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* Copyright 2013 Google Inc.
* Use of this source code is governed by a BSD-style license that can be
* found in the LICENSE file.
#ifndef SkOnce_DEFINED
#define SkOnce_DEFINED
#include <atomic>
#include <utility>
#include "SkTypes.h"
// SkOnce provides call-once guarantees for Skia, much like std::once_flag/std::call_once().
// There should be no particularly error-prone gotcha use cases when using SkOnce.
// It works correctly as a class member, a local, a global, a function-scoped static, whatever.
class SkOnce {
constexpr SkOnce() = default;
template <typename Fn, typename... Args>
void operator()(Fn&& fn, Args&&... args) {
auto state = fState.load(std::memory_order_acquire);
if (state == Done) {
// If it looks like no one has started calling fn(), try to claim that job.
if (state == NotStarted && fState.compare_exchange_strong(state, Claimed,
std::memory_order_relaxed)) {
// Great! We'll run fn() then notify the other threads by releasing Done into fState.
return, std::memory_order_release);
// Some other thread is calling fn().
// We'll just spin here acquiring until it releases Done into fState.
while (fState.load(std::memory_order_acquire) != Done) { /*spin*/ }
enum State : uint8_t { NotStarted, Claimed, Done};
std::atomic<uint8_t> fState{NotStarted};
#endif // SkOnce_DEFINED