Add smart magnify gesture support for macOS (#2554)

* Add smart magnification gesture

* Deliver position of smart magnification event

* Document smart magnification event

* Revert "Deliver position of smart magnification event"

This reverts commit ac0e61a9a4.

* Remove mention of touchpad from smart magnification event

* Update change log

* Mention minimum macOS version supporting smart magnification

* Improve doc

src/event.rs

@@ -444,6 +444,25 @@ pub enum WindowEvent<'a> {
         phase: TouchPhase,
     },
 
+    /// Smart magnification event.
+    ///
+    /// On a Mac, smart magnification is triggered by a double tap with two fingers
+    /// on the trackpad and is commonly used to zoom on a certain object
+    /// (e.g. a paragraph of a PDF) or (sort of like a toggle) to reset any zoom.
+    /// The gesture is also supported in Safari, Pages, etc.
+    ///
+    /// The event is general enough that its generating gesture is allowed to vary
+    /// across platforms. It could also be generated by another device.
+    ///
+    /// Unfortunatly, neither [Windows](https://support.microsoft.com/en-us/windows/touch-gestures-for-windows-a9d28305-4818-a5df-4e2b-e5590f850741)
+    /// nor [Wayland](https://wayland.freedesktop.org/libinput/doc/latest/gestures.html)
+    /// support this gesture or any other gesture with the same effect.
+    ///
+    /// ## Platform-specific
+    ///
+    /// - Only available on **macOS 10.8** and later.
+    SmartMagnify { device_id: DeviceId },
+
     /// Touchpad rotation event with two-finger rotation gesture.
     ///
     /// Positive delta values indicate rotation counterclockwise and
@@ -594,6 +613,9 @@ impl Clone for WindowEvent<'static> {
                 delta: *delta,
                 phase: *phase,
             },
+            SmartMagnify { device_id } => SmartMagnify {
+                device_id: *device_id,
+            },
             TouchpadRotate {
                 device_id,
                 delta,
@@ -700,6 +722,7 @@ impl<'a> WindowEvent<'a> {
                 delta,
                 phase,
             }),
+            SmartMagnify { device_id } => Some(SmartMagnify { device_id }),
             TouchpadRotate {
                 device_id,
                 delta,

src/platform_impl/macos/view.rs

@@ -858,6 +858,20 @@ declare_class!(
             AppState::queue_event(EventWrapper::StaticEvent(window_event));
         }
 
+        #[sel(smartMagnifyWithEvent:)]
+        fn smart_magnify_with_event(&self, _event: &NSEvent) {
+            trace_scope!("smartMagnifyWithEvent:");
+
+            let window_event = Event::WindowEvent {
+                window_id: self.window_id(),
+                event: WindowEvent::SmartMagnify {
+                    device_id: DEVICE_ID,
+                },
+            };
+
+            AppState::queue_event(EventWrapper::StaticEvent(window_event));
+        }
+
         #[sel(rotateWithEvent:)]
         fn rotate_with_event(&self, event: &NSEvent) {
             trace_scope!("rotateWithEvent:");