1 /* 2 * Copyright 2013 Google Inc. 3 * 4 * Use of this source code is governed by a BSD-style license that can be 5 * found in the LICENSE file. 6 */ 7 8 #ifndef SkOnce_DEFINED 9 #define SkOnce_DEFINED 10 11 #include <atomic> 12 #include <utility> 13 #include "SkTypes.h" 14 15 // SkOnce provides call-once guarantees for Skia, much like std::once_flag/std::call_once(). 16 // 17 // There should be no particularly error-prone gotcha use cases when using SkOnce. 18 // It works correctly as a class member, a local, a global, a function-scoped static, whatever. 19 20 class SkOnce { 21 public: 22 constexpr SkOnce() = default; 23 24 template <typename Fn, typename... Args> 25 void operator()(Fn&& fn, Args&&... args) { 26 auto state = fState.load(std::memory_order_acquire); 27 28 if (state == Done) { 29 return; 30 } 31 32 // If it looks like no one has started calling fn(), try to claim that job. 33 if (state == NotStarted && fState.compare_exchange_strong(state, Claimed, 34 std::memory_order_relaxed, 35 std::memory_order_relaxed)) { 36 // Great! We'll run fn() then notify the other threads by releasing Done into fState. 37 fn(std::forward<Args>(args)...); 38 return fState.store(Done, std::memory_order_release); 39 } 40 41 // Some other thread is calling fn(). 42 // We'll just spin here acquiring until it releases Done into fState. 43 while (fState.load(std::memory_order_acquire) != Done) { /*spin*/ } 44 } 45 46 private: 47 enum State : uint8_t { NotStarted, Claimed, Done}; 48 std::atomic<uint8_t> fState{NotStarted}; 49 }; 50 51 #endif // SkOnce_DEFINED 52
