Xlua对c#的vector3等结构体的优化
参考文章：：
https://www.jianshu.com/p/07dc38e85923
https://gameinstitute.qq.com/community/detail/111993
https://gameinstitute.qq.com/community/detail/125117
一、lua如何操作Vector3，中间做了什么操作？
1.获取Vector3对象
由于Vector3的方法、属性都是成员方法、属性(如x、y、z、Slerp)，那么调用这些方法前需要先获取Vector3对应的对象。比如Vector3()新建、transform.position获取等。
以transform.position为例：_g_get_position方法创建了一个CSharpStruct类型的ud,并把x,y,z存在这个ud里。这个ud的元表指向Vector3的obj_meta，通过这个元表可以访问Vector3Wrap注册的所有方法、属性，进而间接访问到Vector3。
ud ={[] = -1[] = 12 //存储3个float值的大小，的对应xyz，float是4字节32位。[][0] = x[][1] = y[][0] = z[] = obj_meta}
static int _g_get_position(RealStatePtr L){try {ObjectTranslator translator = ObjectTranslatorPool.Instance.Find(L);UnityEngine.Transform gen_to_be_invoked = (UnityEngine.Transform)translator.FastGetCSObj(L, 1);translator.PushUnityEngineVector3(L, gen_to_be_invoked.position);} catch(System.Exception gen_e) {return LuaAPI.luaL_error(L, "c# exception:" + gen_e);}return 1;}public void PushUnityEngineVector3(RealStatePtr L, UnityEngine.Vector3 val){if (UnityEngineVector3_TypeID == -1){bool is_first;UnityEngineVector3_TypeID = getTypeId(L, typeof(UnityEngine.Vector3), out is_first);}//创建一个大小为(12+4+4)字节的userdata，元表Vector3的元表IntPtr buff = LuaAPI.xlua_pushstruct(L, 12, UnityEngineVector3_TypeID);if (!CopyByValue.Pack(buff, 0, val)) //把vector3拆成3个float传入cc，在c的结构体buff存储数据{throw new Exception("pack fail fail for UnityEngine.Vector3 ,value="+val);}}
xlua.c里的接口：
LUA_API void *xlua_pushstruct(lua_State *L, unsigned int size, int meta_ref) {CSharpStruct *css = (CSharpStruct *)lua_newuserdata(L, size + sizeof(int) + sizeof(unsigned int));css->fake_id = -1;css->len = size;lua_rawgeti(L, LUA_REGISTRYINDEX, meta_ref);lua_setmetatable(L, -2);return css;}LUALIB_API int xlua_pack_float3(void *p, int offset, float f1, float f2, float f3) {CSharpStruct *css = (CSharpStruct *)p;if (css->fake_id != -1 || css->len < offset + sizeof(float) * 3) {return 0;} else {float *pos = (float *)(&(css->data[0]) + offset);pos[0] = f1;pos[1] = f2;pos[2] = f3;return 1;}}
2.设置transform.position
代码如下：
主要是UnPack方法调用xlua的xlua_unpack_float3方法，从上面的ud结构里取到x,y,c的值压栈并赋值给UnPack的x,y,z参数，再由UnPack组装一个新的Vevtor3返回给_s_set_position进行赋值。
static int _s_set_position(RealStatePtr L){try {ObjectTranslator translator = ObjectTranslatorPool.Instance.Find(L);UnityEngine.Transform gen_to_be_invoked = (UnityEngine.Transform)translator.FastGetCSObj(L, 1);UnityEngine.Vector3 gen_value;translator.Get(L, 2, out gen_value);gen_to_be_invoked.position = gen_value;} catch(System.Exception gen_e) {return LuaAPI.luaL_error(L, "c# exception:" + gen_e);}return 0;}public void Get(RealStatePtr L, int index, out UnityEngine.Vector3 val){LuaTypes type = LuaAPI.lua_type(L, index);if (type == LuaTypes.LUA_TUSERDATA ){IntPtr buff = LuaAPI.lua_touserdata(L, index);if (!CopyByValue.UnPack(buff, 0, out val)){throw new Exception("unpack fail for UnityEngine.Vector3");}}else if (type ==LuaTypes.LUA_TTABLE){CopyByValue.UnPack(this, L, index, out val);}else{val = (UnityEngine.Vector3)objectCasters.GetCaster(typeof(UnityEngine.Vector3))(L, index, null);}}public static bool UnPack(IntPtr buff, int offset, out UnityEngine.Vector3 field){field = default(UnityEngine.Vector3);float x = default(float);float y = default(float);float z = default(float);if(!LuaAPI.xlua_unpack_float3(buff, offset, out x, out y, out z)){return false;}field.x = x;field.y = y;field.z = z;return true;}
当然，如果你传的不是Vector3类型的usreData就更简单了，例如{x = 1, y = 2, z = 3},直接调用lua_rawget就可以取到了。
public static void UnPack(ObjectTranslator translator, RealStatePtr L, int idx, out UnityEngine.Vector3 val){val = new UnityEngine.Vector3();int top = LuaAPI.lua_gettop(L);if (Utils.LoadField(L, idx, "x")){translator.Get(L, top + 1, out val.x);}LuaAPI.lua_pop(L, 1);if (Utils.LoadField(L, idx, "y")){translator.Get(L, top + 1, out val.y);}LuaAPI.lua_pop(L, 1);if (Utils.LoadField(L, idx, "z")){translator.Get(L, top + 1, out val.z);}LuaAPI.lua_pop(L, 1);}public static bool LoadField(RealStatePtr L, int idx, string field_name){idx = idx > 0 ? idx : LuaAPI.lua_gettop(L) + idx + 1;// abs of indexLuaAPI.xlua_pushasciistring(L, field_name);LuaAPI.lua_rawget(L, idx);return !LuaAPI.lua_isnil(L, -1);}
xlua.c的解包方法：
LUALIB_API int xlua_unpack_float3(void *p, int offset, float *f1, float *f2, float *f3) {CSharpStruct *css = (CSharpStruct *)p;if (css->fake_id != -1 || css->len < offset + sizeof(float) * 3) {return 0;} else {float *pos = (float *)(&(css->data[0]) + offset);*f1 = pos[0];*f2 = pos[1];*f3 = pos[2];return 1;}}
3.Vector.x：取x值
流程其实跟transform.position差不多，先把自身的ud转成Vector3,再把Vector3.x压栈返回给lua。
static int _g_get_x(RealStatePtr L){try {ObjectTranslator translator = ObjectTranslatorPool.Instance.Find(L);UnityEngine.Vector3 gen_to_be_invoked;translator.Get(L, 1, out gen_to_be_invoked);LuaAPI.lua_pushnumber(L, gen_to_be_invoked.x);} catch(System.Exception gen_e) {return LuaAPI.luaL_error(L, "c# exception:" + gen_e);}return 1;}
4.调用总结：
lua跟c#的Vector3并没有直接的调用，而是通过中间层ud(CSharpStruct)进行中转。
这里面的PushUnityEngineVector3、Get方法都是在wrap生成时动态生成的。
每个GCOptimize标记的方法都会生成对应的Push、Get、Pack、UnPack方法，这些方法会调用到xlua.c里对应的c方法对结构体进行解包、封包。
二、xlua为什么要这么做？
Vector3Wrap的__CreateInstance方法使用的是translator.PushUnityEngineVector3对Vector对象压栈。那为什么不用正常的translator.Push压栈呢？
static int __CreateInstance(RealStatePtr L){try {ObjectTranslator translator = ObjectTranslatorPool.Instance.Find(L);if(LuaAPI.lua_gettop(L) == 4 && LuaTypes.LUA_TNUMBER == LuaAPI.lua_type(L, 2) && LuaTypes.LUA_TNUMBER == LuaAPI.lua_type(L, 3) && LuaTypes.LUA_TNUMBER == LuaAPI.lua_type(L, 4)){float _x = (float)LuaAPI.lua_tonumber(L, 2);float _y = (float)LuaAPI.lua_tonumber(L, 3);float _z = (float)LuaAPI.lua_tonumber(L, 4);UnityEngine.Vector3 gen_ret = new UnityEngine.Vector3(_x, _y, _z);translator.PushUnityEngineVector3(L, gen_ret);return 1;}}catch(System.Exception gen_e) {return LuaAPI.luaL_error(L, "c# exception:" + gen_e);}}
translator.Push里会调用addObject把对象实例跟对象下标做一个绑定，这个唯一的下标用于在lua中获取到c#的对象实例。Struct是值类型，值类型和引用类型(object)的转换会涉及装箱、拆箱操作。
int addObject(object obj, bool is_valuetype, bool is_enum){int index = objects.Add(obj);if (is_enum){enumMap[obj] = index;}else if (!is_valuetype){reverseMap[obj] = index;}return index;}
对装箱、拆箱不了解的可以参考文章：https://www.cnblogs.com/yukaizhao/archive/2011/10/18/csharp_box_unbox_1.html
所以xlua其实是对Vector3等值类型的结构体做了一个优化，避免了装箱和拆箱优化。
但是，虽然xlua对结构体做了优化，但还是会有很多开销，例如：在c#需要push x\y\z参数，在lua需要创建表保存x\y\z值，需要查询元表等。
那么如何优化呢？直接在函数中传递三个float，要比传递Vector3要更快。
例如void SetPos(GameObject obj, Vector3pos)改为void SetPos(GameObject obj, float x, floaty, float z)。
对于c#的Struct还是少用，尽量封装静态方法调用，性能会更好。