mtimer/system/on_receive_fields.lua

local m = mtimer
local f = mtimer.show_formspec

minetest.register_on_player_receive_fields(function(player, formname, fields)
    local meta = player:get_meta()
    local name = player:get_player_name()


    -- Select what formspec to show basing on main menu button
    if formname == 'mtimer:main_menu' then
        if fields.set_visibility then f.set_visibility(name) end
        if fields.set_position then f.set_position(name) end
    end


    -- Set timer visibility
    if formname == 'mtimer:set_visibility' then
        local attr = m.meta.visible
        if fields.visible then meta:set_string(attr.key, 'true') end
        if fields.invisible then meta:set_string(attr.key, 'false') end
        if fields.default then meta:set_string(attr.key, attr.default) end
    end


    -- Set timer position
    if formname == 'mtimer:set_position' then
        local attr = m.meta.position
        for p,_ in pairs(fields) do
            if p == 'default' then
                meta:set_string(attr.key, attr.default)
            elseif p:gsub('_.*', '') == 'pos' then
                local new_pos = p:gsub('pos_', '')
                if new_pos ~= 'xx' then meta:set_string(attr.key, new_pos) end
            end
        end
    end


    -- Back to menu from all formspecs
    if fields.back then f.main_menu(name) end


    -- DEBUG: Print all player meta data
    print(dump(meta:to_table()))
end)