derive(Clone)]
 pub struct MonitorId(CGDirectDisplayID);
@@ -51,8 +55,35 @@ impl MonitorId {
         unimplemented!()
     }
 
-    #[inline]
     pub fn get_hidpi_factor(&self) -> f32 {
-        1.0
+        let screen = match self.get_nsscreen() {
+            Some(screen) => screen,
+            None => return 1.0, // default to 1.0 when we can't find the screen
+        };
+
+        unsafe { NSScreen::backingScaleFactor(screen) as f32 }
+    }
+
+    pub(crate) fn get_nsscreen(&self) -> Option<id> {
+        unsafe {
+            let native_id = self.get_native_identifier();
+            let screens = NSScreen::screens(nil);
+            let count: NSUInteger = msg_send![screens, count];
+            let key = IdRef::new(NSString::alloc(nil).init_str("NSScreenNumber"));
+            let mut matching_screen: Option<id> = None;
+            for i in 0..count {
+                let screen = msg_send![screens, objectAtIndex: i as NSUInteger];
+                let device_description = NSScreen::deviceDescription(screen);
+                let value: id = msg_send![device_description, objectForKey:*key];
+                if value != nil {
+                    let screen_number: NSUInteger = msg_send![value, unsignedIntegerValue];
+                    if screen_number as u32 == native_id {
+                        matching_screen = Some(screen);
+                        break;
+                    }
+                }
+            }
+            matching_screen
+        }
     }
 }