Lua和C++交互

1.  
   static int averageFunc(lua_State *L)
2.  
   {
3.  
   int n = lua_gettop(L);//获取栈顶元素，表示参数数量
4.  
   double sum = 0;
5.  
   int i;
6.  
   /* 循环求参数之和 */
7.  
   for (i = 1; i <= n; i )
8.  
   sum = lua_tonumber(L, i);
9.  
   lua_pushnumber(L, sum / n); //压入平均值
10.  
    lua_pushnumber(L, sum); //压入和
11.  
    return 2; //返回两个结果
12.  
    }
13.  
    static int sayHelloFunc(lua_State* L)
14.  
    {
15.  
    printf("hello world!");
16.  
    return 0;
17.  
    }
18.  
    static const struct luaL_Reg myLib[] =
19.  
    {
20.  
    {"average", averageFunc},
21.  
    {"sayHello", sayHelloFunc},
22.  
    {NULL, NULL} //数组中最后一对必须是{NULL, NULL}，用来表示结束
23.  
    }
24.  
    int luaopen_mLualib(lua_State *L)
25.  
    {
26.  
    luaL_register(L, "ss", myLib);
27.  
    return 1; // 把myLib表压入了栈中，所以就需要返回1
28.  
    }

 

例子讲解：

local spr = cc.Sprite:create()

cc是一张表(table)，表(table)是lua里面唯一的数据结构，cc存于_G中，_G是lua里面的全局表，cc表中存了所有cocos绑定的常量和“类”。

Sprite也是个表(table)，Sprite表是怎样跟c 代码关联起来的呢，这就要分析c 的绑定代码了。首先来看看Sprite表里面的内容

1.  
   ["Sprite"] = {--table: 0x0032cce0
2.  
   [".isclass"] = "true",
3.  
   [".metatable"] = table: 0x0032c690
4.  
   },

通过打印发现，Sprite表里面就一条数据key=[".isclass"] value=true，这个表明这个此table是cocos绑定的一个类型。看来秘密都在元表(metatable)里面。元表(metatable)在注册表(LUA_REGISTRYINDEX)里面，关于lua的全局表(_G)、注册表(Register)、元表(metatable)，这里不再介绍了，不理解的需要先去学习一下。下面来打印一下Sprite的元表(metatable)里面的内容

1.  
   ["cc.Sprite"] = {--table: 0x0032c690
2.  
   ["getTextureRect"] = "function: 0x00b9b430",
3.  
   ["getBatchNode"] = "function: 0x00b9afe8",
4.  
   ["setTextureAtlas"] = "function: 0x00b9b770",
5.  
   ["new"] = "function: 0x00b9b168",
6.  
   ["setDisplayFrameWithAnimationName"] = "function: 0x00b9b750",
7.  
   ["create"] = "function: 0x003284a8",
8.  
   ["__lt"] = "function: 0x0032cbf0",
9.  
   ["getTexture"] = "function: 0x00b9b2b8",
10.  
    ["__sub"] = "function: 0x0032cc58",
11.  
    ["initWithFile"] = "function: 0x00b9b450",
12.  
    ["isTextureRectRotated"] = "function: 0x00b9b3e8",
13.  
    ["removeAllChildrenWithCleanup"] = "function: 0x00b9b050",
14.  
    ["setTextureRect"] = "function: 0x00b9b098",
15.  
    ["setDirty"] = "function: 0x00b9b398",
16.  
    ["getSpriteFrame"] = "function: 0x00b9b790",
17.  
    ["__newindex"] = "function: 0x0032c710",
18.  
    ["__eq"] = "function: 0x0032cc30",
19.  
    ["setPolygonInfo"] = "function: 0x00328488",
20.  
    ["initWithPolygon"] = "function: 0x00328440",
21.  
    ["__le"] = "function: 0x0032cc10",
22.  
    ["setFlippedX"] = "function: 0x00b9af60",
23.  
    ["createWithSpriteFrame"] = "function: 0x00b9be50",
24.  
    ["createWithSpriteFrameName"] = "function: 0x00b9be00",
25.  
    ["tolua_ubox"] = "table: 0x002c86f0" , -- loop table,
26.  
    ["createWithTexture"] = "function: 0x00b9b5c0",
27.  
    ["setTexture"] = "function: 0x00b9b298",
28.  
    ["setVertexRect"] = "function: 0x00b9b5a0",
29.  
    ["isFlippedY"] = "function: 0x00b9b558",
30.  
    ["isFlippedX"] = "function: 0x00b9b518",
31.  
    ["getResourceType"] = "function: 0x00b9afa8",
32.  
    ["setFlippedY"] = "function: 0x00b9b2f8",
33.  
    ["setBlendFunc"] = "function: 0x003283f8",
34.  
    ["__div"] = "function: 0x0032c6b8",
35.  
    ["getTextureAtlas"] = "function: 0x00b9b490",
36.  
    ["__index"] = "function: 0x0032c6f0",
37.  
    ["setAtlasIndex"] = "function: 0x00b9b358",
38.  
    ["getAtlasIndex"] = "function: 0x00b9b6b8",
39.  
    ["isDirty"] = "function: 0x00b9b338",
40.  
    ["getResourceName"] = "function: 0x00b9b7d8",
41.  
    ["getOffsetPosition"] = "function: 0x00b9b030",
42.  
    ["setBatchNode"] = "function: 0x00b9b6d8",
43.  
    ["__gc"] = "function: 0x002c1a90",
44.  
    ["__call"] = "function: 0x0032cc98",
45.  
    ["__mul"] = "function: 0x0032cc78",
46.  
    ["initWithSpriteFrame"] = "function: 0x00b9b4d8",
47.  
    ["__add"] = "function: 0x0032c730",
48.  
    ["initWithTexture"] = "function: 0x00b9afc8",
49.  
    ["setSpriteFrame"] = "function: 0x00b9b1b0",
50.  
    ["initWithSpriteFrameName"] = "function: 0x00b9b650",
51.  
    ["isFrameDisplayed"] = "function: 0x00b9b698",
52.  
    ["getBlendFunc"] = "function: 0x00b9b6f8",
53.  
    [".metatable"] = table: 0x002ca9c8
54.  
    }

cc.Sprite:create，在元表(metatable)的前半部分找到到了create方法，下面来看一看create方法里面的内容：

1.  
   int lua_cocos2dx_Sprite_create(lua_State* tolua_S)
2.  
   {
3.  
   int argc = 0;
4.  
   bool ok = true;
5.  
   //lua_gettop获取栈顶正索引，也表示栈内的元素个数
6.  
   argc = lua_gettop(tolua_S)-1;//获取栈顶下面的正索引
7.  
   do{
8.  
   if (argc == 1){//表示栈内有两个元素
9.  
   std::string arg0;
10.  
    ok &= luaval_to_std_string(tolua_S, 2,&arg0, "cc.Sprite:create");
11.  
    if (!ok) { break; }
12.  
    cocos2d::Sprite* ret = cocos2d::Sprite::create(arg0);
13.  
    object_to_luaval<cocos2d::Sprite>(tolua_S, "cc.Sprite",(cocos2d::Sprite*)ret);
14.  
    return 1;
15.  
    }
16.  
    } while (0);
17.  
    luaL_error(tolua_S, "%s has wrong number of arguments: %d, was expecting %d", "cc.Sprite:create",argc, 2);
18.  
    return 0;
19.  
    }

create方法当然也是一个c函数。lua_gettop()用来获取create函数参数数量，第一个参数是Sprite(table)自己，减1后是真正参数数量，所以create方法必须要用:号调用，元表中其它的方法也一样。函数先用luaval_to_std_string判断传入参数是否是std::string类型，其核心就是调用tolua_isstring函数来进行判定。然后就是调用cocos2d::Sprite::create()创建c 对象，然后调用object_to_luaval<>()函数把c 指针压入lua的栈中，理解这个函数是理解cocos lua的运作流程的关键。
 

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