Add a window swap operation (#899)

Swap the active window with the a neighboring column's active window.


---------

Co-authored-by: Ivan Molodetskikh <yalterz@gmail.com>
Take into account PR comments

- no longer behave like an expel when a swap is made in a direction
  where there is no column to swap with
- fix janky animation

5 files changed, 183 insertions, 2 deletions

diff --git a/src/input/mod.rs b/src/input/mod.rs
index 263f533d..576bc2f5 100644
--- a/src/input/mod.rs
+++ b/src/input/mod.rs
@@ -36,6 +36,7 @@ use touch_move_grab::TouchMoveGrab;
 use self::move_grab::MoveGrab;
 use self::resize_grab::ResizeGrab;
 use self::spatial_movement_grab::SpatialMovementGrab;
+use crate::layout::scrolling::ScrollDirection;
 use crate::niri::State;
 use crate::ui::screenshot_ui::ScreenshotUi;
 use crate::utils::spawning::spawn;
@@ -1132,6 +1133,22 @@ impl State {
                 // FIXME: granular
                 self.niri.queue_redraw_all();
             }
+            Action::SwapWindowRight => {
+                self.niri
+                    .layout
+                    .swap_window_in_direction(ScrollDirection::Right);
+                self.maybe_warp_cursor_to_focus();
+                // FIXME: granular
+                self.niri.queue_redraw_all();
+            }
+            Action::SwapWindowLeft => {
+                self.niri
+                    .layout
+                    .swap_window_in_direction(ScrollDirection::Left);
+                self.maybe_warp_cursor_to_focus();
+                // FIXME: granular
+                self.niri.queue_redraw_all();
+            }
             Action::SwitchPresetColumnWidth => {
                 self.niri.layout.toggle_width();
             }
diff --git a/src/layout/mod.rs b/src/layout/mod.rs
index 4923282b..9fa4681a 100644
--- a/src/layout/mod.rs
+++ b/src/layout/mod.rs
@@ -55,6 +55,7 @@ pub use self::monitor::MonitorRenderElement;
 use self::monitor::{Monitor, WorkspaceSwitch};
 use self::workspace::{OutputId, Workspace};
 use crate::animation::Clock;
+use crate::layout::scrolling::ScrollDirection;
 use crate::niri_render_elements;
 use crate::render_helpers::renderer::NiriRenderer;
 use crate::render_helpers::snapshot::RenderSnapshot;
@@ -2025,6 +2026,13 @@ impl<W: LayoutElement> Layout<W> {
         monitor.expel_from_column();
     }
 
+    pub fn swap_window_in_direction(&mut self, direction: ScrollDirection) {
+        let Some(monitor) = self.active_monitor() else {
+            return;
+        };
+        monitor.swap_window_in_direction(direction);
+    }
+
     pub fn center_column(&mut self) {
         let Some(monitor) = self.active_monitor() else {
             return;
@@ -4382,6 +4390,10 @@ mod tests {
         ]
     }
 
+    fn arbitrary_scroll_direction() -> impl Strategy<Value = ScrollDirection> {
+        prop_oneof![Just(ScrollDirection::Left), Just(ScrollDirection::Right)]
+    }
+
     #[derive(Debug, Clone, Copy, Arbitrary)]
     enum Op {
         AddOutput(#[proptest(strategy = "1..=5usize")] usize),
@@ -4462,6 +4474,9 @@ mod tests {
         },
         ConsumeWindowIntoColumn,
         ExpelWindowFromColumn,
+        SwapWindowInDirection(
+            #[proptest(strategy = "arbitrary_scroll_direction()")] ScrollDirection,
+        ),
         CenterColumn,
         CenterWindow {
             #[proptest(strategy = "proptest::option::of(1..=5usize)")]
@@ -4984,6 +4999,7 @@ mod tests {
                 }
                 Op::ConsumeWindowIntoColumn => layout.consume_into_column(),
                 Op::ExpelWindowFromColumn => layout.expel_from_column(),
+                Op::SwapWindowInDirection(direction) => layout.swap_window_in_direction(direction),
                 Op::CenterColumn => layout.center_column(),
                 Op::CenterWindow { id } => {
                     let id = id.filter(|id| layout.has_window(id));
diff --git a/src/layout/monitor.rs b/src/layout/monitor.rs
index 2de3c760..57ca7f53 100644
--- a/src/layout/monitor.rs
+++ b/src/layout/monitor.rs
@@ -9,7 +9,7 @@ use smithay::backend::renderer::element::utils::{
 use smithay::output::Output;
 use smithay::utils::{Logical, Point, Rectangle, Size};
 
-use super::scrolling::{Column, ColumnWidth};
+use super::scrolling::{Column, ColumnWidth, ScrollDirection};
 use super::tile::Tile;
 use super::workspace::{
     OutputId, Workspace, WorkspaceAddWindowTarget, WorkspaceId, WorkspaceRenderElement,
@@ -707,6 +707,10 @@ impl<W: LayoutElement> Monitor<W> {
         self.active_workspace().expel_from_column();
     }
 
+    pub fn swap_window_in_direction(&mut self, direction: ScrollDirection) {
+        self.active_workspace().swap_window_in_direction(direction);
+    }
+
     pub fn center_column(&mut self) {
         self.active_workspace().center_column();
     }
diff --git a/src/layout/scrolling.rs b/src/layout/scrolling.rs
index 89975736..e54beccb 100644
--- a/src/layout/scrolling.rs
+++ b/src/layout/scrolling.rs
@@ -239,6 +239,16 @@ pub enum WindowHeight {
     Preset(usize),
 }
 
+/// Horizontal direction for an operation
+///
+/// As operations often have a symmetrical counterpart, e.g. focus-right/focus-left, methods
+/// on `Scrolling` can sometimes be factored using the direction of the operation as a parameter.
+#[derive(Debug, Clone, Copy, PartialEq)]
+pub enum ScrollDirection {
+    Left,
+    Right,
+}
+
 impl<W: LayoutElement> ScrollingSpace<W> {
     pub fn new(
         view_size: Size<f64, Logical>,
@@ -1749,6 +1759,132 @@ impl<W: LayoutElement> ScrollingSpace<W> {
         new_col.tiles[0].animate_move_from(offset);
     }
 
+    pub fn swap_window_in_direction(&mut self, direction: ScrollDirection) {
+        if self.columns.is_empty() {
+            return;
+        }
+
+        // if this is the first (resp. last column), then this operation is equivalent
+        // to an `consume_or_expel_window_left` (resp. `consume_or_expel_window_right`)
+        match direction {
+            ScrollDirection::Left => {
+                if self.active_column_idx == 0 {
+                    return;
+                }
+            }
+            ScrollDirection::Right => {
+                if self.active_column_idx == self.columns.len() - 1 {
+                    return;
+                }
+            }
+        }
+
+        let source_column_idx = self.active_column_idx;
+        let target_column_idx = self.active_column_idx.wrapping_add_signed(match direction {
+            ScrollDirection::Left => -1,
+            ScrollDirection::Right => 1,
+        });
+
+        // if both source and target columns contain a single tile, then the operation is equivalent
+        // to a simple column move
+        if self.columns[source_column_idx].tiles.len() == 1
+            && self.columns[target_column_idx].tiles.len() == 1
+        {
+            return self.move_column_to(target_column_idx);
+        }
+
+        let source_tile_idx = self.columns[source_column_idx].active_tile_idx;
+        let target_tile_idx = self.columns[target_column_idx].active_tile_idx;
+        let source_column_drained = self.columns[source_column_idx].tiles.len() == 1;
+
+        // capture the original positions of the tiles
+        let (mut source_pt, mut target_pt) = (
+            self.columns[source_column_idx].render_offset()
+                + self.columns[source_column_idx].tile_offset(source_tile_idx),
+            self.columns[target_column_idx].render_offset()
+                + self.columns[target_column_idx].tile_offset(target_tile_idx),
+        );
+        source_pt.x += self.column_x(source_column_idx);
+        target_pt.x += self.column_x(target_column_idx);
+
+        let transaction = Transaction::new();
+
+        // If the source column contains a single tile, this will also remove the column.
+        // When this happens `source_column_drained` will be set and the column will need to be
+        // recreated with `add_tile`
+        let source_removed = self.remove_tile_by_idx(
+            source_column_idx,
+            source_tile_idx,
+            transaction.clone(),
+            None,
+        );
+
+        {
+            // special case when the source column disappears after removing its last tile
+            let adjusted_target_column_idx =
+                if direction == ScrollDirection::Right && source_column_drained {
+                    target_column_idx - 1
+                } else {
+                    target_column_idx
+                };
+
+            self.add_tile_to_column(
+                adjusted_target_column_idx,
+                Some(target_tile_idx),
+                source_removed.tile,
+                false,
+            );
+
+            let RemovedTile {
+                tile: target_tile, ..
+            } = self.remove_tile_by_idx(
+                adjusted_target_column_idx,
+                target_tile_idx + 1,
+                transaction.clone(),
+                None,
+            );
+
+            if source_column_drained {
+                // recreate the drained column with only the target tile
+                self.add_tile(
+                    Some(source_column_idx),
+                    target_tile,
+                    true,
+                    source_removed.width,
+                    source_removed.is_full_width,
+                    None,
+                )
+            } else {
+                // simply add the removed target tile to the source column
+                self.add_tile_to_column(
+                    source_column_idx,
+                    Some(source_tile_idx),
+                    target_tile,
+                    false,
+                );
+            }
+        }
+
+        // update the active tile in the modified columns
+        self.columns[source_column_idx].active_tile_idx = source_tile_idx;
+        self.columns[target_column_idx].active_tile_idx = target_tile_idx;
+
+        // Animations
+        self.columns[target_column_idx].tiles[target_tile_idx]
+            .animate_move_from(source_pt - target_pt);
+
+        // FIXME: this stop_move_animations() causes the target tile animation to "reset" when
+        // swapping. It's here as a workaround to stop the unwanted animation of moving the source
+        // tile down when adding the target tile above it. This code needs to be written in some
+        // other way not to trigger that animation, or to cancel it properly, so that swap doesn't
+        // cancel all ongoing target tile animations.
+        self.columns[source_column_idx].tiles[source_tile_idx].stop_move_animations();
+        self.columns[source_column_idx].tiles[source_tile_idx]
+            .animate_move_from(target_pt - source_pt);
+
+        self.activate_column(target_column_idx);
+    }
+
     pub fn center_column(&mut self) {
         if self.columns.is_empty() {
             return;
diff --git a/src/layout/workspace.rs b/src/layout/workspace.rs
index b11b46ac..a931c694 100644
--- a/src/layout/workspace.rs
+++ b/src/layout/workspace.rs
@@ -15,7 +15,8 @@ use smithay::wayland::shell::xdg::SurfaceCachedState;
 
 use super::floating::{FloatingSpace, FloatingSpaceRenderElement};
 use super::scrolling::{
-    Column, ColumnWidth, InsertHint, InsertPosition, ScrollingSpace, ScrollingSpaceRenderElement,
+    Column, ColumnWidth, InsertHint, InsertPosition, ScrollDirection, ScrollingSpace,
+    ScrollingSpaceRenderElement,
 };
 use super::tile::{Tile, TileRenderSnapshot};
 use super::{ActivateWindow, InteractiveResizeData, LayoutElement, Options, RemovedTile, SizeFrac};
@@ -969,6 +970,13 @@ impl<W: LayoutElement> Workspace<W> {
         self.scrolling.expel_from_column();
     }
 
+    pub fn swap_window_in_direction(&mut self, direction: ScrollDirection) {
+        if self.floating_is_active.get() {
+            return;
+        }
+        self.scrolling.swap_window_in_direction(direction);
+    }
+
     pub fn center_column(&mut self) {
         if self.floating_is_active.get() {
             self.floating.center_window(None);