SDL: add multitouch support (#1599)

Partially inspired by the Android implementation.

After I added it, I suddenly realized we could've had fake multitouch on desktop for years. There's a right button on the mouse, after all.
A possible alternative would be to simply fake right mouse button as being to fingers at once, as opposed to having to truly press left and right at the same time.

Also removes an outdated OSX workaround.

Author: Frenzie

ffi/SDL2_0.lua

@@ -54,6 +54,10 @@ local function SDL_Linked_Version_AtLeast(x, y, z)
     return SDL_VersionNum(getSDLVersion()) >= SDL_VersionNum(x, y, z)
 end
 
+-- Used as the device ID for mouse events simulated with touch input.
+-- Defined in SDL_touch.h as #define SDL_TOUCH_MOUSEID ((Uint32)-1)
+local SDL_TOUCH_MOUSEID = ffi.cast('uint32_t', -1);
+
 local S = {
     w = 0, h = 0,
     screen = nil,
@@ -201,6 +205,58 @@ local function genEmuEvent(evtype, code, value)
     table.insert(inputQueue, ev)
 end
 
+-- Keep track of active pointers so we can feed ABS_MT_SLOT 0 and 1 to the frontend for multitouch to work.
+-- down, a boolean denoting whether the pointer is currently down for tracking mouse button status.
+local pointers = {}
+local function setPointerDownState(slot, down)
+    if not pointers[slot] then
+        pointers[slot] = { down = down }
+    else
+        pointers[slot].down = down
+    end
+end
+
+-- For the moment we pretend there can only be one touchscreen/trackpad/whatever at a time.
+-- It's probably close enough to the truth unless you run into a tester.
+local function getFingerSlot(event)
+    if not pointers[tonumber(event.tfinger.fingerId)] then
+        local num_touch_fingers = SDL.SDL_GetNumTouchFingers(event.tfinger.touchId)
+        if num_touch_fingers > 1 then
+            for i=0,num_touch_fingers-1 do
+                if SDL.SDL_GetTouchFinger(event.tfinger.touchId, i).id == event.tfinger.fingerId then
+                    pointers[tonumber(event.tfinger.fingerId)] = { slot = i }
+                end
+            end
+        else
+            pointers[tonumber(event.tfinger.fingerId)] = { slot = 0 }
+        end
+    end
+    return pointers[tonumber(event.tfinger.fingerId)].slot
+end
+
+local function genTouchDownEvent(event, slot, x, y)
+    genEmuEvent(C.EV_ABS, C.ABS_MT_SLOT, slot)
+    genEmuEvent(C.EV_ABS, C.ABS_MT_TRACKING_ID, tonumber(event.tfinger.fingerId))
+    genEmuEvent(C.EV_ABS, C.ABS_MT_POSITION_X, x)
+    genEmuEvent(C.EV_ABS, C.ABS_MT_POSITION_Y, y)
+    genEmuEvent(C.EV_SYN, C.SYN_REPORT, 0)
+end
+
+local function genTouchUpEvent(event, slot, x, y)
+    genEmuEvent(C.EV_ABS, C.ABS_MT_SLOT, slot)
+    genEmuEvent(C.EV_ABS, C.ABS_MT_TRACKING_ID, -1)
+    genEmuEvent(C.EV_ABS, C.ABS_MT_POSITION_X, x)
+    genEmuEvent(C.EV_ABS, C.ABS_MT_POSITION_Y, y)
+    genEmuEvent(C.EV_SYN, C.SYN_REPORT, 0)
+end
+
+local function genTouchMoveEvent(event, slot, x, y)
+    genEmuEvent(C.EV_ABS, C.ABS_MT_SLOT, slot)
+    genEmuEvent(C.EV_ABS, C.ABS_MT_POSITION_X, x)
+    genEmuEvent(C.EV_ABS, C.ABS_MT_POSITION_Y, y)
+    genEmuEvent(C.EV_SYN, C.SYN_REPORT, 0)
+end
+
 local function handleWindowEvent(event_window)
     -- The next buffer might always contain garbage, and on X11 without
     -- compositing the buffers will be damaged just by moving the window
@@ -273,8 +329,7 @@ local function handleJoyAxisMotionEvent(event)
 end
 
 local SDL_BUTTON_LEFT = 1
-
-local is_in_touch = false
+local SDL_BUTTON_RIGHT = 3
 
 function S.waitForEvent(sec, usec)
     local event = ffi.new("union SDL_Event")
@@ -293,64 +348,74 @@ function S.waitForEvent(sec, usec)
     end
 
     -- if we got an event, examine it here and generate events for koreader
-    if ffi.os == "OSX" and (event.type == SDL.SDL_FINGERMOTION or
-        event.type == SDL.SDL_FINGERDOWN or
-        event.type == SDL.SDL_FINGERUP) then
-        -- noop for trackpad finger inputs which interfere with emulated mouse inputs
-        do end -- luacheck: ignore 541
-    elseif event.type == SDL.SDL_KEYDOWN then
+    if event.type == SDL.SDL_KEYDOWN then
         genEmuEvent(C.EV_KEY, event.key.keysym.sym, 1)
     elseif event.type == SDL.SDL_KEYUP then
         genEmuEvent(C.EV_KEY, event.key.keysym.sym, 0)
     elseif event.type == SDL.SDL_TEXTINPUT then
         genEmuEvent(C.EV_SDL, SDL.SDL_TEXTINPUT, ffi.string(event.text.text))
-    elseif event.type == SDL.SDL_MOUSEMOTION
+    elseif event.type == SDL.SDL_MOUSEMOTION --and event.motion.which ~= SDL_TOUCH_MOUSEID
         or event.type == SDL.SDL_FINGERMOTION then
         local is_finger = event.type == SDL.SDL_FINGERMOTION
-        if is_in_touch then
-            if is_finger then
-                if event.tfinger.dx ~= 0 then
-                    genEmuEvent(C.EV_ABS, C.ABS_MT_POSITION_X,
-                        event.tfinger.x * S.w)
-                end
-                if event.tfinger.dy ~= 0 then
-                    genEmuEvent(C.EV_ABS, C.ABS_MT_POSITION_Y,
-                        event.tfinger.y * S.h)
-                end
-            else
-                if event.motion.xrel ~= 0 then
-                    genEmuEvent(C.EV_ABS, C.ABS_MT_POSITION_X,
-                        event.motion.x)
-                end
-                if event.motion.yrel ~= 0 then
-                    genEmuEvent(C.EV_ABS, C.ABS_MT_POSITION_Y,
-                        event.motion.y)
-                end
+        local slot
+
+        if is_finger then
+            slot = getFingerSlot(event)
+            genTouchMoveEvent(event, slot, event.tfinger.x * S.w, event.tfinger.y * S.h)
+        else
+            if pointers[0] and pointers[0].down then
+                slot = 0 -- left mouse button down
+                genTouchMoveEvent(event, slot, event.motion.x, event.motion.y)
+            end
+            if pointers[1] and pointers[1].down then
+                slot = 1 -- right mouse button down
+                genTouchMoveEvent(event, slot, event.motion.x, event.motion.y)
             end
-            genEmuEvent(C.EV_SYN, C.SYN_REPORT, 0)
         end
-    elseif event.type == SDL.SDL_MOUSEBUTTONUP
+    elseif event.type == SDL.SDL_MOUSEBUTTONUP and event.button.which ~= SDL_TOUCH_MOUSEID
         or event.type == SDL.SDL_FINGERUP then
-        is_in_touch = false
-        genEmuEvent(C.EV_ABS, C.ABS_MT_TRACKING_ID, -1)
-        genEmuEvent(C.EV_SYN, C.SYN_REPORT, 0)
-    elseif event.type == SDL.SDL_MOUSEBUTTONDOWN
+        local is_finger = event.type == SDL.SDL_FINGERUP
+        local slot
+        if is_finger then
+            slot = getFingerSlot(event)
+        elseif event.button.button == SDL_BUTTON_RIGHT then
+            slot = 1
+        else -- SDL_BUTTON_LEFT
+            slot = 0
+        end
+
+        local x = is_finger and event.tfinger.x * S.w or event.button.x
+        local y = is_finger and event.tfinger.y * S.h or event.button.y
+        genTouchUpEvent(event, slot, x, y)
+        if is_finger then
+            pointers[tonumber(event.tfinger.fingerId)] = nil
+        else
+            pointers[slot] = nil
+        end
+    elseif event.type == SDL.SDL_MOUSEBUTTONDOWN and event.button.which ~= SDL_TOUCH_MOUSEID
         or event.type == SDL.SDL_FINGERDOWN then
         local is_finger = event.type == SDL.SDL_FINGERDOWN
-        if not is_finger and event.button.button ~= SDL_BUTTON_LEFT then
-            -- Not a left-click?
+        if not is_finger and not (event.button.button == SDL_BUTTON_LEFT or event.button.button == SDL_BUTTON_RIGHT) then
+            -- We don't do anything with extra buttons for now.
             return false, C.ENOSYS
         end
+
         -- use mouse click to simulate single tap
-        is_in_touch = true
-        genEmuEvent(C.EV_ABS, C.ABS_MT_TRACKING_ID, 0)
-        genEmuEvent(C.EV_ABS, C.ABS_MT_POSITION_X,
-            is_finger and event.tfinger.x * S.w or event.button.x)
-        genEmuEvent(C.EV_ABS, C.ABS_MT_POSITION_Y,
-            is_finger and event.tfinger.y * S.h or event.button.y)
-        genEmuEvent(C.EV_SYN, C.SYN_REPORT, 0)
+        local slot
+        if is_finger then
+            slot = getFingerSlot(event)
+        elseif event.button.button == SDL_BUTTON_RIGHT then
+            slot = 1
+        else -- SDL_BUTTON_LEFT
+            slot = 0
+        end
+        local x = is_finger and event.tfinger.x * S.w or event.button.x
+        local y = is_finger and event.tfinger.y * S.h or event.button.y
+        setPointerDownState(slot, true)
+        genTouchDownEvent(event, slot, x, y)
     elseif event.type == SDL.SDL_MULTIGESTURE then
         genEmuEvent(C.EV_SDL, SDL.SDL_MULTIGESTURE, event.mgesture)
+        genEmuEvent(C.EV_SYN, C.SYN_REPORT, 0)
     elseif event.type == SDL.SDL_MOUSEWHEEL then
         genEmuEvent(C.EV_SDL, SDL.SDL_MOUSEWHEEL, event.wheel)
     elseif event.type == SDL.SDL_DROPFILE then