Binding With Metatable And Closures
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Binding With Metatable And Closures |
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Here is an example of how to make a Lua binding to Thomas Boutell's GD Graphics Library[1] using closures containing a metatable. The original example of how to use GD in C can be found on Tom's web page[2].
Image.new is a constructor that returns a userdata containing the image
to be maninpulated. The userdata's metatable has a garbage collection event
to destroy the image, and a index event for calling the methods needed to
draw the image and save the image to a file. Only three methods are implimented:
ColorAllocate, Line and PNG.
#include <stdio.h>
#include "lua.h"
#include "lauxlib.h"
#include "lualib.h"
#include "gd.h"
/*
==============================================================================
Example Lua bindings for GD
==============================================================================
*/
#define IMAGE "Image"
static gdImagePtr checkimage (lua_State *L, int index)
{
luaL_checktype(L, index, LUA_TUSERDATA);
lua_getmetatable(L, index);
if( ! lua_equal(L, lua_upvalueindex(1), -1) )
luaL_typerror(L, index, IMAGE); /* die */
lua_pop(L, 1);
return (gdImagePtr)lua_unboxpointer(L, index);
}
static gdImagePtr pushimage (lua_State *L, gdImagePtr im)
{
lua_boxpointer(L, im);
lua_pushvalue(L, lua_upvalueindex(1));
lua_setmetatable(L, -2);
return im;
}
static int image_new (lua_State *L)
{
int x = luaL_checkint(L, 1);
int y = luaL_checkint(L, 2);
pushimage(L, gdImageCreate(x, y));
return 1;
}
static int image_destroy (lua_State *L)
{
gdImagePtr im = (gdImagePtr)lua_unboxpointer(L, 1);
if (im) gdImageDestroy(im);
return 0;
}
static int image_color_allocate (lua_State *L)
{
gdImagePtr im = checkimage(L, 1);
int r = luaL_checkint(L, 2);
int g = luaL_checkint(L, 3);
int b = luaL_checkint(L, 4);
lua_pushnumber(L, gdImageColorAllocate(im, r, g, b));
return 1;
}
static int image_line (lua_State *L)
{
gdImagePtr im = checkimage(L, 1);
int x1 = luaL_checkint(L, 2);
int y1 = luaL_checkint(L, 3);
int x2 = luaL_checkint(L, 4);
int y2 = luaL_checkint(L, 5);
int colour = luaL_checkint(L, 6);
gdImageLine(im, x1, y1, x2, y2, colour);
return 0;
}
static int image_png (lua_State *L)
{
/* Output the image to the disk file in PNG format. */
gdImagePtr im = checkimage(L, 1);
const char *name = luaL_checkstring(L, 2);
FILE *pngout = fopen( name, "wb");
gdImagePng(im, pngout);
fclose(pngout);
return 0;
}
static const luaL_reg meta_methods[] = {
{"__gc", image_destroy },
{0,0}
};
static const luaL_reg image_methods[] = {
{"new", image_new},
{"colorallocate", image_color_allocate},
{"line", image_line},
{"PNG", image_png},
{0,0}
};
#define newtable(L) (lua_newtable(L), lua_gettop(L))
int Image_register (lua_State *L)
{
int metatable, methods;
lua_pushliteral(L, IMAGE); /* name of Image table */
methods = newtable(L); /* Image methods table */
metatable = newtable(L); /* Image metatable */
lua_pushliteral(L, "__index"); /* add index event to metatable */
lua_pushvalue(L, methods);
lua_settable(L, metatable); /* metatable.__index = methods */
lua_pushliteral(L, "__metatable"); /* hide metatable */
lua_pushvalue(L, methods);
lua_settable(L, metatable); /* metatable.__metatable = methods */
luaL_openlib(L, 0, meta_methods, 0); /* fill metatable */
luaL_openlib(L, 0, image_methods, 1); /* fill Image methods table */
lua_settable(L, LUA_GLOBALSINDEX); /* add Image to globals */
return 0;
}
int main(int argc, char *argv[])
{
lua_State *L = lua_open();
luaopen_base(L);
luaopen_table(L);
luaopen_io(L);
luaopen_string(L);
luaopen_math(L);
luaopen_debug(L);
Image_register(L);
if(argc>1) lua_dofile(L, argv[1]);
lua_close(L);
return 0;
}
This code can be compiled for Lua 5.0 as follows:
gcc gd.c -L/usr/local/lib/ -llua -llualib -lgd -lpng
for n,v in pairs(Image) do print(n,v) end size = 256 im = Image.new(size, size) print(im) white = im:colorallocate(255, 255, 255) for i = 0,size-1,1 do c = im:colorallocate(0, i, i) im:line(0, 0, size-1 , i, c) end im:PNG'test.png' -- debug.debug()
$ ./a gd.lua line function: 0x100553b8 PNG function: 0x10055500 colorallocate function: 0x10055418 new function: 0x10054ff8 userdata: 0x10055e98
This is the image created by the test code.
The next example shows how to use the index and newindex events to read and write structure members BindingWithMembersAndMethods