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source/scriptinterface/FunctionWrapper.h

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/* Copyright (C) 2020 Wildfire Games.
* This file is part of 0 A.D.
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year.

Stan: year.
*
* 0 A.D. is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 2 of the License, or
* (at your option) any later version.
*
* 0 A.D. is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with 0 A.D. If not, see <http://www.gnu.org/licenses/>.
*/
#ifndef INCLUDED_FUNCTIONWRAPPER
#define INCLUDED_FUNCTIONWRAPPER
/**
* This file introduces templates to conveniently wrap C++ functions in JSNative functions,
* and to conveniently call JS functions from C++ code.
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Pretty sure you are missing some std includes?

Stan: Pretty sure you are missing some std includes?
* This _is_ rather template heavy, so compilation times beware.
* The C++ code can have arbitrary arguments and arbitrary return types, so long
* as they can be converted to/from JS using ScriptInterface::ToJSVal (FromJSVal respectively).
*/
#include "ScriptInterface.h"
// Misconfigured template arguments might lead to code calling this
// (which obviously won't work).
// I can't use a static_assert as that would auto-fire, so don't provide
// the implementation and linking will fail.
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Inverted.

Stan: Inverted.
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It's not, because I need the template to be defined above where they are used (and the logic of the code builds up progressively), so I don't have a choice.

wraitii: It's not, because I need the template to be defined above where they are used (and the logic of…
template<> inline void* ScriptInterface::getPrivate<void>(JSContext* cx, JS::CallArgs&);
/**
* Some types can't be default-constructed, and thus can't be used in the magic-template below.
* This in particular concerns HandleValue, which is rather annoying.
* To work around that, we replace HandleValue with a "passthrough" type, and the calling code
* will simply pass the CallArgs value directly.
*/
struct passthrough_t{
passthrough_t() : t(std::unique_ptr<JS::HandleValue>(new JS::HandleValue(JS::NullHandleValue))) {};
std::unique_ptr<JS::HandleValue> t;
};
template<typename T>
using is_passthrough = typename std::is_same<T, passthrough_t>::value;
template<> inline bool ScriptInterface::FromJSVal<passthrough_t>(JSContext*, JS::HandleValue val, passthrough_t& o) {
o.t = std::unique_ptr<JS::HandleValue>(new JS::HandleValue(val));
return true;
};
/**
* Some functions return JS::Value. One could SFINAE this in call(), but this works too.
*/
template<>
inline void ScriptInterface::ToJSVal<JS::Value>(JSContext*, JS::MutableHandleValue ret, const JS::Value& val)
{
ret.set(val);
}
namespace ScriptWrapperHelper {
/**
* Convenient struct to get info on a [const] function pointer.
*/
template<typename ptr>
struct args_info;
template<typename ...Types>
using _args_info_t = std::tuple<typename
// We can't construct const-references, so just construct plain objects.
std::remove_const<typename std::remove_reference<
typename std::conditional<std::is_same<JS::HandleValue, Types>::value, passthrough_t, Types>::type
>::type>::type...>;
// TODO: would be nice to find a way around the duplication here.
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How much work? in what direction?

Stan: How much work? in what direction?
wraitiiAuthorUnsubmitted
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Basically need to change the signature of FromJSVal. I'll clarify.

wraitii: Basically need to change the signature of FromJSVal. I'll clarify.
template<typename C, typename R, typename ...Types>
struct args_info<R(C::*)(Types ...)>
{
static const size_t nb_args = sizeof...(Types);
using return_type = R;
using object_type = C;
using convertible_args = _args_info_t<Types...>;
};
template<typename C, typename R, typename ...Types>
struct args_info<R(C::*)(Types ...) const>
{
static const size_t nb_args = sizeof...(Types);
using return_type = R;
using object_type = C;
using convertible_args = _args_info_t<Types...>;
};
template<typename R, typename ...Types>
struct args_info<R(*)(Types ...)>
{
static const size_t nb_args = sizeof...(Types);
using return_type = R;
using object_type = void;
using convertible_args = _args_info_t<Types...>;
};
/**
* Based on https://stackoverflow.com/a/32223343
* make_index_sequence is not defined in C++11... Only C++14.
* TODO C++14: remove this.
*/
template <size_t... Ints>
struct index_sequence
{
using type = index_sequence;
using value_type = size_t;
static constexpr std::size_t size() noexcept { return sizeof...(Ints); }
};
template <class Sequence1, class Sequence2>
struct _merge_and_renumber;
template <size_t... I1, size_t... I2>
struct _merge_and_renumber<index_sequence<I1...>, index_sequence<I2...>>
: index_sequence<I1..., (sizeof...(I1)+I2)...> { };
template <size_t N>
struct make_index_sequence
: _merge_and_renumber<typename make_index_sequence<N/2>::type,
typename make_index_sequence<N - N/2>::type> { };
template<> struct make_index_sequence<0> : index_sequence<> { };
template<> struct make_index_sequence<1> : index_sequence<0> { };
/**
* This helper is a recursive template call that converts the N-1th argument
* of the function from a JS value to its proper C++ type.
*/
template<int index_tuple, int index_vals, int N, typename tuple>
struct convertFromJS
{
bool operator()(ScriptInterface& interface, JSContext* cx, JS::CallArgs& val, tuple& outs)
{
if (index_vals >= val.length())
{
std::get<index_tuple>(outs) = typename std::tuple_element<index_tuple, tuple>::type{};
return true;
}
if (!interface.FromJSVal(cx, val[index_vals], std::get<index_tuple>(outs)))
return false;
return convertFromJS<index_tuple+1, index_vals+1, N, tuple>()(interface, cx, val, outs);
}
};
// Specialization for the base case "no arguments" -> return True.
template<int index_vals, int N, typename tuple>
struct convertFromJS<N, index_vals, N, tuple> { bool operator()(ScriptInterface&, JSContext*, JS::CallArgs&, tuple&) { return true; } };
// Wrapper around convertFromJS to handle the optional CxPrivate* first argument.
template<size_t N, typename ...Types>
static bool convertFromJS2(ScriptInterface& interface, JSContext* cx, JS::CallArgs& val, std::tuple<ScriptInterface::CxPrivate*, Types...>& outs)
{
std::get<0>(outs) = ScriptInterface::GetScriptInterfaceAndCBData(cx);
return convertFromJS<1, 0, N, std::tuple<ScriptInterface::CxPrivate*, Types...>>()(interface, cx, val, outs);
}
template<size_t N, typename ...Types>
static bool convertFromJS2(ScriptInterface& interface, JSContext* cx, JS::CallArgs& val, std::tuple<Types...>& outs)
{
return convertFromJS<0, 0, N, std::tuple<Types...>>()(interface, cx, val, outs);
}
template<typename T>
static auto pickVal(T arg) -> typename std::enable_if<!std::is_same<T, passthrough_t>::value, T>::type { return arg; }
template<typename T>
static auto pickVal(T arg) -> typename std::enable_if< std::is_same<T, passthrough_t>::value, JS::HandleValue>::type { return *arg.t.get(); }
/**
* These four templates take a function pointer, its arguments, call it,
* and set the return value of the CallArgs to whatever it returned, if anything.
* It's tag-dispatched for the "returns_void" and the regular return case.
*/
template <typename funcPtr, funcPtr callable, typename T, size_t... Is, typename... types>
static void call(T* object, ScriptInterface* scriptInterface, JS::CallArgs& callArgs, std::tuple<types...>& args, std::false_type, index_sequence<Is...>)
{
// This is perfectly readable, what are you talking about.
auto ret = ((*object).* callable)(pickVal(std::move(std::get<Is>(args)))...);
scriptInterface->ToJSVal(scriptInterface->GetContext(), callArgs.rval(), ret);
}
template <typename funcPtr, funcPtr callable, typename T, size_t... Is, typename... types>
static void call(T* object, ScriptInterface* UNUSED(scriptInterface), JS::CallArgs&, std::tuple<types...>& args, std::true_type, index_sequence<Is...>)
{
// Void return specialization, just call the function.
((*object).* callable)(pickVal(std::move(std::get<Is>(args)))...);
}
// Global-scope variants. This mixes templates and overloading, which is a no-no, but it works.
// TODO C++17: if constexpr would work around this nicely.
template <typename funcPtr, funcPtr callable, typename T, size_t... Is, typename... types>
static void call(void*, ScriptInterface* scriptInterface, JS::CallArgs& callArgs, std::tuple<types...>& args, std::false_type, index_sequence<Is...>)
{
auto ret = (*callable)(pickVal(std::move(std::get<Is>(args)))...);
scriptInterface->ToJSVal(scriptInterface->GetContext(), callArgs.rval(), ret);
}
template <typename funcPtr, funcPtr callable, typename T, size_t... Is, typename... types>
static void call(void*, ScriptInterface* UNUSED(scriptInterface), JS::CallArgs&, std::tuple<types...>& args, std::true_type, index_sequence<Is...>)
{
(*callable)(pickVal(std::move(std::get<Is>(args)))...);
}
/**
* Call a c++ function (templateparam callable), on @param object (and if void, in the global scope).
*/
template <typename funcPtr, funcPtr callable, typename T = void, size_t N = args_info<funcPtr>::nb_args, typename tuple = typename args_info<funcPtr>::convertible_args>
static bool JSToCppCall(JSContext* cx, JS::CallArgs& args, T* object = nullptr)
{
ScriptInterface* scriptInterface = ScriptInterface::GetScriptInterfaceAndCBData(cx)->pScriptInterface;
// This is where the magic happens: instantiate a tuple to store the converted JS arguments,
// then 'unpack' the tuple to call the C++ function, convert & store the return value.
tuple outs;
if (!convertFromJS2<N>(*scriptInterface, cx, args, outs))
return false;
static_assert(std::is_same<typename args_info<funcPtr>::object_type, T>::value,
"C++ function called from JS with the wrong object type.");
// TODO: We have limited failure handling here. It's non trivial in a generic sense since we may return a value.
// We could either try-catch and throw exceptions,
// or come C++17 return an std::optional/maybe or some kind of [bool, val] structured binding.
// For now, rely on code calling a JS error, but that's meh for code that doesn't expect to be called from JS.
using returns_void = std::is_same<typename args_info<funcPtr>::return_type, void>;
call<funcPtr, callable, T>(object, scriptInterface, args, outs, returns_void{}, make_index_sequence<N>{});
return !ScriptInterface::IsExceptionPending(cx);
}
/**
* Call a C++ Function from a JS Function.
* TODO: this can get rewritten as <auto> and deduced come c++14
*/
template <typename funcPtr, funcPtr callable,
typename ObjType = typename args_info<funcPtr>::object_type,
ObjType*(*thisGetter)(JSContext* cx, JS::CallArgs& callArgs) = ScriptInterface::getPrivate<ObjType>>
static bool scriptMethod(JSContext* cx, unsigned argc, JS::Value* vp)
{
JSAutoRequest rq(cx);
JS::CallArgs args = JS::CallArgsFromVp(argc, vp);
// TODO C++17: this should be if constexpr-ed out (I'm relying on optimisations for now).
ObjType* obj = thisGetter != nullptr ? thisGetter(cx, args) : nullptr;
if (thisGetter != nullptr && !obj)
return false;
if (!JSToCppCall<funcPtr, callable>(cx, args, obj))
return false;
return true;
}
}
/**
* This could be in the above namespace, but it's convenient to be able to friend these by only early-declaring the class.
*/
class ScriptWrapper
{
template <typename T>
using args_info = ScriptWrapperHelper::args_info<T>;
public:
// flags must be a constant expression so until C++17 template-arguments it is.
template <typename funcPtr, funcPtr callable, u16 flags = JSPROP_ENUMERATE|JSPROP_READONLY|JSPROP_PERMANENT>
static JSFunctionSpec Wrap(const char* name)
{
return JS_FN(name, (&ScriptWrapperHelper::scriptMethod<funcPtr, callable>), args_info<funcPtr>::nb_args, flags);
}
/**
* Wrap a function, and register it on the root function object (usually 'Engine')
* For global & static function, there is no 'this' computed to call the pointer on.
* For method functors, 'this' is by default computed using getPrivate.
* Otherwise, you can pass your own "this getter".
*/
template <typename funcPtr, funcPtr callable, typename ObjType = typename args_info<funcPtr>::object_type, ObjType*(*thisGetter)(JSContext* cx, JS::CallArgs& callArgs) = std::is_same<ObjType, void>::value ? nullptr : ScriptInterface::getPrivate<ObjType>>
static void WrapAndRegister(const ScriptInterface& itf, const char* name)
{
itf.Register(name, &(ScriptWrapperHelper::scriptMethod<funcPtr, callable, ObjType, thisGetter>), args_info<funcPtr>::nb_args);
}
};
#endif // INCLUDED_FUNCTIONWRAPPER
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