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 "include/private/base/SkThreadAnnotations.h" 12 13 #include <atomic> 14 #include <cstdint> 15 #include <utility> 16 17 // SkOnce provides call-once guarantees for Skia, much like std::once_flag/std::call_once(). 18 // 19 // There should be no particularly error-prone gotcha use cases when using SkOnce. 20 // It works correctly as a class member, a local, a global, a function-scoped static, whatever. 21 22 class SkOnce { 23 public: 24 constexpr SkOnce() = default; 25 26 template <typename Fn, typename... Args> operator()27 void operator()(Fn&& fn, Args&&... args) { 28 auto state = fState.load(std::memory_order_acquire); 29 30 if (state == Done) { 31 return; 32 } 33 34 // If it looks like no one has started calling fn(), try to claim that job. 35 if (state == NotStarted && fState.compare_exchange_strong(state, Claimed, 36 std::memory_order_relaxed, 37 std::memory_order_relaxed)) { 38 // Great! We'll run fn() then notify the other threads by releasing Done into fState. 39 fn(std::forward<Args>(args)...); 40 return fState.store(Done, std::memory_order_release); 41 } 42 43 // Some other thread is calling fn(). 44 // We'll just spin here acquiring until it releases Done into fState. 45 SK_POTENTIALLY_BLOCKING_REGION_BEGIN; 46 while (fState.load(std::memory_order_acquire) != Done) { /*spin*/ } 47 SK_POTENTIALLY_BLOCKING_REGION_END; 48 } 49 50 private: 51 enum State : uint8_t { NotStarted, Claimed, Done}; 52 std::atomic<uint8_t> fState{NotStarted}; 53 }; 54 55 #endif // SkOnce_DEFINED 56