Add action to switch layouts

Allow users to trigger layout change with custom bindings.

resources/default-config.kdl

@@ -230,6 +230,14 @@ binds {
     Mod+Shift+Minus { set-window-height "-10%"; }
     Mod+Shift+Equal { set-window-height "+10%"; }
 
+    // Actions to switch layouts.
+    // Note: if you uncomment these, make sure you do NOT have
+    // a matching layout switch hotkey configured in xkb options above.
+    // Having both at once on the same hotkey will break the switching,
+    // since it will switch twice upon pressing the hotkey (once by xkb, once by niri).
+    // Mod+Space       { switch-layout "next"; }
+    // Mod+Shift+Space { switch-layout "prev"; }
+
     Print { screenshot; }
     Ctrl+Print { screenshot-screen; }
     Alt+Print { screenshot-window; }

src/config.rs

@@ -288,6 +288,7 @@ pub enum Action {
     SwitchPresetColumnWidth,
     MaximizeColumn,
     SetColumnWidth(#[knuffel(argument, str)] SizeChange),
+    SwitchLayout(#[knuffel(argument)] LayoutAction),
 }
 
 #[derive(Debug, Clone, Copy, PartialEq)]
@@ -298,6 +299,12 @@ pub enum SizeChange {
     AdjustProportion(f64),
 }
 
+#[derive(knuffel::DecodeScalar, Debug, Clone, Copy, PartialEq)]
+pub enum LayoutAction {
+    Next,
+    Prev,
+}
+
 #[derive(knuffel::Decode, Debug, PartialEq)]
 pub struct DebugConfig {
     #[knuffel(child, unwrap(argument), default = 1.)]

src/input.rs

@@ -17,7 +17,7 @@ use smithay::input::pointer::{
 use smithay::utils::SERIAL_COUNTER;
 use smithay::wayland::tablet_manager::{TabletDescriptor, TabletSeatTrait};
 
-use crate::config::{Action, Binds, Modifiers};
+use crate::config::{Action, Binds, LayoutAction, Modifiers};
 use crate::niri::State;
 use crate::screenshot_ui::ScreenshotUi;
 use crate::utils::{center, get_monotonic_time, spawn};
@@ -90,6 +90,7 @@ impl State {
                             | Action::ChangeVt(_)
                             | Action::Suspend
                             | Action::PowerOffMonitors
+                            | Action::SwitchLayout(_)
                     )
                 {
                     return;
@@ -186,6 +187,16 @@ impl State {
                             self.niri.layout.toggle_fullscreen(&window);
                         }
                     }
+                    Action::SwitchLayout(action) => {
+                        self.niri
+                            .seat
+                            .get_keyboard()
+                            .unwrap()
+                            .with_kkb_state(self, |mut state| match action {
+                                LayoutAction::Next => state.cycle_next_layout(),
+                                LayoutAction::Prev => state.cycle_prev_layout(),
+                            });
+                    }
                     Action::MoveColumnLeft => {
                         self.niri.layout.move_left();
                         // FIXME: granular