example: add example to include a lua file to C code

Signed-off-by: Neo Xu <neo.xu1990@gmail.com>

examples/luaC.lua

@@ -1,4 +1,4 @@
--- Everything is already setup in C code, check mixed.c
+-- Everything is already setup in C code, check lua_in_c.c
 -- This file is just to trigger the demo
 
 embed_lua_in_c()

simulator/lua_in_C.c

@@ -9,38 +9,32 @@
 #define _STRINGIZE(...) #__VA_ARGS__
 #define STRINGIZE(...) _STRINGIZE(__VA_ARGS__)
 
-static const char lua_code_string[] = STRINGIZE(
-    local root = lvgl.get_child_by_id("luaUIroot")
-    root.w = lvgl.VER_RES(),
-    root.h = lvgl.HOR_RES(),
-
-    btn = Button(root, {
-        id = "Button in Lua",
-        Label {
-            text = "Hello, lua, C and lvgl!",
-            align = lvgl.ALIGN_CENTER
-        }
-    }):center()
+static const char lua_code_string[] = {
+    /* Must start with -- */
+    "--"
+#include "ui.lua"
+};
+
+static const char button_lua_code[] = STRINGIZE(
+    local btn = lvgl.get_child_by_id('Button in Lua')
+    local label = btn:get_child(0)
+    label.text = 'Button clicked'
 );
 
 static void button_clicked(lv_event_t *e)
 {
   (void)e;
   lua_State *L = lv_event_get_user_data(e);
   LV_LOG_USER("Button clicked");
-  luaL_dostring(L, "\
-    local btn = lvgl.get_child_by_id('Button in Lua')\n\
-    local label = btn:get_child(0)\n\
-    label.text = 'Button clicked'\n\
-  ");
+  luaL_dostring(L, button_lua_code);
 }
 
 static int embed_lua_in_c(lua_State *L)
 {
   /* We create a root obj in C and create other UIs in lua. */
   lv_obj_t *root = lv_obj_create(lv_screen_active());
   luavgl_add_lobj(L, root)->lua_created = false;
-  lv_obj_set_id(root, lv_strdup("luaUIroot"));
+  lv_obj_set_id(root, "luaUIroot");
   int ret = luaL_dostring(L, lua_code_string);
   if (ret != 0) {
     LV_LOG_USER("luaL_dostring error: %d", ret);

simulator/ui.lua

@@ -0,0 +1,15 @@
+STRINGIZE((function() --\n
+    local root = lvgl.get_child_by_id("luaUIroot")
+    root.w = lvgl.VER_RES()
+    root.h = lvgl.HOR_RES()
+
+    Button(root, {
+        id = "Button in Lua",
+        Label {
+            text = "Hello, lua, C and lvgl!",
+            align = lvgl.ALIGN_CENTER
+        }
+    }):center()
+
+-- Keep this comment
+end)())