OpenEuphoria

Re: Callback Wrapping Help

• Posted by ghaberek (admin) Mar 15, 2023
• 757 views

These are function pointer types (mostly use for callbacks, but there are other uses). The asterisk inside the parentheses is part of the type name (because it's a function pointer) not the return value.

typedef size_t (*sfShapeGetPointCountCallback)(void*);        ///< Type of the callback used to get the number of points in a shape 
typedef sfVector2f (*sfShapeGetPointCallback)(size_t, void*); ///< Type of the callback used to get a point of a shape 

This function declaration uses the function pointer types above to take function pointers as a parameter, which it can then use to call that function (hence "callback").

CSFML_GRAPHICS_API sfShape* sfShape_create(sfShapeGetPointCountCallback getPointCount, 
                                           sfShapeGetPointCallback getPoint, 
                                           void* userData); 

First we need to declare sfVector2f struct so FFI knows how to correctly return the value from sfShapeGetPointCallback().

typedef struct { 
    float x; 
    float y; 
} sfVector2f; 

constant SF_VECTOR2F = define_c_struct({ 
    C_FLOAT, -- x 
    C_FLOAT  -- y 
}) 

The we need to declare our own functions that accept the parameters in the typedef's above. I had to dig into the ShapeStruct header to figure out the parameter names and I found some examples demonstrating how to implement custom shapes.

-- Our custom shape data (we can use userData parameter as a shape id). 
-- You could use this approach to define any number of custom shapes. 
sequence sfShapes = { 
    {{-10.0,-10.0},{10.0,-10.0},{10.0,10.0},{-10.0,10.0}} -- a square 
} 
constant SHAPE_SQUARE = 1 -- shape id of the square above 
 
-- Callback that provides the point count of the shape 
function sfShapeGetPointCountCallback( atom userData ) 
 
    atom shapeId = userData -- should be 1 (SHAPE_SQUARE) 
 
    return length( sfShapes[shapeId] ) -- should return 4 
end function 
 
-- Declare the call back with its parameter and return types 
constant sfShapeGetPointCountCallback_cb = call_back( routine_id("sfShapeGetPointCountCallback"), {C_POINTER}, C_SIZE_T ) 
 
-- Callback that provides the points of the shape 
function sfShapeGetPointCallback( atom index, atom userData ) 
 
    atom shapeId = userData -- should be 1 
    atom x = sfShapes[shapeId][1] 
    atom y = sfShapes[shapeId][2] 
 
    return {x,y} -- should return two floats (struct sfVector2f) 
end function 
 
-- Declare the call back with its parameter and return types 
constant sfShapeGetPointCallback_cb = call_back( routine_id("sfShapeGetPointCallback"), {C_SIZE_T,C_POINTER}, SF_VECTOR2F ) 

Then we declare the the function as you did.

constant C_CALLBACK = C_POINTER -- I'm going to add this to std/ffi.e so we can distinguish callbacks from other pointers 
 
constant xsfShape_create = define_c_func(gfx,"+sfShape_create",{C_CALLBACK,C_CALLBACK,C_POINTER},C_POINTER) 
 
public function sfShape_create(atom getPointCount, atom getPoint, atom userData) 
    return c_func(xsfShape_create,{getPointCount,getPoint,userData}) 
end function 

Finally, we would call it like this:

atom shape = sfShape_create( sfShapeGetPointCountCallback_cb, sfShapeGetPointCallback_cb, SHAPE_SQUARE ) 

-Greg