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03-UnrealEngine/Rendering/RenderFeature/FRenderCommandFence.md
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03-UnrealEngine/Rendering/RenderFeature/FRenderCommandFence.md
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---
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title: FRenderCommandFence
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date: 2025-06-25 23:11:25
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excerpt:
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tags:
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rating: ⭐
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---
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在 Unreal Engine 5 (UE5) 中，`FRenderCommandFence` 是一个关键的**线程同步工具**，主要用于协调**游戏线程（Game Thread）** 和**渲染线程（Render Thread）** 之间的执行顺序。它的核心作用是确保渲染相关的操作（如资源创建、更新或销毁）在特定时间点前完成，避免多线程环境下的竞态条件。
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---
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### 核心作用
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1. **跨线程同步**
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- 游戏线程向渲染线程提交渲染命令后，这些命令不会立即执行（渲染线程有自己的任务队列）。
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- `FRenderCommandFence` 允许游戏线程**阻塞等待**，直到所有已提交的渲染命令（包括栅栏之前的所有命令）在渲染线程中执行完毕。
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2. **确保资源安全**
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- 当游戏线程需要操作渲染资源（如纹理、网格）时，必须确保渲染线程不再使用这些资源（例如释放 `UTexture`）。
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- 通过栅栏同步，可以安全地销毁或修改资源，避免访问无效内存。
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---
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### 关键方法
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- **`BeginFence()`**
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在渲染命令队列中插入一个“栅栏标记”，表示同步点。调用后游戏线程可以继续执行其他任务。
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- **`Wait(bool bProcessGameThreadTasks = false)`**
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- 阻塞游戏线程，直到渲染线程处理到栅栏位置（即执行完栅栏之前的所有渲染命令）。
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- `bProcessGameThreadTasks`: 若为 `true`，等待期间允许游戏线程处理其他任务（如消息泵）。
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- **`IsFenceComplete()`**
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非阻塞检查栅栏是否已完成（渲染线程是否已越过栅栏点）。
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---
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### 典型使用场景
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#### 1. 安全释放渲染资源
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```c++
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// 游戏线程代码
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void ReleaseTexture(UTexture* Texture)
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{
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// 将资源释放命令提交到渲染线程
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ENQUEUE_RENDER_COMMAND(ReleaseTextureCommand)(
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[Texture](FRHICommandListImmediate& RHICmdList) {
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Texture->ReleaseResource();
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}
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);
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// 创建栅栏并等待释放完成
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FRenderCommandFence Fence;
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Fence.BeginFence();
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Fence.Wait(); // 阻塞直到渲染线程执行完 ReleaseResource()
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}
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```
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#### 2. 确保渲染操作完成后再继续
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```c++
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// 更新动态纹理后确保数据已提交到GPU
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void UpdateDynamicTexture()
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{
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// 提交更新命令到渲染线程
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ENQUEUE_RENDER_COMMAND(UpdateTexture)(
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[...](...) { /* 更新纹理数据 */ }
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);
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FRenderCommandFence Fence;
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Fence.BeginFence();
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Fence.Wait(); // 等待纹理更新完成
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// 此时可以安全读取纹理或进行后续操作
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}
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```
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#### 3. 异步加载资源时同步
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```c++
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void LoadAssetAsync()
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{
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StreamableManager.RequestAsyncLoad(..., [this]()
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{
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// 资源加载完成后，确保渲染线程初始化完毕
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FRenderCommandFence Fence;
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Fence.BeginFence();
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Fence.Wait(); // 等待渲染线程处理完资源初始化
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OnAssetLoaded();
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});
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}
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```
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---
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### 注意事项
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1. **性能影响**
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`Wait()` 会阻塞游戏线程，过度使用可能导致帧率下降。应仅在必要时同步（如资源卸载）。
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2. **替代方案**
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- 对于非紧急任务，可用 `AsyncTask` 或 `TaskGraph` 异步处理。
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- 使用 `FGraphEvent` 实现无阻塞等待。
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3. **与 `FlushRenderingCommands()` 的区别**
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`FlushRenderingCommands()` 会强制**立即刷新**整个渲染命令队列（更重操作），而 `FRenderCommandFence` 只等待到特定同步点，更轻量可控。
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---
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### 总结
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`FRenderCommandFence` 是 UE5 多线程架构中的**安全阀**，通过它开发者可以：
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✅ 确保渲染操作在指定时间点前完成
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✅ 安全操作渲染资源（创建/更新/销毁）
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✅ 避免游戏线程与渲染线程的竞态条件
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@ -57,10 +57,107 @@ rating: ⭐
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- TerrainAndSpawnablesManagement()
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### ReadbacksManagement()
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相关线程安全变量：
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- **bProcessingHeightRetrieval**
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- **bProcessingHeightRetrievalRT**
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- ThreadSafe
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- bool
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- **bProcessingHeightRetrieval**
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- **bProcessingHeightRetrievalRT**
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- FSWShareableSamplePoints
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- PointsPendingReadBacks
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- FSWColorRead
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- ReadBackHeightData
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- FRenderCommandFence
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- HeightReadBackFence
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```c++
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void AShaderWorldActor::ReadbacksManagement()
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{
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if (!bProcessingHeightRetrieval.IsValid())
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bProcessingHeightRetrieval = MakeShared<FThreadSafeBool, ESPMode::ThreadSafe>();
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if (!bProcessingHeightRetrievalRT.IsValid())
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bProcessingHeightRetrievalRT = MakeShared<FThreadSafeBool, ESPMode::ThreadSafe>();
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//PointsPendingReadBacks存储从GPU回读的采样坐标位置;在RetrieveHeightAt()调用HeightReadBackFence.BeginFence(true)开启渲染栅栏，不开启这段逻辑不会执行。
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if ((*bProcessingHeightRetrieval.Get()) && (*bProcessingHeightRetrievalRT.Get()) && HeightReadBackFence.IsFenceComplete() && PointsPendingReadBacks.IsValid())
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{
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// ReadBackHeightData uin8 贴图数据
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if (!ReadBackHeightData.IsValid())
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return;
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//For now we do Compute samples on a rendertarget 5x5, therefore 25 positions evaluated per request.
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//每次请求可以读取25个点。
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const int NumOfVertex = 25;
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PositionsOfReadBacks.Empty();
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PositionsOfReadBacks.AddUninitialized(25);
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uint8* ReadData8 = (uint8*)ReadBackHeightData->ReadData.GetData();
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uint16 MaterialIndice = 0;
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for (int32 k = 0; k < NumOfVertex; k++)
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{
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FVector3f& PositionSample = PositionsOfReadBacks[k];
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if (RendererAPI == EGeoRenderingAPI::OpenGL)
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{
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int X = k % 5;
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int Y = k / 5;
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int index = X + Y * 5;
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Y = (5 - 1) - Y;
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index = X + Y * 5;
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PositionSample = FVector3f(PointsPendingReadBacks->PositionsXY[2 * k], PointsPendingReadBacks->PositionsXY[2 * k + 1], GetHeightFromGPURead(&ReadData8[index * 4], MaterialIndice) / HeightScale);
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}
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else
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PositionSample = FVector3f(PointsPendingReadBacks->PositionsXY[2 * k], PointsPendingReadBacks->PositionsXY[2 * k + 1], GetHeightFromGPURead(&ReadData8[k * 4], MaterialIndice) / HeightScale);
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}
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if(HeightRetrieveDelegate.ExecuteIfBound(PositionsOfReadBacks))
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{
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//SW_LOG("HeightRetrieveDelegate.ExecuteIfBound(PositionsOfReadBacks) PositionsOfReadBacks[0] %s",*PositionsOfReadBacks[0].ToString())
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}
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else
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{
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//SW_LOG("Fail HeightRetrieveDelegate.ExecuteIfBound(PositionsOfReadBacks)")
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}
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bProcessingHeightRetrieval->AtomicSet(false);
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bProcessingHeightRetrievalRT->AtomicSet(false);
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}
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//For now MinMax is here
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//处理FClipMapMeshElement的最大最小队列？
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if (UseSegmented())
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{
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if(WorldHasBounds_OnRebuild)
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{
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FClipMapMeshElement& MeshEl = FarLOD_BoundedWorld;
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if (MeshEl.MinMaxQueue.Num() > 0)
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{
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MeshEl.ProcessMinMaxQueue(SWorldSubsystem);
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}
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}
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for (FClipMapMeshElement& MeshEl : Meshes)
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{
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if (MeshEl.MinMaxQueue.Num() > 0)
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{
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MeshEl.ProcessMinMaxQueue(SWorldSubsystem);
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}
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}
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}
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}
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```
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### CollisionManagement()
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### SpawnablesManagement()
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### TerrainAndSpawnablesManagement()
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## PreEditChange() / PostEditChangeProperty()
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**PreEditChange()** 主要针对以下两个变量的设置：
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