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// Copyright 2020 The Dawn Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
#include <gmock/gmock.h>
#include "common/Assert.h"
#include <memory>
#include <set>
namespace testing {
template <typename F>
class MockCallback;
// Helper class for mocking callbacks used for Dawn callbacks with |void* userdata|
// as the last callback argument.
// Example Usage:
// MockCallback<WGPUDeviceLostCallback> mock;
// void* foo = XYZ; // this is the callback userdata
// wgpuDeviceSetDeviceLostCallback(device, mock.Callback(), mock.MakeUserdata(foo));
// EXPECT_CALL(mock, Call(_, foo));
template <typename R, typename... Args>
class MockCallback<R (*)(Args...)> : public ::testing::MockFunction<R(Args...)> {
using CallbackType = R (*)(Args...);
// Helper function makes it easier to get the callback using |foo.Callback()|
// unstead of MockCallback<CallbackType>::Callback.
static CallbackType Callback() {
return CallUnboundCallback;
void* MakeUserdata(void* userdata) {
auto mockAndUserdata =
std::unique_ptr<MockAndUserdata>(new MockAndUserdata{this, userdata});
// Add the userdata to a set of userdata for this mock. We never
// remove from this set even if a callback should only be called once so that
// repeated calls to the callback still forward the userdata correctly.
// Userdata will be destroyed when the mock is destroyed.
auto it = mUserdatas.insert(std::move(mockAndUserdata));
return it.first->get();
struct MockAndUserdata {
MockCallback* mock;
void* userdata;
static R CallUnboundCallback(Args... args) {
std::tuple<Args...> tuple = std::make_tuple(args...);
constexpr size_t ArgC = sizeof...(Args);
static_assert(ArgC >= 1, "Mock callback requires at least one argument (the userdata)");
// Get the userdata. It should be the last argument.
auto userdata = std::get<ArgC - 1>(tuple);
static_assert(std::is_same<decltype(userdata), void*>::value,
"Last callback argument must be void* userdata");
// Extract the mock.
ASSERT(userdata != nullptr);
auto* mockAndUserdata = reinterpret_cast<MockAndUserdata*>(userdata);
MockCallback* mock = mockAndUserdata->mock;
ASSERT(mock != nullptr);
// Replace the userdata
std::get<ArgC - 1>(tuple) = mockAndUserdata->userdata;
// Forward the callback to the mock.
return mock->CallImpl(std::make_index_sequence<ArgC>{}, std::move(tuple));
// This helper cannot be inlined because we dependent on the templated index sequence
// to unpack the tuple arguments.
template <size_t... Is>
R CallImpl(const std::index_sequence<Is...>&, std::tuple<Args...> args) {
return this->Call(std::get<Is>(args)...);
std::set<std::unique_ptr<MockAndUserdata>> mUserdatas;
} // namespace testing