plugins: add an example for using a wheel to trigger button scrolling

See https://gitlab.freedesktop.org/libinput/libinput/-/issues/1206

Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1356>

plugins/10-wheel-to-button.lua

@@ -0,0 +1,64 @@
+-- SPDX-License-Identifier: MIT
+--
+-- This is an example libinput plugin
+--
+-- This plugin maps a downwards mouse wheel to a button down event and
+-- an upwards wheel movement to a button up event.
+
+-- UNCOMMENT THIS LINE TO ACTIVATE THE PLUGIN
+-- libinput:register({1})
+
+-- The button we want to press on wheel events
+local wheel_button = evdev.BTN_EXTRA
+local button_states = {}
+
+local function evdev_frame(device, frame, timestamp)
+    local events = {}
+    local modified = false
+
+    for _, v in ipairs(frame) do
+        if v.usage == evdev.REL_WHEEL then
+            -- REL_WHEEL is inverted, neg value -> down, pos value -> up
+            if v.value < 0 then
+                if not button_states[device] then
+                    table.insert(events, { usage = wheel_button, value = 1 })
+                    button_states[device] = true
+                end
+            else
+                if button_states[device] then
+                    table.insert(events, { usage = wheel_button, value = 0 })
+                    button_states[device] = false
+                end
+            end
+            modified = true
+        -- Because REL_WHEEL is no longer a wheel, the high-res
+        -- events are dropped
+        elseif v.usage == evdev.REL_WHEEL_HI_RES then
+            modified = true
+        else
+            table.insert(events, v)
+        end
+    end
+
+    if modified then
+        return events
+    else
+        return nil
+    end
+end
+
+local function device_new(device)
+    local usages = device:usages()
+    if usages[evdev.REL_WHEEL] then
+        button_states[device] = false
+        if not usages[wheel_button] then
+            device:enable_evdev_usage(wheel_button)
+        end
+        device:connect("evdev-frame", evdev_frame)
+        device:connect("device-removed", function(dev)
+            button_states[dev] = nil
+        end)
+    end
+end
+
+libinput:connect("new-evdev-device", device_new)

plugins/meson.build

@@ -7,6 +7,7 @@ plugins = [
 	'10-delay-motion.lua',
 	'10-disable-feature.lua',
 	'10-copilot-key-override.lua',
+	'10-wheel-to-button.lua',
 ]
 
 fs = import('fs')