如何使用xlua实现协程，示例代码如下：

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1 local unpack = unpack or table.unpack; 2 local function async_to_sync(async_func, callback_pos) 3     return function(...) 4         local _co = coroutine.running() or error ('this function must be run in coroutine') 5         local rets 6         local waiting = false 7         local function cb_func(...) 8             if waiting then 9                 assert(coroutine.resume(_co, ...))10             else11                 rets = {...}12             end13         end14         local params = {...}15         table.insert(params, callback_pos or (#params + 1), cb_func)16         async_func(unpack(params))17         if rets == nil then18             waiting = true19             rets = {
   coroutine.yield()}20         end21         22         return unpack(rets)23     end24 end25 26 -- 异步协程27 local gameobject = CS.UnityEngine.GameObject('XLua_Coroutine_Runner');28 CS.UnityEngine.Object.DontDestroyOnLoad(gameobject);29 local csCoroutineRunner = gameobject:AddComponent(typeof(CS.XLuaCoroutineRunner));30 local function AsyncYieldReturn(toYield, cb)31     csCoroutineRunner:YieldAndCallback(toYield, cb)32 end33 local yieldReturn = async_to_sync(AsyncYieldReturn);34 35 -- 协程36 local co = coroutine.create(function()37     local url = CS.Common.LoadHelp.GetAssetBundleStreamingUrl("xxx.json");38     local www = CS.UnityEngine.WWW(url);39     yieldReturn(www);40     41     local text = www.text;42     print(text);43 end);44 45 coroutine.resume(co);