Change workspace based on scroll events

Works, I'm not sure how many pixels to interpret as a change to the
workspace, and perhaps it should accumulate multiple deltas with a
timer. Assuming some lower level of the stack isn't doing that already.
I only see `ScrollDelta::Pixels` events, not `Lines`, though maybe
that's relevant with a different type of input device.

The behavior would also be clearer with animation, though it doesn't
seem to bad
(https://github.com/pop-os/cosmic-workspaces-epoch/issues/32).

Fixes https://github.com/pop-os/cosmic-workspaces-epoch/issues/34.

src/main.rs

@@ -16,6 +16,7 @@ use cosmic::{
         self,
         event::wayland::{Event as WaylandEvent, LayerEvent, OutputEvent},
         keyboard::key::{Key, Named},
+        mouse::ScrollDelta,
         Size, Subscription, Task,
     },
     iced_core::window::Id as SurfaceId,
@@ -98,6 +99,7 @@ enum Msg {
     Config(CosmicWorkspacesConfig),
     BgConfig(cosmic_bg_config::state::State),
     UpdateToplevelIcon(String, Option<PathBuf>),
+    OnScroll(wl_output::WlOutput, ScrollDelta),
     Ignore,
 }
 
@@ -537,6 +539,29 @@ impl Application for App {
                     }
                 }
             }
+            Msg::OnScroll(output, delta) => {
+                // TODO assumes only one active workspace per output
+                let mut workspaces = self.workspaces_for_output(&output).collect::<Vec<_>>();
+                if let Some(workspace_idx) = workspaces.iter().position(|i| i.is_active) {
+                    if let ScrollDelta::Pixels { x: _, y } = delta {
+                        // XXX accumulate delta, with timer?
+                        let workspace = if y <= -4. {
+                            // Next workspace on output
+                            workspaces[workspace_idx + 1..].iter().next()
+                        } else if y >= 4. {
+                            // Previous workspace on output
+                            workspaces[..workspace_idx].iter().last()
+                        } else {
+                            None
+                        };
+                        if let Some(workspace) = workspace {
+                            self.send_wayland_cmd(backend::Cmd::ActivateWorkspace(
+                                dbg!(workspace).handle.clone(),
+                            ));
+                        }
+                    }
+                }
+            }
             Msg::Ignore => {}
         }
 

src/view/mod.rs

@@ -103,7 +103,10 @@ pub(crate) fn layer_surface<'a>(
                 .width(iced::Length::Fill),
         ),
     };
-    container.into()
+    let output = surface.output.clone();
+    widget::mouse_area(container)
+        .on_scroll(move |delta| Msg::OnScroll(output.clone(), delta))
+        .into()
 }
 
 fn close_button(on_press: Msg) -> cosmic::Element<'static, Msg> {