Objc method observer

gistfile1.lua

objc = require("objc")
ffi  = require("ffi")
setmetatable(_G, {__index=objc})
objc.debug=false
ffi.cdef("IMP class_replaceMethod(Class cls, SEL name, IMP imp, const char *types);")
local C = ffi.C

function observeMethod(class, sel, lambda)
    local method = C.class_getInstanceMethod(class, SEL(sel))
    if method == nil then
        error("Error! Method not found")
    end
    
    local impTypeStr = objc.impSignatureForMethod(method)
    if impTypeStr == nil then
        return nil
    end

    local originalImp = C.method_getImplementation(method)
    local trampoline = ffi.cast(impTypeStr, function(self, sel, ...)
        lambda(self, ...)
        return originalImp(self, sel, ...)
    end)
    C.class_replaceMethod(class, SEL(sel), ffi.cast("IMP", trampoline), C.method_getTypeEncoding(method))
end

observeMethod(Cube, "render:", function(self, state)
    print("Render called")
end)