macOS: Fix cursor hiding thread unsafety (#611)

CHANGELOG.md

@@ -5,6 +5,7 @@
 - Added NetBSD support.
 - **Breaking:** On iOS, `UIView` is now the default root view. `WindowBuilderExt::with_root_view_class` can be used to set the root view objective-c class to `GLKView` (OpenGLES) or `MTKView` (Metal/MoltenVK).
 - On iOS, the `UIApplication` is not started until `Window::new` is called.
+- Fixed thread unsafety with cursor hiding on macOS.
 
 # Version 0.16.2 (2018-07-07)
 

src/platform/macos/window.rs

@@ -1,8 +1,9 @@
 use std;
+use std::cell::{Cell, RefCell};
 use std::ops::Deref;
 use std::os::raw::c_void;
 use std::sync::Weak;
-use std::cell::{Cell, RefCell};
+use std::sync::atomic::{Ordering, AtomicBool};
 
 use cocoa;
 use cocoa::appkit::{
@@ -45,6 +46,7 @@ use window::MonitorId as RootMonitorId;
 #[derive(Debug, Copy, Clone, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct Id(pub usize);
 
+// TODO: It's possible for delegate methods to be called asynchronously, causing data races / `RefCell` panics.
 pub struct DelegateState {
     view: IdRef,
     window: IdRef,
@@ -526,7 +528,7 @@ pub struct Window2 {
     pub window: IdRef,
     pub delegate: WindowDelegate,
     pub input_context: IdRef,
-    cursor_hidden: Cell<bool>,
+    cursor_hidden: AtomicBool,
 }
 
 unsafe impl Send for Window2 {}
@@ -1007,13 +1009,13 @@ impl Window2 {
         let cursor_class = class!(NSCursor);
         // macOS uses a "hide counter" like Windows does, so we avoid incrementing it more than once.
         // (otherwise, `hide_cursor(false)` would need to be called n times!)
-        if hide != self.cursor_hidden.get() {
+        if hide != self.cursor_hidden.load(Ordering::Acquire) {
             if hide {
                 let _: () = unsafe { msg_send![cursor_class, hide] };
             } else {
                 let _: () = unsafe { msg_send![cursor_class, unhide] };
             }
-            self.cursor_hidden.replace(hide);
+            self.cursor_hidden.store(hide, Ordering::Release);
         }
     }