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目录

概念

Async函数应用案例

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概念

   Async 函数的原型如下 

template<typename TFunction>
auto Async(EAsyncExecution::Type ExecutionType, TFunction&& Function) -> decltype(Function);

   Async 函数是一个模板函数，接受两个主要参数：

（1）EAsyncExecution::Type ExecutionType：用于指定任务执行的方式或线程相关的属性。可指定如下参数：

/** Execute in Task Graph (for short running tasks). */
TaskGraph,/** Execute in Task Graph on the main thread (for short running tasks). */
TaskGraphMainThread,/** Execute in separate thread if supported (for long running tasks). */
Thread,/** Execute in separate thread if supported or supported post fork (see FForkProcessHelper::CreateThreadIfForkSafe) (for long running tasks). */
ThreadIfForkSafe,/** Execute in global queued thread pool. */
ThreadPool,

（2）TFunction&& Function：这是一个可调用对象，通常可以是函数指针、lambda 表达式等，定义了具体要执行的任务逻辑。lambda表达式示例如下：

Async(EAsyncExecution::Thread, []() {// 这里放置需要在单独线程中执行的任务逻辑，比如执行一些耗时计算int sum = 0;for (int i = 0; i < 1000; i++){sum += i;}
});

Async函数应用案例

同时提交100个耗时任务。

如下代码所示，通过循环 100 次调用 AsyncTask 向 ENamedThreads::AnyThread提交异步任务，每个任务只是执行 FPlatformProcess::Sleep(2)，也就是让执行该任务的线程休眠 2 秒。

通过执行结果可以看到，执行任务的线程有很多重复的线程。由于提交了大量这样的任务，会使得线程池中大量的线程被占用并处于阻塞休眠状态。在这 2 秒内，这些线程无法去执行其他任务，而如果此时还有其他需要线程资源来执行的任务，就没办法获取到可用线程，进而导致整个程序的运行像是卡住了一样，无法顺利推进后续业务逻辑。

改进： 

通过循环 100 次调用 Async 函数提交异步任务，每个任务执行 FPlatformProcess::Sleep(10) 让线程休眠 10 秒，但虚幻引擎内部有一套相对复杂且智能的线程调度机制。在这种机制下，它会尝试合理地分配线程资源来执行这些任务，不会像 BlockThreadPool 函数那样简单粗暴地让大量线程同时进入阻塞休眠状态。虚幻引擎可能会根据系统当前的负载情况、线程优先级等因素，对任务进行排队或者分批次地安排线程去执行，避免一次性将所有线程资源耗尽，使得其他必要的任务依然有机会获取到线程来执行，从而保证程序整体上还能继续运行，不会出现明显的卡顿或卡住情况。

执行结果如下，可以看到执行任务的线程是100个不同的线程。

为了获取每个任务的结果，继续做如下改进：

void UThreadSubsystem::GetAsyncFuture()
{AsyncTask(ENamedThreads::AnyThread, [this]() {TArray<TFuture<int32>> FutureResults;for (size_t i = 0; i < 100; i++){FutureResults.AddDefaulted();FutureResults.Last()=Async(EAsyncExecution::Thread, [this, i]()->int32 {int32 SleepTime = i % 5;FPlatformProcess::Sleep(SleepTime);uint32 CurrentID = FPlatformTLS::GetCurrentThreadId();FString CurrentThread = FThreadManager::Get().GetThreadName(CurrentID);FString Info = FString::Printf(TEXT("-- Thread Finished -- CurrentThreadID:[%d] -- CurrentThreadName:[%s] -- SleepTime:%d"), CurrentID, *CurrentThread, SleepTime);PrintLogInThread(Info);return SleepTime;});}PrintLogInThread(TEXT("Task construction completed"));for (auto& TmpFuture : FutureResults){int32 SleepTime = TmpFuture.Get();PrintLogInThread(FString::FromInt(SleepTime));}PrintLogInThread(TEXT("Task execution completed"));});
}

首先通过 AsyncTask 函数将一个 lambda 表达式提交到 ENamedThreads::AnyThread 所代表的任意可用线程中执行。在 lambda 表达式中捕获了当前类指针 this，以便在这个异步任务内部能够访问类的成员函数和成员变量。

然后在 AsyncTask 函数内部创建了一个 TArray<TFuture<int32>> 类型的数组 FutureResults，用于存放 100 个 TFuture 对象，每个对象将与一个内层异步任务相关联，后续通过这些对象来获取对应异步任务的执行结果。

在for循环中，使用 FutureResults.AddDefaulted() 向 FutureResults 数组添加一个默认初始化的 TFuture<int32> 对象，然后通过 FutureResults.Last() 获取数组中刚添加的这个默认元素，并将 Async 函数返回的 TFuture 对象赋值给它。这里的 Async 函数用于创建实际执行具体任务逻辑的内层异步任务，传入参数 EAsyncExecution::Thread 表示每个内层异步任务会在单独的线程中执行。传递的 lambda 表达式定义了具体的任务逻辑，它捕获了当前类指针 this 和循环变量 i，并指定返回值类型为 int32。

最后通过一个 for 循环遍历 FutureResults 数组中的每个 TFuture 对象，然后调用 TmpFuture.Get() 来获取对应的内层异步任务结果。

执行效果如下：

应用案例代码：

 “ThreadSubsystem.h”

// Fill out your copyright notice in the Description page of Project Settings.#pragma once#include "CoreMinimal.h"
#include "SimpleRunnable.h"
#include "HAL/ThreadManager.h"
#include "ThreadSubsystem.generated.h"UCLASS()
class STUDY_API UThreadSubsystem : public UGameInstanceSubsystem
{GENERATED_BODY()public:virtual bool ShouldCreateSubsystem(UObject* Outer) const override;virtual void Initialize(FSubsystemCollectionBase& Collection) override;virtual void Deinitialize() override;public:UFUNCTION(BlueprintCallable)void BlockThreadPool();  //阻塞主线程的测试函数UFUNCTION(BlueprintCallable)void InitAsync();UFUNCTION(BlueprintCallable)void GetAsyncFuture();void PrintLogInThread(FString Info);
};

 “ThreadSubsystem.cpp” 

// Fill out your copyright notice in the Description page of Project Settings.#include "Thread/ThreadSubsystem.h"bool UThreadSubsystem::ShouldCreateSubsystem(UObject* Outer) const
{return true;
}void UThreadSubsystem::Initialize(FSubsystemCollectionBase& Collection)
{Super::Initialize(Collection);//InitSimpleThread();
}void UThreadSubsystem::Deinitialize()
{//ReleaseSimpleThread();Super::Deinitialize();
}void UThreadSubsystem::BlockThreadPool()
{for (size_t i = 0; i < 100; i++){AsyncTask(ENamedThreads::AnyThread, []() {FPlatformProcess::Sleep(2);uint32 CurrentID = FPlatformTLS::GetCurrentThreadId();FString CurrentThread = FThreadManager::Get().GetThreadName(CurrentID);FString Info = FString::Printf(TEXT("-- Thread Finished -- CurrentThreadID:[%d] -- CurrentThreadName:[%s]"), CurrentID, *CurrentThread);AsyncTask(ENamedThreads::GameThread, [Info]() {UE_LOG(LogTemp, Warning, TEXT("ThreadLog:[%s]"), *Info);});});}
}void UThreadSubsystem::InitAsync()
{for (size_t i = 0; i < 100; i++){Async(EAsyncExecution::Thread, [this]() {FPlatformProcess::Sleep(10);uint32 CurrentID = FPlatformTLS::GetCurrentThreadId();FString CurrentThread = FThreadManager::Get().GetThreadName(CurrentID);FString Info = FString::Printf(TEXT("-- Thread Finished -- CurrentThreadID:[%d] -- CurrentThreadName:[%s]"), CurrentID, *CurrentThread);PrintLogInThread(Info);});}
}void UThreadSubsystem::GetAsyncFuture()
{AsyncTask(ENamedThreads::AnyThread, [this]() {TArray<TFuture<int32>> FutureResults;for (size_t i = 0; i < 100; i++){FutureResults.AddDefaulted();FutureResults.Last()=Async(EAsyncExecution::Thread, [this, i]()->int32 {int32 SleepTime = i % 5;FPlatformProcess::Sleep(SleepTime);uint32 CurrentID = FPlatformTLS::GetCurrentThreadId();FString CurrentThread = FThreadManager::Get().GetThreadName(CurrentID);FString Info = FString::Printf(TEXT("-- Thread Finished -- CurrentThreadID:[%d] -- CurrentThreadName:[%s] -- SleepTime:%d"), CurrentID, *CurrentThread, SleepTime);PrintLogInThread(Info);return SleepTime;});}PrintLogInThread(TEXT("Task construction completed"));for (auto& TmpFuture : FutureResults){int32 SleepTime = TmpFuture.Get();PrintLogInThread(FString::FromInt(SleepTime));}PrintLogInThread(TEXT("Task execution completed"));});
}void UThreadSubsystem::PrintLogInThread(FString Info)
{AsyncTask(ENamedThreads::GameThread, [Info]() {UE_LOG(LogTemp, Warning, TEXT("ThreadLog:[%s]"), *Info);});
}