Rework Drag-And-Drop API (#4079)

* Add cursor position drag and drop events.
* Reword drag events to match pointer ones.
* appkit: Use `convertPoint_fromView` for coordinate conversion.
* appkit: use ProtocolObject<dyn NSDraggingInfo>.
* x11: store dnd.position as pair of i16

  It's what translate_coords takes anyway, so the extra precision is
  misleading if we're going to cast it to i16 everywhere it's used.

  We can also simplify the "unpacking" from the XdndPosition message--we
  can and should use the value of 16 as the shift instead of
  size_of::<c_short> * 2 or something like that, because the specification
  gives us the constant 16.
* x11: store translated DnD coords.
* x11: don't emit DragLeave without DragEnter.
* windows: only emit DragEnter if valid.
* windows: in DnD, always set pdwEffect.

  It appears other apps (like Chromium) set pdwEffect on Drop too:
  61a391b86b/ui/base/dragdrop/drop_target_win.cc
* docs: make it clearer that drag events are for dragged *files*.
* examples/dnd: handle RedrawRequested event.

Co-authored-by: amrbashir <amr.bashir2015@gmail.com>

examples/dnd.rs

@@ -0,0 +1,64 @@
+use std::error::Error;
+
+use winit::application::ApplicationHandler;
+use winit::event::WindowEvent;
+use winit::event_loop::{ActiveEventLoop, EventLoop};
+use winit::window::{Window, WindowAttributes, WindowId};
+
+#[path = "util/fill.rs"]
+mod fill;
+#[path = "util/tracing.rs"]
+mod tracing;
+
+fn main() -> Result<(), Box<dyn Error>> {
+    tracing::init();
+
+    let event_loop = EventLoop::new()?;
+
+    let app = Application::new();
+    Ok(event_loop.run_app(app)?)
+}
+
+/// Application state and event handling.
+struct Application {
+    window: Option<Box<dyn Window>>,
+}
+
+impl Application {
+    fn new() -> Self {
+        Self { window: None }
+    }
+}
+
+impl ApplicationHandler for Application {
+    fn can_create_surfaces(&mut self, event_loop: &dyn ActiveEventLoop) {
+        let window_attributes =
+            WindowAttributes::default().with_title("Drag and drop files on me!");
+        self.window = Some(event_loop.create_window(window_attributes).unwrap());
+    }
+
+    fn window_event(
+        &mut self,
+        event_loop: &dyn ActiveEventLoop,
+        _window_id: WindowId,
+        event: WindowEvent,
+    ) {
+        match event {
+            WindowEvent::DragLeft { .. }
+            | WindowEvent::DragEntered { .. }
+            | WindowEvent::DragMoved { .. }
+            | WindowEvent::DragDropped { .. } => {
+                println!("{:?}", event);
+            },
+            WindowEvent::RedrawRequested => {
+                let window = self.window.as_ref().unwrap();
+                window.pre_present_notify();
+                fill::fill_window(window.as_ref());
+            },
+            WindowEvent::CloseRequested => {
+                event_loop.exit();
+            },
+            _ => {},
+        }
+    }
+}

examples/window.rs

@@ -541,11 +541,12 @@ impl ApplicationHandler for Application {
                 info!("Smart zoom");
             },
             WindowEvent::TouchpadPressure { .. }
-            | WindowEvent::HoveredFileCancelled
+            | WindowEvent::DragLeft { .. }
             | WindowEvent::KeyboardInput { .. }
             | WindowEvent::PointerEntered { .. }
-            | WindowEvent::DroppedFile(_)
-            | WindowEvent::HoveredFile(_)
+            | WindowEvent::DragEntered { .. }
+            | WindowEvent::DragMoved { .. }
+            | WindowEvent::DragDropped { .. }
             | WindowEvent::Destroyed
             | WindowEvent::Moved(_) => (),
         }